Nitrogen limitation inhibits marine diatom adaptation to high temperatures

Asymmetrical evolution of cross inhibition in zooplankton: insights from contrasting phosphorus limitation and salinization exposure sequences

Understanding the evolutionary responses of organisms to multiple stressors is crucial for predicting the ecological consequences of intensified anthropogenic activities. While previous studies have documented the effects of selection history on organisms' abilities to cope with new stressors, the impact of the sequence in which stressors occur on evolutionary outcomes remains less understood. In this study, we examined the evolutionary responses of a metazoan rotifer species to two prevalent freshwater stressors: nutrient limitation and increased salinization. We subjected rotifer populations with distinct selection histories (salt-adapted, low phosphorus-adapted and ancestral clones) to a reciprocal common garden experiment and monitored their population growth rates. Our results revealed an asymmetric evolutionary response to phosphorus (P) limitation and increased salinity. Specifically, adaptation to low P conditions reduced rotifer tolerance to increased salinity, whereas adaptation to saline conditions did not show such cross-inhibitory effects. Instead, the addition of moderate concentrations of salt enhanced the growth of the salt-adapted population in low P conditions, potentially as a consequence of evolved cross-tolerance. Our findings, therefore, underscore the importance of considering historical stressor regimes to improve our understanding and predictions of organismal responses to multiple stressors and also have significant implications for ecosystem management.

Toward a More Dynamic Metabolic Theory of Ecology to Predict Climate Change Effects on Biological Systems

AbstractThe metabolic theory of ecology (MTE) aims to link biophysical constraints on individual metabolic rates to the emergence of patterns at the population and ecosystem scales. Because MTE links temperature's kinetic effects on individual metabolism to ecological processes at higher levels of organization, it holds great potential to mechanistically predict how complex ecological systems respond to warming and increased temperature fluctuations under climate change. To scale up from individuals to ecosystems, applications of classical MTE implicitly assume that focusing on steady-state dynamics and averaging temperature responses across individuals and populations adequately capture the dominant attributes of biological systems. However, in the context of climate change, frequent perturbations from steady state and rapid changes in thermal performance curves via plasticity and evolution are almost guaranteed. Here, we explain how some of the assumptions made when applying MTE's simplest canonical expression can lead to blind spots in understanding how temperature change affects biological systems and how this presents an opportunity for formal expansion of the theory. We review existing advances in this direction and provide a decision tree for identifying when dynamic modifications to classical MTE are needed for certain research questions. We conclude with empirical and theoretical challenges to be addressed in a more dynamic MTE for understanding biological change in an increasingly uncertain world.

What Does It Mean to Be(Come) Arctic? Functional and Genetic Traits of Arctic- and Temperate-Adapted Diatoms

Climate change-induced warming is expected to drive phytoplankton poleward as they track suitable thermal conditions. However, successful establishment in new environments requires adaptation to multiple abiotic factors beyond temperature alone. As little is known about how polar species differ in key functional and genetic traits, simple predictions of poleward movement rely on large assumptions about performance in other relevant dimensions other than thermal responses (e.g., light regime, nutrient uptake). To identify evolutionary bottlenecks of poleward range shifts, we assessed a range of thermal, resource acquisition, and genetic traits for multiple strains of the diatom Thalassiosira rotula from the temperate North Sea, as well as multiple strains of the closely related Arctic Thalassiosira gravida. We found a broader thermal range for the temperate diatoms and a mean optimum temperature of 10.3°C ± 0.8°C and 18.4°C ± 2.4°C for the Arctic and temperate diatoms, respectively, despite similar maximum growth rates. Photoperiod reaction norms had an optimum photoperiod of approximately 17 h for temperate diatoms, whereas the Arctic diatoms exhibited their highest growth performance at a photoperiod of 24 h. Nitrate uptake kinetics showed high intraspecific variation without a habitat-specific signal. The screening for convergent amino acid substitutions (CAAS) of the studied diatom strains and other publicly available transcriptomes revealed 26 candidate genes in which potential habitat-specific genetic adaptation occurred. The identified genes include subunits of the DNA polymerase and multiple transcription factors (zinc-finger proteins). Our findings suggest that the thermal range of the temperate diatom would enable poleward migration, while the extreme polar photoperiods might pose a barrier to the Arctic. Additionally, the identified genetic adaptations are particularly abundant in Arctic diatoms as they may contribute to competitive advantages in polar habitats beyond those detected with our physiological assays, hampering the establishment of temperate diatoms in Arctic habitats.

The trade-offs associated with the adaptions of marine microalgae to high CO2 and warming

Trade-offs play vital roles in evolutionary theory, linking organism performance to changing environments in the context of global change. Marine microalgae, as one of the most important groups of primary producers in the biosphere, exhibit significant trade-offs across multiple traits in response to environmental changes, such as elevated CO2 (and consequent ocean acidification) and warming. In this review, we synthesize recent findings on the trade-offs associated with both short-term phenotypic acclimation and long-term genotypic adaptation of marine microalgae. Specifically, we discuss distinct classes of trade-offs (i.e., allocation trade-offs, acquisition trade-offs and specialist-generalist trade-offs) between multiple traits, such as growth rate, photosynthesis, nutrient acquisition, and stress tolerance. We also explored the underlying mechanisms driving these trade-offs. Finally, we discuss the broader ecological consequences of these trade-offs, such as potential shifts in species composition and ecosystem functions, and outline key research directions to better predict marine ecosystem responses to future global change scenarios.

Interactions between ocean acidification and multiple environmental drivers on the biochemical traits of marine primary producers: A meta-analysis

Ocean acidification (OA) interacts with multiple environmental drivers, such as temperature, nutrients, and ultraviolet radiation (UVR), posing a threat to marine primary producers. In this study, we conducted a quantitative meta-analysis of 1001 experimental assessments from 68 studies to examine the combined effects of OA and multiple environmental drivers (e.g., light, nutrient) on the biochemical compositions of marine primary producers. The results revealed significant positive effects of each environmental driver and their interactions with OA according to Hedge's d analysis. The results revealed significant positive effects of multiple environmental drivers and their interactions with OA. Additive effects dominated (71%), with smaller proportions of antagonistic (20%) and synergistic interactions (9%). The antagonistic interactions, although fewer, had a substantial impact, causing OA and other environmental drivers to interact antagonistically. Significant differences were observed among taxonomic groups: haptophytes and rhodophytes were negatively affected, while bacillariophytes were positively affected by OA. Our findings also indicated that the interactions between OA and multiple environmental drivers varied depending on specific type of the environmental driver, suggesting a modulating role of OA on the biochemical compositions of marine primary producers in response to global change. In summary, our study elucidates the complex interactions between OA and multiple environmental drivers on marine primary producers, highlighting the varied impacts on biochemical compositions and elemental stoichiometry.

