The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO2 and pH

Long-term successional dynamics and response strategies of harmful algal blooms to environmental changes in Tolo Harbour

The production and succession of harmful algae blooms (HABs) are attributed more to excessive nutrient concentrations and unbalanced nutrient stoichiometry than to other environmental drivers as the absence of long-term monitoring data. This study analyzed HABs succession patterns and key drivers in Tolo Harbour from 1986 to 2023, leveraging nearly 40 years of data. Effective governmental measures significantly improved water quality, with dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), 5-day biochemical oxygen demand (BOD5), and Escherichia coli (E. coli) concentrations decreasing by 53 %, 80 %, 45 %, and 59 %, respectively. Annual HABs events dropped from 28 to 3, and species diversity declined from 6 to 2. However, toxic species frequency rose from 21 % to 46 %. Dinoflagellates emerged as dominant initial species, with a shift in secondary dominance from diatoms to ochrophytes and toxin types from diarrhetic shellfish poisoning (DSP) to hemolytic toxins (HT). These shifts likely result from combined human and natural influences. Model simulations confirmed that red tide outbreaks, species succession, and shifts in toxin types were driven by declining pH, rising temperatures, unbalanced nitrogen-phosphorus ratios, organic nutrient increases, and algal antagonism. The study emphasizes the importance of the dual reduction of both DIN and DIP, meanwhile inorganic and organic nutrients, suggesting that overly focusing on or distract from one nutrient (e.g., DIP or DON) could lead to unintended ecological consequences, like the proliferation of rare and toxic species. We highlight the combined impacts of climate change (warming and ocean acidification) and anthropogenic activities (nutrient pollution and eutrophication) on HABs, particularly the number and toxin production. This research links policy changes to HABs dynamics, offering strategic recommendations for managing red tides and contribute novel perspectives on the impact of nutrient reduction in comparable bay ecosystems.

Eukaryotic phytoplankton drive a decrease in primary production in response to elevated CO2 in the tropical and subtropical oceans

Ocean acidification caused by increasing anthropogenic CO2 is expected to impact marine phytoplankton productivity, yet the extent and even direction of these changes are not well constrained. Here, we investigate the responses of phytoplankton community composition and productivity to acidification across the western North Pacific. Consistent reductions in primary production were observed under acidified conditions in the North Pacific Subtropical Gyre and the northern South China Sea, whereas no significant changes were found at the northern boundary of the subtropical gyre. While prokaryotic phytoplankton showed little or positive responses to high CO2, small (<20 µm) eukaryotic phytoplankton which are primarily limited by low ambient nitrogen drove the observed decrease in community primary production. Extrapolating these results to global tropical and subtropical oceans predicts a potential decrease of about 5 Pg C y-1 in primary production in low Chl-a oligotrophic regions, which are anticipated to experience both acidification and stratification in the future.

Quantifying evolutionary changes to temperature-CO2 growth response surfaces in Skeletonema marinoi after adaptation to extreme conditions

Global warming and ocean acidification are having an unprecedented impact on marine ecosystems, yet we do not yet know how phytoplankton will respond to simultaneous changes in multiple drivers. To better comprehend the combined impact of oceanic warming and acidification, we experimentally estimated how evolution shifted the temperature-CO2 growth response surfaces of two strains of Skeletonema marinoi that were each previously adapted to four different temperature × CO2 combinations. These adapted strains were then grown under a factorial combination of five temperatures and five CO2 concentrations to capture the temperature-CO2 response surfaces for their unacclimated growth rates. The development of the first complete temperature-CO2 response surfaces showed the optimal CO2 concentration for growth to be substantially higher than expected future CO2 levels (~6000 ppm). There was minimal variation in the optimal CO2 concentration across the tested temperatures, suggesting that temperature will have a greater influence on growth rates compared to enhanced CO2. Optimal temperature did not show a unimodal response to CO2, either due to the lack of acclimation or the highly efficient CO2 concentrating mechanisms, which diatoms (e.g. Skeletonema) can up-/downregulate depending on the CO2 conditions. We also found that both strains showed evidence of evolutionary shifts as a result of adaptation to temperature and CO2. The evolutionary response differed between strains, underscoring how genetic differences (perhaps related to historical regimes) can impact phytoplankton performance. Understanding how a dominant algal species responds to multiple drivers provides insight into real-world scenarios and helps construct theoretical predictions of environmental change.

