Southern Ocean phytoplankton physiology in a changing climate

Effects of temperature and nitrogen sources on physiological performance of the coccolithophore Emiliania huxleyi

Both temperature and nutrient levels are rising in worldwide ocean ecosystems, and they strongly influence biological responses of phytoplankton. However, few studies have addressed the interactive effects of temperature and nitrogen sources on physiological performance of the coccolithophore Emiliania huxleyi. In this study, we evaluated algal growth, photosynthesis and respiration, elemental composition, enzyme activity, and calcification under a matrix of two temperatures gradients (ambient temperature 20 °C and high temperature 24 °C) and two nitrogen sources (nitrate (NO3-) and ammonium (NH4+)). When the algae was cultured with NO3- medium, high temperature reduced algal photosynthesis and nitrate reductase activity, but it did not change other indicators significantly relative to ambient temperature. In addition, E. huxleyi preferred NO3- as the growth medium, whereas NH4+ had negative effects on physiological parameters. In the NH4+ medium, the growth rate, photosynthesis and photosynthetic rate, nitrate reductase activity, and particulate organic carbon and particulate organic nitrogen production rate of the algae decreased as temperature increased. Conversely, high temperature increased cellular particulate organic carbon, cellular particulate organic nitrogen, and particulate inorganic carbon levels. In summary, our findings indicate that the distribution and abundance of microalgae could be greatly affected under warming ocean temperature and different nutrient conditions.

Long-term studies on West Antarctic Peninsula phytoplankton blooms suggest range shifts between temperate and polar species

The Western Antarctic Peninsula (WAP) experiences one of the highest rates of sea surface warming globally, leading to potential changes in biological communities. Long-term phytoplankton monitoring in Potter Cove (PC, King George Island, South Shetlands) from the 1990s to 2009 revealed consistently low biomass values, and sporadic blooms dominated by cold-water microplankton diatoms. However, a significant change occurred between 2010 and 2020, marked by a notable increase in intense phytoplankton blooms in the region. During this period, the presence of a nanoplankton diatom, Shionodiscus gaarderae, was documented for the first time. In some instances, this species even dominated the blooms. S. gaarderae is recognized for producing blooms in temperate waters in both hemispheres. However, its blooming in the northern Southern Ocean may suggest either a recent introduction or a range shift associated with rising temperatures in the WAP, a phenomenon previously observed in experimental studies. The presence of S. gaarderae could be viewed as a warning sign of significant changes already underway in the northern WAP plankton communities. This includes the potential replacement of microplankton diatoms by smaller nanoplankton species. This study, based on observations along the past decade, and compared to the previous 20 years, could have far-reaching implications for the structure of the Antarctic food web.

Nanoplankton: The dominant vector for carbon export across the Atlantic Southern Ocean in spring

Across the Southern Ocean, large (≥20 μm) diatoms are generally assumed to be the primary vector for carbon export, although this assumption derives mainly from summertime observations. Here, we investigated carbon production and export potential during the Atlantic Southern Ocean's spring bloom from size-fractionated measurements of net primary production (NPP), nitrogen (nitrate, ammonium, urea) and iron (labile inorganic iron, organically complexed iron) uptake, and a high-resolution characterization of phytoplankton community composition. The nanoplankton-sized (2.7 to 20 μm) diatom, Chaetoceros spp., dominated the biomass, NPP, and nitrate uptake across the basin (40°S to 56°S), which we attribute to their low iron requirement, rapid response to increased light, and ability to escape grazing when aggregated into chains. We estimate that the spring Chaetoceros bloom accounted for >25% of annual export production across the Atlantic Southern Ocean, a finding consistent with recent observations from other regions highlighting the central role of the phytoplankton "middle class" in carbon export.

Saturating growth rate against phosphorus concentration explained by macromolecular allocation

IMPORTANCE

The Monod equation has been used to represent the relationship between growth rate and the environmental nutrient concentration under the limitation of this respective nutrient. This model often serves as a means to connect microorganisms to their environment, specifically in ecosystem and global models. Here, we use a simple model of a marine microorganism cell to illustrate the model's ability to capture the same relationship as Monod, while highlighting the additional physiological details our model provides. In this study, we focus on the relationship between growth rate and phosphorus concentration and find that RNA allocation largely contributes to the commonly observed trend. This work emphasizes the potential role our model could play in connecting microorganisms to the surrounding environment while using realistic physiological representations.

Physical-biological drivers modulating phytoplankton seasonal succession along the Northern Antarctic Peninsula

The Northern Antarctic Peninsula (NAP) shows shifts in phytoplankton distribution and composition along its warming marine ecosystems. However, despite recent efforts to mechanistically understand these changes, little focus has been given to the phytoplankton seasonal succession, remaining uncertainties regarding to distribution patterns of emerging taxa along the NAP. To fill this gap, we collected phytoplankton (pigment and microscopy analysis) and physico-chemical datasets during spring and summer (November, February and March) of 2013/2014 and 2014/2015 off the NAP. Satellite measurements (sea surface temperature, sea ice concentration and chlorophyll-a) were used to extend the temporal coverage of analysis associated with the in situ sampling. We improved the quantification and distribution pattern of emerging taxa, such as dinoflagellates and cryptophytes, and described a contrasting seasonal behavior and distinct fundamental niche between centric and pennate diatoms. Cryptophytes and pennate diatoms preferentially occupied relatively shallower mixing layers compared with centric diatoms and dinoflagellates, suggesting differences between these groups in distribution and environment occupation over the phytoplankton seasonal succession. Under colder conditions, negative sea surface temperature anomalies were associated with positive anomalies of sea ice concentration and duration. Therefore, based on sea ice-phytoplankton growth relationship, large phytoplankton biomass accumulation was expected during the spring/summer of 2013/2014 and 2014/2015 along the NAP. However, there was a decoupling between sea ice concentration/duration and phytoplankton biomass, characterizing two seasonal periods of low biomass accumulation (negative chlorophyll-a anomalies), associated with the top-down control in the region. These results provide an improved mechanistic understanding on physical-biological drivers modulating phytoplankton seasonal succession along the Antarctic coastal waters.