Warming underpins community turnover in temperate freshwater and terrestrial communities

Rising temperatures are leading to increased prevalence of warm-affinity species in ecosystems, known as thermophilisation. However, factors influencing variation in thermophilisation rates among taxa and ecosystems, particularly freshwater communities with high diversity and high population decline, remain unclear. We analysed compositional change over time in 7123 freshwater and 6201 terrestrial, mostly temperate communities from multiple taxonomic groups. Overall, temperature change was positively linked to thermophilisation in both realms. Extirpated species had lower thermal affinities in terrestrial communities but higher affinities in freshwater communities compared to those persisting over time. Temperature change's impact on thermophilisation varied with community body size, thermal niche breadth, species richness and baseline temperature; these interactive effects were idiosyncratic in the direction and magnitude of their impacts on thermophilisation, both across realms and taxonomic groups. While our findings emphasise the challenges in predicting the consequences of temperature change across communities, conservation strategies should consider these variable responses when attempting to mitigate climate-induced biodiversity loss.

Environmental forcing of phytoplankton carbon-to-diversity ratio and carbon-to-chlorophyll ratio: A case study in Jiaozhou Bay, the Yellow Sea

The relationships between phytoplankton carbon (C) biomass and diversity (i.e., C-to-H' ratio) and chlorophyll a (i.e., C-to-Chl a ratio) are good indicators of marine ecosystem functioning and stability. Here we conducted four cruises spanning 2 years in Jiaozhou Bay to explore the dynamics of C-to-H' and C-to-Chl a ratios. The results showed that the phytoplankton C biomass and diversity were dominated by diatoms, followed by dinoflagellates. The average C-to-H' ratio ranged from 84.10 to 912.17, with high values occurring in the northern region of the bay. In contrast, the average C-to-Chl a ratio ranged between 15.55 and 89.47, and high values primarily appeared in the northern or northeastern part of the bay. In addition, the redundancy analysis showed that temperature and phosphate (DIP) were significantly correlated with both ratios in most cases, indicating that temperature and DIP may be key factors affecting the dynamics of C-to-H' and C-to-Chl a ratios.

Natural ocean iron fertilization and climate variability over geological periods

Marine primary producers are largely dependent on and shape the Earth's climate, although their relationship with climate varies over space and time. The growth of phytoplankton and associated marine primary productivity in most of the modern global ocean is limited by the supply of nutrients, including the micronutrient iron. The addition of iron via episodic and frequent events drives the biological carbon pump and promotes the sequestration of atmospheric carbon dioxide (CO2 ) into the ocean. However, the dependence between iron and marine primary producers adaptively changes over different geological periods due to the variation in global climate and environment. In this review, we examined the role and importance of iron in modulating marine primary production during some specific geological periods, that is, the Great Oxidation Event (GOE) during the Huronian glaciation, the Snowball Earth Event during the Cryogenian, the glacial-interglacial cycles during the Pleistocene, and the period from the last glacial maximum to the late Holocene. Only the change trend of iron bioavailability and climate in the glacial-interglacial cycles is consistent with the Iron Hypothesis. During the GOE and the Snowball Earth periods, although the bioavailability of iron in the ocean and the climate changed dramatically, the changing trend of many factors contradicted the Iron Hypothesis. By detangling the relationship among marine primary productivity, iron availability and oceanic environments in different geological periods, this review can offer some new insights for evaluating the impact of ocean iron fertilization on removing CO2 from the atmosphere and regulating the climate.

Dissecting the contributions of organic nitrogen aerosols to global atmospheric nitrogen deposition and implications for ecosystems

Atmospheric deposition of particulate organic nitrogen (ONp) is a significant process in the global nitrogen cycle and may be pivotally important for N-limited ecosystems. However, past models largely overlooked the spatial and chemical inhomogeneity of atmospheric ONp and were thus deficient in assessing global ONp impacts. We constructed a comprehensive global model of atmospheric gaseous and particulate organic nitrogen (ON), including the latest knowledge on emissions and secondary formations. Using this model, we simulated global atmospheric ONp abundances consistent with observations. Our estimated global atmospheric ON deposition was 26 Tg N yr-1, predominantly in the form of ONp (23 Tg N yr-1) and mostly from wildfires (37%), oceans (22%) and aqueous productions (17%). Globally, ONp contributed as much as 40% to 80% of the total N deposition downwind of biomass-burning regions. Atmospheric ONp deposition thus constituted the dominant external N supply to the N-limited boreal forests, tundras and the Arctic Ocean, and its importance may be amplified in a future warming climate.

Effect of ocean warming on pigment and photosynthetic carbon fixation of plankton assemblage in Pingtan Island of Southeast China

Temperature plays an important role in affecting the physiological traits of marine plankton. In this study, we conducted an outdoor incubation experiment to investigate the effects of elevated temperature on Chl a, photosynthetic carbon fixation and the composition of plankton communities in the surface seawater around Pingtan Island, the northwest Taiwan Strait in Autumn 2022. After 3-4 days of incubation, elevated temperature (1-4 °C higher than ambient temperature) led to a decrease in Chl a concentration across all three stations, did not result in significant increases in the particulate organic carbon (POC) and nitrogen (PON) concentrations in seawater with high nitrate concentrations, whereas increased POC and PON concentrations in nitrate-limited seawater. These findings suggest that the effect of temperature on the POC and PON contents of plankton is affected by the availability of nitrate. Diatoms were the dominant phytoplankton group in all three stations. Our results indicate that ocean warming has a potential to increase the POC contents of marine plankton per volume of seawater, which may increase the ability of phytoplankton to absorb atmospheric CO2 and to alleviate global warming.

Warming modulates the photosynthetic performance of Thalassiosira pseudonana in response to UV radiation

Diatoms form a major component of phytoplankton. These eukaryotic organisms are responsible for approximately 40% of primary productivity in the oceans and contribute significantly to the food web. Here, the influences of ultraviolet radiation (UVR) and ocean warming on diatom photosynthesis were investigated in Thalassiosira pseudonana. The organism was grown at two temperatures, namely, 18°C, the present surface water temperature in summer, and 24°C, an estimate of surface temperature in the year 2,100, under conditions of high photosynthetically active radiation (P, 400-700 nm) alone or in combination with UVR (P + UVR, 295-700 nm). It was found that the maximum photochemical yield of PSII (Fv/Fm) in T. pseudonana was significantly decreased by the radiation exposure with UVR at low temperature, while the rise of temperature alleviated the inhibition induced by UVR. The analysis of PSII subunits turnover showed that high temperature alone or worked synergistically with UVR provoking fast removal of PsbA protein (KPsbA), and also could maintain high PsbD pool in T. pseudonana cells. With the facilitation of PSII repair process, less non-photochemical quenching (NPQ) occurred at high temperature when cells were exposed to P or P + UVR. In addition, irrespective of radiation treatments, high temperature stimulated the induction of SOD activity, which partly contributed to the higher PSII repair rate constant (Krec) as compared to KPsbA. Our findings suggest that the rise in temperature could benefit the photosynthetic performance of T. pseudonana via modulation of its PSII repair cycle and protective capacity, affecting its abundance in phytoplankton in the future warming ocean.