Elevated pCO2 may increase the edible safety risk of clams exposed to toxic Alexandrium spp

Toxic harmful algal blooms (HABs) have received increasing attention owing to their threat to the health of aquatic life and seafood consumers. This study evaluated the impacts of elevated atmospheric partial pressure of CO2 (pCO2) on the production of paralytic shellfish toxins (PSTs) in different Alexandrium spp. strains, together with its further effects on the bioaccumulation/elimination dynamics of PSTs in bivalves contaminated with PSTs from toxic dinoflagellates. Our results showed that elevated pCO2 stimulated the growth of the two Alexandrium spp. (A. catenella and A. pacificum) isolated from the northern and southern coastal areas of China, respectively, and affected PST production including content and toxicity of the two strains differently. Further PSTs bioaccumulation/elimination in PSTs-contaminated Manila clam, Ruditapes philippinarum under high pCO2 also occurred. It is worth noting the biotransformation of neosaxitoxin (NEO) with high toxicity through trophic transfer with effect of elevated pCO2. When in microalgae cultured under the control (410 ppm) and elevated pCO2 conditions (495 and 850 ppm), the proportion of NEO in the PST content produced by A. catenella was reduced from 11.1 to 6.4 and 2.6 %, while the proportion of NEO in A. pacificum was increased from 3.1 to 3.6 and 4.7 %, respectively. NEO accounted for >50 % of total PST contents in clams, which were biotransformed via transfer from dinoflagellates and higher pCO2 enhanced this biotransformation leading to increased NEO accumulation. The negatively affected elimination of PSTs, especially NEO, in clams fed with A. catenella or A. pacificum, indicates that the detoxification of PSTs-contaminated clams may be more difficult under elevated pCO2. This study provides reference for developing models to assess the safety of bivalves under the co-stress of environmental change and toxic HABs, suggesting that ocean acidification may lead to the higher safety risk of Manila clams exposed to toxic HAB dinoflagellates.

Acidification alleviates the inhibition of hyposaline stress on physiological performance of tropical seagrass Thalassia hemprichii

Since the Industrial Revolution, increasing atmospheric CO2 concentrations have had a substantial negative impact influence on coastal ecosystems because of direct effects including ocean acidification and indirect effects such as extreme rainfall events. Using a two-factor crossover indoor simulation experiment, this study examined the combined effects of acidification and hyposaline stress on Thalassia hemprichii. Seawater acidification increased the photosynthetic pigment content of T. hemprichii leaves and promoted seagrass growth rate. Hyposaline stress slowed down seagrass growth and had an impact on the osmotic potential and osmoregulatory substance content of seagrass leaves. Acidification and salinity reduction had significant interaction effects on the photosynthesis rate, photosynthetic pigment content, chlorophyll fluorescence parameters, and osmotic potential of T. hemprichii, but not on the growth rate. Overall, these findings have shown that the hyposaline stress inhibitory effect on the T. hemprichii physiological performance and growth may be reduced by acidification.

Mechanisms underlying the alleviated cadmium toxicity in marine diatoms adapted to ocean acidification

Anthropogenic activities have significantly increased the influx of carbon dioxide and metals into the marine environment. Combining ocean acidification (OA) and metal pollution may lead to unforeseen biological and ecological consequences. Several studies have shown that OA reduces cadmium (Cd) toxicity in marine diatoms. Although these studies have shed light on the physiological and transcriptomic responses of diatoms exposed to Cd, many aspects of the mechanisms underlying the reduced metal accumulation in diatoms remain unknown. This study aims to address this unresolved question by comparing Cd subcellular distribution, antioxidant enzyme activity, relative expression of metal transporters, surface potential, surface composition, and transmembrane potential in the diatom Phaeodactylum tricornutum grown under ambient and 1200 µatm pCO2 conditions. Our findings reveal that diatoms grown in acidified seawater exhibit higher surface potential and higher plasma membrane depolarization. These changes and the competing effects of increased H+ concentration result in a blunted response of P. tricornutum to the Cd challenge. Consequently, this study offers a new explanation for mitigating Cd toxicity by marine diatoms adapted to OA.