Mechanistic constraints on the trade-off between photosynthesis and respiration in response to warming

Phytoplankton are responsible for half of all oxygen production and drive the ocean carbon cycle. Metabolic theory predicts that increasing global temperatures will cause phytoplankton to become more heterotrophic and smaller. Here, we uncover the metabolic trade-offs between cellular space, energy, and stress management driving phytoplankton thermal acclimation and how these might be overcome through evolutionary adaptation. We show that the observed relationships between traits such as chlorophyll, lipid content, C:N, and size can be predicted on the basis of the metabolic demands of the cell, the thermal dependency of transporters, and changes in membrane lipids. We suggest that many of the observed relationships are not fixed physiological constraints but rather can be altered through adaptation. For example, the evolution of lipid metabolism can favor larger cells with higher lipid content to mitigate oxidative stress. These results have implications for rates of carbon sequestration and export in a warmer ocean.

Comparative experimental evolution reveals species-specific idiosyncrasies in marine phytoplankton adaptation to warming

A number of experimental studies have demonstrated that phytoplankton can display rapid thermal adaptation in response to warmed environments. While these studies provide insight into the evolutionary responses of single species, they tend to employ different experimental techniques. Consequently, our ability to compare the potential for thermal adaptation across different, ecologically relevant, species remains limited. Here, we address this limitation by conducting simultaneous long-term warming experiments with the same experimental design on clonal isolates of three phylogenetically diverse species of marine phytoplankton; the cyanobacterium Synechococcus sp., the prasinophyte Ostreococcus tauri and the diatom Phaeodoactylum tricornutum. Over the same experimental time period, we observed differing levels of thermal adaptation in response to stressful supra-optimal temperatures. Synechococcus sp. displayed the greatest improvement in fitness (i.e., growth rate) and thermal tolerance (i.e., temperature limits of growth). Ostreococcus tauri was able to improve fitness and thermal tolerance, but to a lesser extent. Finally, Phaeodoactylum tricornutum showed no signs of adaptation. These findings could help us understand how the structure of phytoplankton communities may change in response to warming, and possible biogeochemical implications, as some species show relatively more rapid adaptive shifts in their thermal tolerance.

Mixoplankton and mixotrophy: future research priorities

Phago-mixotrophy, the combination of photoautotrophy and phagotrophy in mixoplankton, organisms that can combine both trophic strategies, have gained increasing attention over the past decade. It is now recognized that a substantial number of protistan plankton species engage in phago-mixotrophy to obtain nutrients for growth and reproduction under a range of environmental conditions. Unfortunately, our current understanding of mixoplankton in aquatic systems significantly lags behind our understanding of zooplankton and phytoplankton, limiting our ability to fully comprehend the role of mixoplankton (and phago-mixotrophy) in the plankton food web and biogeochemical cycling. Here, we put forward five research directions that we believe will lead to major advancement in the field: (i) evolution: understanding mixotrophy in the context of the evolutionary transition from phagotrophy to photoautotrophy; (ii) traits and trade-offs: identifying the key traits and trade-offs constraining mixotrophic metabolisms; (iii) biogeography: large-scale patterns of mixoplankton distribution; (iv) biogeochemistry and trophic transfer: understanding mixoplankton as conduits of nutrients and energy; and (v) in situ methods: improving the identification of in situ mixoplankton and their phago-mixotrophic activity.

Multi-trait analysis reveals large interspecific differences for phytoplankton in response to thermal change

Understanding the responses of multiple traits in phytoplankton, and identifying interspecific variabilities to thermal changes is crucial for predicting the impacts of ocean warming on phytoplankton distributions and community structures in future scenarios. Here, we applied a trait-based approach by examining the patterns in multi-traits variations (eight traits) and interspecific variabilities in five phytoplankton species (two diatoms, three dinoflagellates) in response to a wide range of ecologically relevant temperatures (14-30 °C). Our results show large inter-traits and interspecific variabilities of thermal reaction norms in all of the tested traits. We also found that the interspecific variability exceeded the variations induced by thermal changes. Constrained variations and trade-offs between traits both revealed substantial interspecific differences and shifted as the temperature changed. Our study helps to understand the species-specific response patterns of multiple traits to ocean warming and to investigate the implications of these responses in the context of global change.

The cellular response to ocean warming in Emiliania huxleyi

Marine phytoplankton contribute substantially to the global flux of carbon from the atmosphere to the deep ocean. Sea surface temperatures will inevitably increase in line with global climate change, altering the performance of marine phytoplankton. Differing sensitivities of photosynthesis and respiration to temperature, will likely shift the strength of the future oceanic carbon sink. To further clarify the molecular mechanisms driving these alterations in phytoplankton function, shotgun proteomic analysis was carried out on the globally-occurring coccolithophore Emiliania huxleyi exposed to moderate- (23°C) and elevated- (28°C) warming. Compared to the control (17°C), growth of E. huxleyi increased under elevated temperatures, with higher rates recorded under moderate- relative to elevated- warming. Proteomic analysis revealed a significant modification of the E. huxleyi cellular proteome as temperatures increased: at lower temperature, ribosomal proteins and photosynthetic machinery appeared abundant, as rates of protein translation and photosynthetic performance are restricted by low temperatures. As temperatures increased, evidence of heat stress was observed in the photosystem, characterized by a relative down-regulation of the Photosystem II oxygen evolving complex and ATP synthase. Acclimation to elevated warming (28°C) revealed a substantial alteration to carbon metabolism. Here, E. huxleyi made use of the glyoxylate cycle and succinate metabolism to optimize carbon use, maintain growth and maximize ATP production in heat-damaged mitochondria, enabling cultures to maintain growth at levels significantly higher than those recorded in the control (17°C). Based on the metabolic changes observed, we can predict that warming may benefit photosynthetic carbon fixation by E. huxleyi in the sub-optimal to optimal thermal range. Past the thermal optima, increasing rates of respiration and costs of repair will likely constrain growth, causing a possible decline in the contribution of this species to the oceanic carbon sink depending on the evolvability of these temperature thresholds.