Shift in algal blooms from micro- to macroalgae around China with increasing eutrophication and climate change

Blooms of microalgal red tides and macroalgae (e.g., green and golden tides caused by Ulva and Sargassum) have caused widespread problems around China in recent years, but there is uncertainty around what triggers these blooms and how they interact. Here, we use 30 years of monitoring data to help answer these questions, focusing on the four main species of microalgae Prorocentrum donghaiense, Karenia mikimotoi, Noctiluca scintillans, and Skeletonema costatum) associated with red tides in the region. The frequency of red tides increased from 1991 to 2003 and then decreased until 2020, with S. costatum red tides exhibiting the highest rate of decrease. Green tides started to occur around China in 1999 and the frequency of green tides has since been on the increase. Golden tides were first reported to occur around China in 2012. The frequency of macroalgal blooms has a negative linear relationship with the frequency and coverage of red tides around China, and a positive correlation with total nitrogen and phosphorus loads as well as with atmospheric CO2 and sea surface temperature (SST). Increased outbreaks of macroalgal blooms are very likely due to worsening levels of eutrophication, combined with rising CO2 and SST, which contribute to the reduced frequency of red tides. The increasing grazing rate of microzooplankton also results in the decline in areas affected by red tides. This study shows a clear shift of algal blooms from microalgae to macroalgae around China over the past 30 years driven by the combination of eutrophication, climate change, and grazing stress, indicating a fundamental change in coastal systems in the region.

Survival of Nutrient-Starved Diatoms Under Ocean Acidification: Perspective from Nutrient Sensing, Cadmium Detection, and Nitrogen Assimilation

Increased anthropogenic emissions of carbon dioxide (CO2) have resulted in ocean acidification (OA) that is intertwined with enhanced ocean stratification. Diatoms are assumed to suffer from a more nutrient-limited condition in the future ocean. This study aimed to explore how OA affects the diatom dynamics under nutrient-poor conditions and the ability of diatoms to perceive nutrients (nitrogen, phosphorus, silicon, and trace metals) and cadmium (Cd) stimuli and assimilate nitrogen when receiving nutrients or Cd supplementation. Our study observed that diatom population grown under OA condition declined faster than those grown under ambient condition. Ocean acidification greatly lower intracellular Ca2+ concentration in diatom cells. Intracellular Ca2+ burst was involved in phosphorus accumulation but not in nitrogen, silicon, essential metals, and cadmium uptake. Our data demonstrate slower NO3- assimilation rates of diatoms grown in acidified seawater. Our study also indicates that diatoms have a poor perception of phosphorus availability under OA condition.

Coccolithophores and diatoms resilient to ocean alkalinity enhancement: A glimpse of hope?

It is increasingly apparent that adequately mitigating anthropogenic climate interference will require ocean carbon dioxide removal (CDR) strategies. Ocean alkalinity enhancement (OAE) is an abiotic ocean CDR approach that aims to increase the ocean's CO₂ uptake capacity through the dispersal of pulverized mineral or dissolved alkali into the surface ocean. However, OAE's effect on marine biota is largely unexplored. Here, we investigate the impacts of moderate (~700 μmol kg^-1) and high (~2700 μmol kg^-1) limestone-inspired alkalinity additions on two biogeochemically and ecologically important phytoplankton functional group representatives: Emiliania huxleyi (calcium carbonate producer) and Chaetoceros sp. (silica producer). The growth rate and elemental ratios of both taxa showed a neutral response to limestone-inspired alkalinization. While our results are encouraging, we also observed abiotic mineral precipitation, which removed nutrients and alkalinity from solution. Our findings offer an evaluation of biogeochemical and physiological responses to OAE and provide evidence supporting the need for continued research into how OAE strategies affect marine ecosystems.

Haemolymph pH of two important mollusc species is susceptible to seawater buffering capacity instead of pH or pCO2

The acid-base status and balance of molluscs are considered to be susceptible to environmental changes, especially in the context of ocean acidification (OA). Here, we studied the effects of manipulated seawater carbonate chemistry on the acid-base status of scallop Chlamys farreri and abalone Haliotis discus hannai. The haemolymph pH of the tested individuals showed a fast response to acidified seawater incubation, and the pH level was restored to a normal value within 1 h of recovery in control seawater. However, no significant correlation (P > 0.05) was found between haemolymph pH and seawater pCO₂ or pH, while the squared Pearson correlation coefficient (R²) ranged from close to zero to 0.41. In addition, although the pCO₂ level of total alkalinity (TA)-lowered seawater was lower than half of that in the control, molluscs eliminated less CO₂ (less than 80%) to TA lowered waters than to the control waters. These findings seem to disagree with the crucial role of seawater pCO₂ in influencing the acid-base balance of molluscs. CO₂ elimination occurs in the microenvironment, and CO₂ first diffuses to limited amounts of seawater that tightly surround the gills, causing dissolved inorganic carbon (DIC) accumulation in the ventilation sites, which leads to a sharp increase in the pCO₂ of the surrounding seawater. Moreover, in this microenvironment, the pCO₂ level increases much faster and more greatly if the environmental seawater is acidified or contains a lower level of TA. Therefore, mollusc acid-base status is influenced by rapidly varying pCO₂ levels at the ventilation site, which is largely independent of that of the rest of the incubating seawater. In summary, CO₂ elimination by molluscs relies heavily on the carbonate chemistry of environmental seawater, and seawater buffering capacity should be taken into consideration instead of considering only pCO₂ or pH in studying the acid-base balance of marine molluscs.

Temperature sensitivity of carbon concentrating mechanisms in the diatom Phaeodactylum tricornutum

Marine diatoms are key primary producers across diverse habitats in the global ocean. Diatoms rely on a biophysical carbon concentrating mechanism (CCM) to supply high concentrations of CO₂ around their carboxylating enzyme, RuBisCO. The necessity and energetic cost of the CCM are likely to be highly sensitive to temperature, as temperature impacts CO₂ concentration, diffusivity, and the kinetics of CCM components. Here, we used membrane inlet mass spectrometry (MIMS) and modeling to capture temperature regulation of the CCM in the diatom Phaeodactylum tricornutum (Pt). We found that enhanced carbon fixation rates by Pt at elevated temperatures were accompanied by increased CCM activity capable of maintaining RuBisCO close to CO₂ saturation but that the mechanism varied. At 10 and 18 °C, diffusion of CO₂ into the cell, driven by Pt's 'chloroplast pump' was the major inorganic carbon source. However, at 18 °C, upregulation of the chloroplast pump enhanced (while retaining the proportion of) both diffusive CO₂ and active HCO₃^- uptake into the cytosol, and significantly increased chloroplast HCO₃^- concentrations. In contrast, at 25 °C, compared to 18 °C, the chloroplast pump had only a slight increase in activity. While diffusive uptake of CO₂ into the cell remained constant, active HCO₃^- uptake across the cell membrane increased resulting in Pt depending equally on both CO₂ and HCO₃^- as inorganic carbon sources. Despite changes in the CCM, the overall rate of active carbon transport remained double that of carbon fixation across all temperatures tested. The implication of the energetic cost of the Pt CCM in response to increasing temperatures was discussed.

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.

A metabolic, phylogenomic and environmental atlas of diatom plastid transporters from the model species Phaeodactylum

Diatoms are an important group of algae, contributing nearly 40% of total marine photosynthetic activity. However, the specific molecular agents and transporters underpinning the metabolic efficiency of the diatom plastid remain to be revealed. We performed in silico analyses of 70 predicted plastid transporters identified by genome-wide searches of Phaeodactylum tricornutum. We considered similarity with Arabidopsis thaliana plastid transporters, transcriptional co-regulation with genes encoding core plastid metabolic pathways and with genes encoded in the mitochondrial genomes, inferred evolutionary histories using single-gene phylogeny, and environmental expression trends using Tara Oceans meta-transcriptomics and meta-genomes data. Our data reveal diatoms conserve some of the ion, nucleotide and sugar plastid transporters associated with plants, such as non-specific triose phosphate transporters implicated in the transport of phosphorylated sugars, NTP/NDP and cation exchange transporters. However, our data also highlight the presence of diatom-specific transporter functions, such as carbon and amino acid transporters implicated in intricate plastid-mitochondria crosstalk events. These confirm previous observations that substrate non-specific triose phosphate transporters (TPT) may exist as principal transporters of phosphorylated sugars into and out of the diatom plastid, alongside suggesting probable agents of NTP exchange. Carbon and amino acid transport may be related to intricate metabolic plastid-mitochondria crosstalk. We additionally provide evidence from environmental meta-transcriptomic/meta- genomic data that plastid transporters may underpin diatom sensitivity to ocean warming, and identify a diatom plastid transporter (J43171) whose expression may be positively correlated with temperature.