Microplastics trigger the Matthew effect on nitrogen assimilation in marine diatoms at an environmentally relevant concentration

Microplastics (MPs, diameter <5 mm) are widely distributed on Earth, especially in the oceans. Diatoms account for ∼40% of marine primary productivity and affect the global biogeochemical cycles of macroelements. However, the effects of MPs on marine nitrogen cycling remain poorly understood, particularly comparisons between nitrogen-replete and nitrogen-limited conditions. We found that MPs trigger the Matthew effect on nitrogen assimilation in diatoms, where MPs inhibited nitrogen assimilation under nitrogen-limited conditions while enhancing nitrogen metabolism under nitrogen-replete conditions in Phaeodactylum tricornutum. Nitrate reductase (NR) and nitrite reductase (NIR) are upregulated, but nitrate transporter (NRT) and glutamine synthetase (GS) are downregulated by MPs under nitrogen-limited conditions. In contrast, NR, NIR, and GS are all upregulated by MPs under nitrogen-replete conditions. MPs accelerate nitrogen anabolic processes with an increase in the accumulation of carbohydrates by 80.7 ± 7.9% and enhance the activities of key nitrogen-metabolizing enzymes (8.20-44.90%) under nitrogen-replete conditions. In contrast, the abundance of carbohydrates decreases by 22.0-34.4%, and NRT activity is inhibited by 79.0-86.5% in nitrogen-limited algae exposed to MPs. Metabolomic and transcriptomic analyses were performed to further explore the molecular mechanisms of reprogrammed nitrogen assimilation, including carbon metabolism, nitrogen transport and ammonia assimilation. The aforementioned spatial redistribution (e.g., the Matthew effect between nitrogen-replete and -limited conditions) of nitrogen assimilation highlights the potential risks of MP contamination in the ocean.

Long-term adaptation to elevated temperature but not CO2 alleviates the negative effects of ultraviolet-B radiation in a marine diatom

Multifaceted changes in marine environments as a result of anthropogenic activities are likely to have a compounding impact on the physiology of marine phytoplankton. Most studies on the combined effects of rising pCO₂, sea surface temperature, and UVB radiation on marine phytoplankton were only conducted in the short-term, which does not allow to test the adaptive capacity of phytoplankton and associated potential trade-offs. Here, we investigated populations of the diatom Phaeodactylum tricornutum that were long-term (∼3.5 years, ∼3000 generations) adapted to elevated CO₂ and/or elevated temperatures, and their physiological responses to short-term (∼2 weeks) exposure of two levels of ultraviolet-B (UVB) radiation. Our results showed that while elevated UVB radiation showed predominantly negative effects on the physiological performance of P. tricornutum regardless of adaptation regimes. Elevated temperature alleviated these effects on most of the measured physiological parameters (e.g., photosynthesis). We also found that elevated CO₂ can modulate these antagonistic interactions, and conclude that long-term adaptation to sea surface warming and rising CO₂ may alter this diatom's sensitivity to elevated UVB radiation in the environment. Our study provides new insights into marine phytoplankton's long-term responses to the interplay of multiple environmental changes driven by climate change.

Predicting effects of multiple interacting global change drivers across trophic levels

Global change encompasses many co-occurring anthropogenic drivers, which can act synergistically or antagonistically on ecological systems. Predicting how different global change drivers simultaneously contribute to observed biodiversity change is a key challenge for ecology and conservation. However, we lack the mechanistic understanding of how multiple global change drivers influence the vital rates of multiple interacting species. We propose that reaction norms, the relationships between a driver and vital rates like growth, mortality, and consumption, provide insights to the underlying mechanisms of community responses to multiple drivers. Understanding how multiple drivers interact to affect demographic rates using a reaction-norm perspective can improve our ability to make predictions of interactions at higher levels of organization-that is, community and food web. Building on the framework of consumer-resource interactions and widely studied thermal performance curves, we illustrate how joint driver impacts can be scaled up from the population to the community level. A simple proof-of-concept model demonstrates how reaction norms of vital rates predict the prevalence of driver interactions at the community level. A literature search suggests that our proposed approach is not yet used in multiple driver research. We outline how realistic response surfaces (i.e., multidimensional reaction norms) can be inferred by parametric and nonparametric approaches. Response surfaces have the potential to strengthen our understanding of how multiple drivers affect communities as well as improve our ability to predict when interactive effects emerge, two of the major challenges of ecology today.

Evidence for evolutionary adaptation of mixotrophic nanoflagellates to warmer temperatures

Mixotrophs, organisms that combine photosynthesis and heterotrophy to gain energy, play an important role in global biogeochemical cycles. Metabolic theory predicts that mixotrophs will become more heterotrophic with rising temperatures, potentially creating a positive feedback loop that accelerates carbon dioxide accumulation in the atmosphere. Studies testing this theory have focused on phenotypically plastic (short-term, non-evolutionary) thermal responses of mixotrophs. However, as small organisms with short generation times and large population sizes, mixotrophs may rapidly evolve in response to climate change. Here, we present data from a 3-year experiment quantifying the evolutionary response of two mixotrophic nanoflagellates to temperature. We found evidence for adaptive evolution (increased growth rates in evolved relative to acclimated lineages) in the obligately phototrophic strain, but not in the facultative phototroph. All lineages showed trends of increased carbon use efficiency, flattening of thermal reaction norms, and a return to homeostatic gene expression. Generally, mixotrophs evolved reduced photosynthesis and higher grazing with increased temperatures, suggesting that evolution may act to exacerbate mixotrophs' effects on global carbon cycling.

Resource limitation determines realized thermal performance of consumers in trophodynamic models

Recent work has demonstrated that changes in resource availability can alter a consumer's thermal performance curve (TPC). When resources decline, the optimal temperature and breadth of thermal performance also decline, leading to a greater risk of warming than predicted by static TPCs. We investigate the effect of temperature on coupled consumer-resource dynamics, focusing on the potential for changes in the consumer TPC to alter extinction risk. Coupling consumer and resource dynamics generally reduces the potential for resource decline to exacerbate the effects of warming via changes to the TPC due to a reduction in top-down control when consumers near the limits of their thermal performance curve. However, if resources are more sensitive to warming, consumer TPCs can be reshaped by declining resources, leading to increased extinction risk. Our work elucidates the role of top-down and bottom-up regulation in determining the extent to which changes in resource density alter consumer TPCs.