A Competitive Advantage of Middle-Sized Diatoms From Increasing Seawater CO2

Diatoms, one of the most important phytoplankton groups, fulfill their carbon demand from seawater mainly by obtaining passively diffused carbon dioxide (CO₂) and/or actively consuming intracellular energy to acquire bicarbonate (HCO₃^–). An anthropogenically induced increase in seawater CO₂ reduces the HCO₃^– requirement of diatoms, potentially saving intracellular energy and benefitting their growth. This effect is commonly speculated to be most remarkable in larger diatoms that are subject to a stronger limitation of CO₂ supply because of their smaller surface-to-volume ratios. However, we constructed a theoretical model for diatoms and revealed a unimodal relationship between the simulated growth rate response (GRR, the ratio of growth rates under elevated and ambient CO₂) and cell size, with the GRR peaking at a cell diameter of ∼7 μm. The simulated GRR of the smallest diatoms was low because the CO₂ supply was nearly sufficient at the ambient level, while the decline of GRR from a cell diameter of 7 μm was simulated because the contribution of seawater CO₂ to the total carbon demand greatly decreased and diatoms became less sensitive to CO₂ increase. A collection of historical data in CO₂ enrichment experiments of diatoms also showed a roughly unimodal relationship between maximal GRR and cell size. Our model further revealed that the “optimal” cell size corresponding to peak GRR enlarged with the magnitude of CO₂ increase but diminished with elevating cellular carbon demand, leading to projection of the smallest optimal cell size in the equatorial Pacific upwelling zone. Last, we need to emphasize that the size-dependent effects of increasing CO₂ on diatoms are multifaceted, while our model only considers the inorganic carbon supply from seawater and optimal allocation of intracellular energy. Our study proposes a competitive advantage of middle-sized diatoms and can be useful in projecting changes in the diatom community in the future acidified high-CO₂ ocean.

Lipid Remodeling Reveals the Adaptations of a Marine Diatom to Ocean Acidification

Ocean acidification is recognized as a major anthropogenic perturbation of the modern ocean. While extensive studies have been carried out to explore the short-term physiological responses of phytoplankton to ocean acidification, little is known about their lipidomic responses after a long-term ocean acidification adaptation. Here we perform the lipidomic analysis of a marine diatom Phaeodactylum tricornutum following long-term (∼400 days) selection to ocean acidification conditions. We identified a total of 476 lipid metabolites in long-term high CO₂ (i.e., ocean acidification condition) and low CO₂ (i.e., ambient condition) selected P. tricornutum cells. Our results further show that long-term high CO₂ selection triggered substantial changes in lipid metabolites by down- and up-regulating 33 and 42 lipid metabolites. While monogalactosyldiacylglycerol (MGDG) was significantly down-regulated in the long-term high CO₂ selected conditions, the majority (∼80%) of phosphatidylglycerol (PG) was up-regulated. The tightly coupled regulations (positively or negatively correlated) of significantly regulated lipid metabolites suggest that the lipid remodeling is an organismal adaptation strategy of marine diatoms to ongoing ocean acidification. Since the composition and content of lipids are crucial for marine food quality, and these changes can be transferred to high trophic levels, our results highlight the importance of determining the long-term adaptation of lipids in marine producers in predicting the ecological consequences of climate change.

The Role of Extracellular Carbonic Anhydrase in Biogeochemical Cycling: Recent Advances and Climate Change Responses

Climate change has been predicted to influence the marine phytoplankton community and its carbon acquisition strategy. Extracellular carbonic anhydrase (eCA) is a zinc metalloenzyme that catalyses the relatively slow interconversion between HCO₃⁻ and CO₂. Early results indicated that sub-nanomolar levels of eCA at the sea surface were sufficient to enhance the oceanic uptake rate of CO₂ on a global scale by 15%, an addition of 0.37 Pg C year⁻¹. Despite its central role in the marine carbon cycle, only in recent years have new analytical techniques allowed the first quantifications of eCA and its activity in the oceans. This opens up new research areas in the field of marine biogeochemistry and climate change. Light and suitable pH conditions, as well as growth stage, are crucial factors in eCA expression. Previous studies showed that phytoplankton eCA activity and concentrations are affected by environmental stressors such as ocean acidification and UV radiation as well as changing light conditions. For this reason, eCA is suggested as a biochemical indicator in biomonitoring programmes and could be used for future response prediction studies in changing oceans. This review aims to identify the current knowledge and gaps where new research efforts should be focused to better determine the potential feedback of phytoplankton via eCA in the marine carbon cycle in changing oceans.