Adaptation of a marine diatom to ocean acidification increases its sensitivity to toxic metal exposure

Most previous studies investigating the interplay of ocean acidification (OA) and heavy metal on marine phytoplankton were only conducted in short-term, which may provide conservative estimates of the adaptive capacity of them. Here, we examined the physiological responses of long-term (~900 generations) OA-adapted and non-adapted populations of the diatom Phaeodactylum tricornutum to different concentrations of the two heavy metals Cd and Cu. Our results showed that long-term OA selected populations exhibited significantly lower growth and reduced photosynthetic activity than ambient CO₂ selected populations at relatively high heavy metal levels. Those findings suggest that the adaptations to high CO₂ results in an increased sensitivity of the marine diatom to toxic metal exposure. This study provides evidence for the costs and the cascading consequences associated with the adaptation of phytoplankton to elevated CO₂ conditions, and improves our understanding of the complex interactions of future OA and heavy metal pollution in marine waters.

Effects of high temperature and nitrogen availability on the growth and composition of the marine diatom Chaetoceros pseudocurvisetus

The assimilation of inorganic nutrients by phytoplankton strongly depends on environmental conditions such as the availability of nitrogen and temperature, especially warming. The acclimation or adaptation of different species to such changes remains poorly understood. Here, we used a multimethod approach to study the viability and physiological and biochemical responses of the marine diatom Chaetoceros pseudocurvisetus to different temperatures (15, 25, and 30 °C) and different N:P ratios. Nitrogen limitation had a greater effect than high temperature on cell growth and reproduction, leading to a marked elongation of setae, decreased phosphorus assimilation, increased lipid accumulation, and decreased protein synthesis. The elongation of setae observed under these conditions may serve to increase the surface area available for the uptake of inorganic and/or organic nitrogen. In contrast, high temperatures (30 °C) had a stronger effect than nitrogen deficiency on cell death, nitrogen assimilation, chlorophyll a accumulation, the cessation of setae formation, and cell lipid remodelling. Significant changes in thylakoid lipids were observed in cells maintained at 30 °C, with increased levels of digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol. These changes may be explained by the role of galactolipids in thylakoid membrane stabilization during heat stress.

The need for unrealistic experiments in global change biology

Climate change is an existential threat, and our ability to conduct experiments on how organisms will respond to it is limited by logistics and resources, making it vital that experiments be maximally useful. The majority of experiments on phytoplankton responses to warming and CO₂ use only two levels of each driver. However, to project the characters of future populations, we need a mechanistic and generalisable explanation for how phytoplankton respond to concurrent changes in temperature and CO₂. This requires experiments with more driver levels, to produce response surfaces that can aid in the development of predictive models. We recommend prioritising experiments or programmes that produce such response surfaces on multiple scales for phytoplankton.

Differential Physiological, Transcriptomic, and Metabolomic Responses of Paspalum wettsteinii Under High-Temperature Stress

Global warming has far-reaching effects on plant growth and development. As a warm-season forage grass, Paspalum wettsteinii is highly adaptable to high temperatures. However, the response mechanism of P. wettsteinii under high-temperature stress is still unclear. Therefore, we investigated the physiological indicators, transcriptome and metabolome of P. wettsteinii under different heat stress treatments. Plant height, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and the contents of soluble sugar, proline, chlorophyll a, and chlorophyll b increased and then decreased, while the malondialdehyde (MDA) content decreased and then increased with increasing heat stress. Transcriptomic analysis revealed that genes related to energy and carbohydrate metabolism, heat shock proteins (HSPs), and transcription factors (TFs), secondary metabolite biosynthesis and the antioxidant system significantly changed to varying degrees. Metabolomic analysis showed that only free fatty acids were downregulated, while amino acids and their derivatives, organic acids, flavonoids, and sugars were both up- and downregulated under heat stress. These combined analyses revealed that growth was promoted at 25-40°C, while at 45°C, excess reactive oxygen species (ROS) damage reduced antioxidant and osmoregulatory effects and inactivated genes associated with the light and electron transport chains (ETCs), as well as damaged the PS II system and inhibited photosynthesis. A small number of genes and metabolites were upregulated to maintain the basic growth of P. wettsteinii. The physiological and biochemical changes in response to high-temperature stress were revealed, and the important metabolites and key genes involved in the response to high temperature were identified, providing an important reference for the physiological and molecular regulation of high-temperature stress in plants.

Additive impacts of ocean acidification and ambient ultraviolet radiation threaten calcifying marine primary producers

Ocean acidification (OA) represents a threat to marine organisms and ecosystems. However, OA rarely exists in isolation but occurs concomitantly with other stressors such as ultraviolet radiation (UVR), whose effects have been neglected in oceanographical observations. Here, we perform a quantitative meta-analysis based on 373 published experimental assessments from 26 studies to examine the combined effects of OA and UVR on marine primary producers. The results reveal predominantly additive stressor interactions (69-84% depending on the UV waveband), with synergistic and antagonistic interactions being rare but significantly different between micro- and macro-algae. In microalgae, variations in interaction type frequencies are related to cell volume, with antagonistic interactions accounting for a higher proportion in larger sized species. Despite additive interactions being most frequent, the small proportion of antagonistic interactions appears to have a stronger power, leading to neutral effects of OA in combination with UVR. High levels of UVR at near in situ conditions in combination with OA showed additive inhibition of calcification, but not when UVR was low. The results also reveal that the magnitude of responses is strongly dependent on experimental duration, with the negative effects of OA on calcification and pigmentation being buffered and amplified by increasing durations, respectively. Tropical primary producers were more vulnerable to OA or UVR alone compared to conspecifics from other climatic regions. Our analysis highlights that further multi-stressor long-term adaptation experiments with marine organisms of different cell volumes (especially microalgae) from different climatic regions are needed to fully disclose future impacts of OA and UVR.

Thermal Acclimation and Adaptation in Marine Protozooplankton and Mixoplankton

Proper thermal adaptation is key to understanding how species respond to long-term changes in temperature. However, this is seldom considered in protozooplankton and mixoplankton experiments. In this work, we studied how two heterotrophic dinoflagellates (Gyrodinium dominans and Oxyrrhis marina), one heterotrophic ciliate (Strombidium arenicola), and one mixotrophic dinoflagellate (Karlodinium armiger) responded to warming. To do so, we compared strains adapted at 16, 19, and 22°C and those adapted at 16°C and exposed for 3 days to temperature increases of 3 and 6°C (acclimated treatments). Neither their carbon, nitrogen or phosphorus (CNP) contents nor their corresponding elemental ratios showed straightforward changes with temperature, except for a modest increase in P contents with temperature in some grazers. In general, the performance of both acclimated and adapted grazers increased from 16 to 19°C and then dropped at 22°C, with a few exceptions. Therefore, our organisms followed the "hotter is better" hypothesis for a temperature rise of 3°C; an increase of >6°C, however, resulted in variable outcomes. Despite the disparity in responses among species and physiological rates, 19°C-adapted organisms, in general, performed better than acclimated-only (16°C-adapted organisms incubated at +3°C). However, at 22°C, most species were at the limit of their metabolic equilibrium and were unable to fully adapt. Nevertheless, adaptation to higher temperatures allowed strains to maintain physiological activities when exposed to sudden increases in temperature (up to 25°C). In summary, adaptation to temperature seems to confer a selective advantage to protistan grazers within a narrow range (i.e., ca. 3°C). Adaptation to much higher increases of temperatures (i.e., +6°C) does not confer any clear physiological advantage (with few exceptions; e.g., the mixotroph K. armiger), at least within the time frame of our experiments.

A reduction in metabolism explains the tradeoffs associated with the long-term adaptation of phytoplankton to high CO2 concentrations

Phytoplankton are responsible for nearly half of global primary productivity and play crucial roles in the Earth's biogeochemical cycles. However, the long-term adaptive responses of phytoplankton to rising CO₂ remains unknown. Here we examine the physiological and proteomics responses of a marine diatom, Phaeodactylum tricornutum, following long-term (c. 900 generations) selection to high CO₂ conditions. Our results show that this diatom responds to long-term high CO₂ selection by downregulating proteins involved in energy production (Calvin cycle, tricarboxylic acid cycle, glycolysis, oxidative pentose phosphate pathway), with a subsequent decrease in photosynthesis and respiration. Nearly similar extents of downregulation of photosynthesis and respiration allow the high CO₂ -adapted populations to allocate the same fraction of carbon to growth, thereby maintaining their fitness during the long-term high CO₂ selection. These results indicate an important role of metabolism reduction under high CO₂ and shed new light on the adaptive mechanisms of phytoplankton in response to climate change.

The marine nitrogen cycle: new developments and global change

The ocean is home to a diverse and metabolically versatile microbial community that performs the complex biochemical transformations that drive the nitrogen cycle, including nitrogen fixation, assimilation, nitrification and nitrogen loss processes. In this Review, we discuss the wealth of new ocean nitrogen cycle research in disciplines from metaproteomics to global biogeochemical modelling and in environments from productive estuaries to the abyssal deep sea. Influential recent discoveries include new microbial functional groups, novel metabolic pathways, original conceptual perspectives and ground-breaking analytical capabilities. These emerging research directions are already contributing to urgent efforts to address the primary challenge facing marine microbiologists today: the unprecedented onslaught of anthropogenic environmental change on marine ecosystems. Ocean warming, acidification, nutrient enrichment and seawater stratification have major effects on the microbial nitrogen cycle, but widespread ocean deoxygenation is perhaps the most consequential for the microorganisms involved in both aerobic and anaerobic nitrogen transformation pathways. In turn, these changes feed back to the global cycles of greenhouse gases such as carbon dioxide and nitrous oxide. At a time when our species casts a lengthening shadow across all marine ecosystems, timely new advances offer us unique opportunities to understand and better predict human impacts on nitrogen biogeochemistry in the changing ocean of the Anthropocene.

Divergent fate of coccolithophores in a warming tropical ecosystem

Rising ocean temperatures will alter the diversity of marine phytoplankton communities, likely leading to modifications in food-web and biogeochemical dynamics. Here we focus on coccolithophores, a prominent group of calcifying phytoplankton that plays a central role in the global carbon cycle. Using both new (2017-2020) and historical (1975-1976) data from the northern Red Sea, we found that during 'mild summers', the most common coccolithophores - Emiliania huxleyi and Gephyrocapsa ericsonii - co-exist at similar densities. Both species then particularly flourish during subsequent winter periods where nutrient availability is higher due to convective mixing. However, during 'hot summers', which have become progressively the norm over the last decades with average surface temperatures exceeding 27°C for long time-periods, G. ericsonii density markedly declined. Moreover, G. ericsonii remains at low background levels even during winter mixing periods, while E. huxleyi succession and development during winter appears unchanged. Further incubation assays using native assemblages confirmed that G. ericsonii's growth over 27°C is significantly reduced relative to E. huxleyi. Additional factors likely contribute to impair G. ericsonii populations at sea, but temperature is a key factor. Our results illustrate the divergent impact of ongoing ocean warming in tropical phytoplankton species.

Biogeochemical extremes and compound events in the ocean

The ocean is warming, losing oxygen and being acidified, primarily as a result of anthropogenic carbon emissions. With ocean warming, acidification and deoxygenation projected to increase for decades, extreme events, such as marine heatwaves, will intensify, occur more often, persist for longer periods of time and extend over larger regions. Nevertheless, our understanding of oceanic extreme events that are associated with warming, low oxygen concentrations or high acidity, as well as their impacts on marine ecosystems, remains limited. Compound events-that is, multiple extreme events that occur simultaneously or in close sequence-are of particular concern, as their individual effects may interact synergistically. Here we assess patterns and trends in open ocean extremes based on the existing literature as well as global and regional model simulations. Furthermore, we discuss the potential impacts of individual and compound extremes on marine organisms and ecosystems. We propose a pathway to improve the understanding of extreme events and the capacity of marine life to respond to them. The conditions exhibited by present extreme events may be a harbinger of what may become normal in the future. As a consequence, pursuing this research effort may also help us to better understand the responses of marine organisms and ecosystems to future climate change.

Marine phytoplankton functional types exhibit diverse responses to thermal change

Marine phytoplankton generate half of global primary production, making them essential to ecosystem functioning and biogeochemical cycling. Though phytoplankton are phylogenetically diverse, studies rarely designate unique thermal traits to different taxa, resulting in coarse representations of phytoplankton thermal responses. Here we assessed phytoplankton functional responses to temperature using empirically derived thermal growth rates from four principal contributors to marine productivity: diatoms, dinoflagellates, cyanobacteria, and coccolithophores. Using modeled sea surface temperatures for 1950-1970 and 2080-2100, we explored potential alterations to each group's growth rates and geographical distribution under a future climate change scenario. Contrary to the commonly applied Eppley formulation, our data suggest phytoplankton functional types may be characterized by different temperature coefficients (Q₁₀), growth maxima thermal dependencies, and thermal ranges which would drive dissimilar responses to each degree of temperature change. These differences, when applied in response to global simulations of future temperature, result in taxon-specific projections of growth and geographic distribution, with low-latitude coccolithophores facing considerable decreases and cyanobacteria substantial increases in growth rates. These results suggest that the singular effect of changing temperature may alter phytoplankton global community structure, owing to the significant variability in thermal response between phytoplankton functional types.