Universal response pattern of phytoplankton growth rates to increasing CO2

Phytoplankton growth rate is a key variable controlling species succession and ecosystem structure throughout the surface ocean. Carbonate chemistry conditions are known to influence phytoplankton growth rates but there is no conceptual framework allowing us to compare growth rate responses across taxa. Here we analyse the literature to show that phytoplankton growth rates follow an optimum curve response pattern whenever the tested species is exposed to a sufficiently large gradient in proton (H⁺ ) concentrations. Based on previous findings with coccolithophores and diatoms, we argue that this 'universal reaction norm' is shaped by the stimulating influence of increasing inorganic carbon substrate (left side of the optimum) and the inhibiting influence of increase H⁺ (right side of the optimum). We envisage that exploration of carbonate chemistry-dependent optimum curves as a default experimental approach will boost our mechanistic understanding of phytoplankton responses to ocean acidification, like temperature curves have already boosted our mechanistic understanding to global warming.

Elevated pCO2 Level Affects the Extracellular Polymer Metabolism of Phaeodactylum tricornutum

Extracellular polymeric substances (EPS) play an important role in diatom physiology and carbon biogeochemical cycling in marine ecosystems. Both the composition and yield of EPS in diatom cells can vary with environmental changes. However, information on intracellular pathways and controls of both biochemical and genetic of EPS is limited. Further, how such changes would affect their critical ecological roles in marine systems is also unclear. Here, we evaluated the physiological characteristics, EPS yields, EPS compositions, and gene expression levels of Phaeodactylum tricornutum under elevated pCO2 levels. Genes and pathways related to EPS metabolism in P. tricornutum were identified. Carbohydrate yields in different EPS fractions increased with elevated pCO2 exposure. Although the proportions of monosaccharide sugars among total sugars did not change, higher abundances of uronic acid were observed under high pCO2 conditions, suggesting the alterations of EPS composition. Elevated pCO2 increased PSII light energy conversion efficiency and carbon sequestration efficiency. The up-regulation of most genes involved in carbon fixation pathways led to increased growth and EPS release. RNA-Seq analysis revealed a number of genes and divergent alleles related to EPS production that were up-regulated by elevated pCO2 levels. Nucleotide diphosphate (NDP)-sugar activation and accelerated glycosylation could be responsible for more EPS responding to environmental signals. Further, NDP-sugar transporters exhibited increased expression levels, suggesting roles in EPS over-production. Overall, these results provide critical data for understanding the mechanisms of EPS production in diatoms and evaluating the metabolic plasticity of these organisms in response to environmental changes.

Diatoms Dominate and Alter Marine Food-Webs When CO2 Rises

Diatoms are so important in ocean food-webs that any human induced changes in their abundance could have major effects on the ecology of our seas. The large chain-forming diatom Biddulphia biddulphiana greatly increases in abundance as pCO2 increases along natural seawater CO2 gradients in the north Pacific Ocean. In areas with reference levels of pCO2, it was hard to find, but as seawater carbon dioxide levels rose, it replaced seaweeds and became the main habitat-forming species on the seabed. This diatom algal turf supported a marine invertebrate community that was much less diverse and completely differed from the benthic communities found at present-day levels of pCO2. Seawater CO2 enrichment stimulated the growth and photosynthetic efficiency of benthic diatoms, but reduced the abundance of calcified grazers such as gastropods and sea urchins. These observations suggest that ocean acidification will shift photic zone community composition so that coastal food-web structure and ecosystem function are homogenised, simplified, and more strongly affected by seasonal algal blooms.

Effects of scrubber washwater discharge on microplankton in the Baltic Sea

In 2020, the global cap of maximum allowable sulphur content in marine fuel will be reduced from the current 3.5% to 0.5%. Another way to reduce the sulphur emissions is to install a seawater scrubber that cleans exhausts but instead release acidic water containing nutrients and contaminants back to the marine environment. In the current study, scrubber washwater was tested on a Baltic Sea microplankton community. A significant increase in chlorophyll a, particulate organic phosphorus (POP), carbon (POC) and nitrogen (PON) were observed when the community was exposed to 10% scrubber washwater for 13 days as compared to the control. A laboratory experiment with the filamentous cyanobacteria Nodularia spumigena and the chain-forming diatom Melosira cf. arctica showed negative responses in photosynthetic activity (EC10 = 8.6% for N. spumigena) and increased primary productivity (EC10 = 5.5% for M. cf. arctica), implying species-specific responses to scrubber washwater discharge.