Restoration, conservation and phytoplankton hysteresis

Phytoplankton growth depends not only upon external factors that are not strongly altered by the presence of phytoplankton, such as temperature, but also upon factors that are strongly influenced by activity of phytoplankton, including photosynthetically active radiation, and the availability of the macronutrients carbon, nitrogen, phosphorus and, for some, silicate. Since phytoplankton therefore modify, and to an extent create, their own habitats, established phytoplankton communities can show resistance and resilience to change, including managed changes in nutrient regimes. Phytoplankton blooms and community structures can be predicted from the overall biogeochemical setting and inputs, but restorations may be influenced by the physiological responses of established phytoplankton taxa to nutrient inputs, temperature, second-order changes in illumination and nutrient recycling. In this review we discuss the contributions of phytoplankton ecophysiology to biogeochemical hysteresis and possible effects on community composition in the face of management, conservation or remediation plans.

Adaptation of a marine diatom to ocean acidification and warming reveals constraints and trade-offs

Ocean acidification and warming are recognized as two major anthropogenic perturbations of the modern ocean. However, little is known about the adaptive response of phytoplankton to them. Here we examine the adaptation of a marine diatom Thalassiosira weissflogii to ocean acidification in combination with ocean warming. Our results show that ocean warming have a greater effect than acidification on the growth of T. weissflogii over the long-term selection experiment (~380 generations), as well as many temperature response traits (e.g., optimum temperatures for photosynthesis, maximal net photosynthetic oxygen evolution rates, activation energy) in thermal reaction norm. These results suggest that ocean warming is the main driver for the evolution of the marine diatom T. weissflogii, rather than oceanacidification. However, the evolution resulting from warming can be constrained by ocean acidification, where ocean warming did not impose any effects at high CO₂ level. Furthermore, adaptations to ocean warming alone or to the combination of ocean acidification and warming come with trade-offs by inhibiting photochemical performances. The constrains and trade-offs associated with the adaptation to ocean acidification and warming demonstrated in this study, should be considered for parameterizing evolutionary responses in eco-evolutionary models of phytoplankton dynamics in a future ocean.

Meta-analysis of the response of marine phytoplankton to nutrient addition and seawater warming

As an indispensable part of the marine ecosystem, phytoplankton are important prey for zooplankton and various marine animals with important commercial value. The influence of seawater warming and eutrophication on phytoplankton communities is well known, but few studies have explained the effects of the interaction between temperature and nutrients on marine phytoplankton. Through meta-analysis and meta-regression, the phytoplankton responses to the effects of nutrient addition and seawater warming were evaluated in this study. Nitrogen (N) addition led to an increase in phytoplankton biomass, while phosphorus (P) had no significant effect on phytoplankton biomass. However, this result may be biased by the uneven distribution of the research area. N limitation is widespread in the areas where these collected studies were conducted, including many parts of North and South Atlantic and West Pacific Oceans. The key limiting nutrient in other areas lacking corresponding experiments, however, remain unclear. The effect of seawater warming was not significant, which indicates the uncertainty about the effect of temperature on phytoplankton. The results of ANOVA show that nutrient addition and seawater warming had similar effects in various marine habitats (coastal regions, estuaries and open seas), while salinity could have caused the difference in the N effects among the three habitats. Furthermore, our results showed that the impact of temperature depends on nutrient conditions, especially N status, which has rarely been considered before. This result demonstrated the importance of evaluating nutrient limitation patterns when studying climate warming. The impact of rising temperatures may need to be reevaluated because N limitation is common.

Adaptive responses of free-living and symbiotic microalgae to simulated future ocean conditions

Marine microalgae are a diverse group of microscopic eukaryotic and prokaryotic organisms capable of photosynthesis. They are important primary producers and carbon sinks but their physiology and persistence are severely affected by global climate change. Powerful experimental evolution technologies are being used to examine the potential of microalgae to respond adaptively to current and predicted future conditions, as well as to develop resources to facilitate species conservation and restoration of ecosystem functions. This review synthesizes findings and insights from experimental evolution studies of marine microalgae in response to elevated temperature and/or pCO₂ . Adaptation to these environmental conditions has been observed in many studies of marine dinoflagellates, diatoms and coccolithophores. An enhancement in traits such as growth and photo-physiological performance and an increase in upper thermal limit have been shown to be possible, although the extent and rate of change differ between microalgal taxa. Studies employing multiple monoclonal replicates showed variation in responses among replicates and revealed the stochasticity of mutations. The work to date is already providing valuable information on species' climate sensitivity or resilience to managers and policymakers but extrapolating these insights to ecosystem- and community-level impacts continues to be a challenge. We recommend future work should include in situ experiments, diurnal and seasonal fluctuations, multiple drivers and multiple starting genotypes. Fitness trade-offs, stable versus plastic responses and the genetic bases of the changes also need investigating, and the incorporation of genome resequencing into experimental designs will be invaluable.

Biological stoichiometry and growth dynamics of a diazotrophic cyanobacteria in nitrogen sufficient and deficient conditions

The role of nitrogen (N) fixation in determining the frequency, magnitude, and extent of harmful algal blooms (HABs) has not been well studied. Dolichospermum is a common HAB species that is diazotrophic (capable of N fixation) and thus growth is often considered never to be limited by low combined N sources. However, N fixation is energetically expensive and its cost during bloom formation has not been quantified. Additionally, it is unknown how acclimation to differing nutrient ratios affects growth and cellular carbon (C):N stoichiometry. Here, we test the hypotheses that diazotrophic cyanobacteria are homeostatic for N because of their ability to fix atmospheric N₂ and that previous acclimation to low N environments will result in more fixed N and lower C:N stoichiometry. Briefly, cultures that varied in resource N:phosphorus (P) ranging from 0.01 to 100 (atom), were seeded with Dolichospermum which were previously acclimated to low and high N:P conditions and then sampled temporally for growth and C:N stoichiometry. We found that Dolichospermum was not homeostatic for N and displayed classic signs of N limitation and elevated C:N stoichiometry, highlighting the necessary growth trade-off within cells when expending energy to fix N. Acclimation to N limited conditions caused differences in both C:N and fixed N at various time points in the experiment. These results highlight the importance of environmentally available N to a diazotrophic bloom, as well as how previous growth conditions can influence population growth during blooms experiencing variable N:P.

Iron Availability Modulates the Response of Endosymbiotic Dinoflagellates to Heat Stress

Warming and nutrient limitation are stressors known to weaken the health of microalgae. In situations of stress, access to energy reserves can minimize physiological damage. Because of its widespread requirements in biochemical processes, iron is an important trace metal, especially for photosynthetic organisms. Lowered iron availability in oceans experiencing rising temperatures may contribute to the thermal sensitivity of reef-building corals, which rely on mutualisms with dinoflagellates to survive. To test the influence of iron concentration on thermal sensitivity, the physiological responses of cultured symbiotic dinoflagellates (genus Breviolum; family Symbiodiniaceae) were evaluated when exposed to increasing temperatures (26 to 30°C) and iron concentrations ranging from replete (500 pM Fe') to limiting (50 pM Fe') under a diurnal light cycle with saturating radiance. Declines in photosynthetic efficiency at elevated temperatures indicated sensitivity to heat stress. Furthermore, five times the amount of iron was needed to reach exponential growth during heat stress (50 pM Fe' at 26-28°C vs. 250 pM Fe' at 30°C). In treatments where exponential growth was reached, Breviolum psygmophilum grew faster than B.minutum, possibly due to greater cellular contents of iron and other trace metals. The metal composition of B.psygmophilum shifted only at the highest temperature (30°C), whereas changes in B.minutum were observed at lower temperatures (28°C). The influence of iron availability in modulating each alga's response to thermal stress suggests the importance of trace metals to the health of coral-algal mutualisms. Ultimately, a greater ability to acquire scarce metals may improve the tolerance of corals to physiological stressors and contribute to the differences in performance associated with hosting one symbiont species over another.

Ecological limits to evolutionary rescue

Environments change, for both natural and anthropogenic reasons, which can threaten species persistence. Evolutionary adaptation is a potentially powerful mechanism to allow species to persist in these changing environments. To determine the conditions under which adaptation will prevent extinction (evolutionary rescue), classic quantitative genetics models have assumed a constantly changing environment. They predict that species traits will track a moving environmental optimum with a lag that approaches a constant. If fitness is negative at this lag, the species will go extinct. There have been many elaborations of these models incorporating increased genetic realism. Here, we review and explore the consequences of four ecological complications: non-quadratic fitness functions, interacting density- and trait-dependence, species interactions and fundamental limits to adaptation. We show that non-quadratic fitness functions can result in evolutionary tipping points and existential crises, as can the interaction between density- and trait-dependent mortality. We then review the literature on how interspecific interactions affect adaptation and persistence. Finally, we suggest an alternative theoretical framework that considers bounded environmental change and fundamental limits to adaptation. A research programme that combines theory and experiments and integrates across organizational scales will be needed to predict whether adaptation will prevent species extinction in changing environments. This article is part of the theme issue 'Integrative research perspectives on marine conservation'.

Different levels of energetic coupling between photosynthesis and respiration do not determine the occurrence of adaptive responses of Symbiodiniaceae to global warming

Disentangling the metabolic functioning of corals' endosymbionts (Symbiodiniaceae) is relevant to understanding the response of coral reefs to warming oceans. In this work, we first question whether there is an energetic coupling between photosynthesis and respiration in Symbiodiniaceae (Symbiodinium, Durusdinium and Effrenium), and second, how different levels of energetic coupling will affect their adaptive responses to global warming. Coupling between photosynthesis and respiration was established by determining the variation of metabolic rates during thermal response curves, and how inhibition of respiration affects photosynthesis. Adaptive (irreversible) responses were studied by exposing two Symbiodinium species with different levels of photosynthesis-respiration interaction to high temperature conditions (32°C) for 1 yr. We found that some Symbiodiniaceae have a high level of energetic coupling; that is, photosynthesis and respiration have the same temperature dependency, and photosynthesis is negatively affected when respiration is inhibited. Conversely, photosynthesis and respiration are not coupled in other species. In any case, prolonged exposure to high temperature caused adjustments in both photosynthesis and respiration, but these changes were fully reversible. We conclude that energetic coupling between photosynthesis and respiration exhibits wide variation amongst Symbiodiniaceae and does not determine the occurrence of adaptive responses in Symbiodiniaceae to temperature increase.

Multiple global change stressor effects on phytoplankton nutrient acquisition in a future ocean

Predicting the effects of multiple global change stressors on microbial communities remains a challenge because of the complex interactions among those factors. Here, we explore the combined effects of major global change stressors on nutrient acquisition traits in marine phytoplankton. Nutrient limitation constrains phytoplankton production in large parts of the present-day oceans, and is expected to increase owing to climate change, potentially favouring small phytoplankton that are better adapted to oligotrophic conditions. However, other stressors, such as elevated pCO2, rising temperatures and higher light levels, may reduce general metabolic and photosynthetic costs, allowing the reallocation of energy to the acquisition of increasingly limiting nutrients. We propose that this energy reallocation in response to major global change stressors may be more effective in large-celled phytoplankton species and, thus, could indirectly benefit large-more than small-celled phytoplankton, offsetting, at least partially, competitive disadvantages of large cells in a future ocean. Thus, considering the size-dependent responses to multiple stressors may provide a more nuanced understanding of how different microbial groups would fare in the future climate and what effects that would have on ecosystem functioning. This article is part of the theme issue 'Conceptual challenges in microbial community ecology'.

Nitrogen sufficiency enhances thermal tolerance in habitat-forming kelp: implications for acclimation under thermal stress

Local and global changes associated with anthropogenic activities are impacting marine and terrestrial ecosystems. Macroalgae, especially habitat-forming species like kelp, play critical roles in temperate coastal ecosystems. However, their abundance and distribution patterns have been negatively affected by warming in many regions around the globe. Along with global change, coastal ecosystems are also impacted by local drivers such as eutrophication. The interaction between global and local drivers might modulate kelp responses to environmental change. This study examines the regulatory effect of NO₃⁻ on the thermal plasticity of the giant kelp Macrocystis pyrifera. To do this, thermal performance curves (TPCs) of key temperature-dependant traits–growth, photosynthesis, NO₃⁻ assimilation and chlorophyll a fluorescence–were examined under nitrate replete and deplete conditions in a short-term incubation. We found that thermal plasticity was modulated by NO₃⁻ but different thermal responses were observed among traits. Our study reveals that nitrogen, a local driver, modulates kelp responses to high seawater temperatures, ameliorating the negative impacts on physiological performance (i.e. growth and photosynthesis). However, this effect might be species-specific and vary among biogeographic regions – thus, further work is needed to determine the generality of our findings to other key temperate macroalgae that are experiencing temperatures close to their thermal tolerance due to climate change.