Microalgae from Cold Environments and Their Possible Biotechnological Applications

Cold environments include deep ocean, alpine, and polar areas. Even if the cold conditions are harsh and extreme for certain habitats, various species have been adapted to survive in them. Microalgae are among the most abundant microbial communities which have adapted to live in low light, low temperature, and ice coverage conditions typical of cold environments by activating different stress-responsive strategies. These species have been shown to have bioactivities with possible exploitation capabilities for human applications. Even if they are less explored compared to species living in more accessible sites, various activities have been highlighted, such as antioxidant and anticancer activities. This review is focused on summarizing these bioactivities and discussing the possible exploitation of cold-adapted microalgae. Thanks to the possibility of mass cultivating algae in controlled photobioreactors, eco-sustainable exploitation is in fact possible by sampling a few microalgal cells without impacting the environment.

Cryptophytes: An emerging algal group in the rapidly changing Antarctic Peninsula marine environments

The western Antarctic Peninsula (WAP) is a climatically sensitive region where foundational changes at the basis of the food web have been recorded; cryptophytes are gradually outgrowing diatoms together with a decreased size spectrum of the phytoplankton community. Based on a 11-year (2008-2018) in-situ dataset, we demonstrate a strong coupling between biomass accumulation of cryptophytes, summer upper ocean stability, and the mixed layer depth. Our results shed light on the environmental conditions favoring the cryptophyte success in coastal regions of the WAP, especially during situations of shallower mixed layers associated with lower diatom biomass, which evidences a clear competition or niche segregation between diatoms and cryptophytes. We also unravel the cryptophyte photo-physiological niche by exploring its capacity to thrive under high light stress normally found in confined stratified upper layers. Such conditions are becoming more frequent in the Antarctic coastal waters and will likely have significant future implications at various levels of the marine food web. The competitive advantage of cryptophytes in environments with significant light level fluctuations was supported by laboratory experiments that revealed a high flexibility of cryptophytes to grow in different light conditions driven by a fast photo-regulating response. All tested physiological parameters support the hypothesis that cryptophytes are highly flexible regarding their growing light conditions and extremely efficient in rapidly photo-regulating changes to environmental light levels. This plasticity would give them a competitive advantage in exploiting an ecological niche where light levels fluctuate quickly. These findings provide new insights on niche separation between diatoms and cryptophytes, which is vital for a thorough understanding of the WAP marine ecosystem.

Effects of global environmental change on microalgal photosynthesis, growth and their distribution

Global climate change (GCC) constitutes a complex challenge posing a serious threat to biodiversity and ecosystems in the next decades. There are several recent studies dealing with the potential effect of increased temperature, decrease of pH or shifts in salinity, as well as cascading events of GCC and their impact on human-environment systems. Microalgae as primary producers are a sensitive compartment of the marine ecosystems to all those changes. However, the potential consequences of these changes for marine microalgae have received relatively little attention and they are still not well understood. Thus, there is an urgent need to explore and understand the effects generated by multiple climatic changes on marine microalgae growth and biodiversity. Therefore, this review aimed to compare and contrast mechanisms that marine microalgae exhibit to directly respond to harsh conditions associated with GCC and the potential consequences of those changes in marine microalgal populations. Literature shows that microalgae responses to environmental stressors such as temperature were affected differently. A stress caused by salinity might slow down cell division, reduces size, ceases motility, and triggers palmelloid formation in microalgae community, but some of these changes are strongly species-specific. UV irradiance can potentially lead to an oxidative stress in microalgae, promoting the production of reactive oxygen species (ROS) or induce direct physical damage on microalgae, then inhibiting the growth of microalgae. Moreover, pH could impact many groups of microalgae being more tolerant of certain pH shifts, while others were sensitive to changes of just small units (such as coccolithophorids) and subsequently affect the species at a higher trophic level, but also total vertical carbon transport in oceans. Overall, this review highlights the importance of examining effects of multiple stressors, considering multiple responses to understand the complexity behind stressor interactions.

Control of Antarctic phytoplankton community composition and standing stock by light availability

Southern Ocean phytoplankton are especially subjected to pronounced seasonal and interannual changes in light availability. Although previous studies have examined the role of light in these environments, very few combined pigment-based taxonomy with flow cytometry to better discriminate the light response of various phytoplankton groups. In particular the different populations within the diverse and important taxonomic group of diatoms require further investigation. Six incubation experiments (9–10 days) were performed during the main productive period with natural seawater collected at the Western Antarctic Peninsula. Standing stock of Phaeocystis spp. cells displayed relatively fast accumulation under all levels of light (low, medium, high; 4–7, 30–50 and 150–200 µmol quanta m−2 s−1), whilst the small- and larger-sized diatom populations (4.5 and 20 µm diameter) exhibited faster accumulation in medium and high light. In contrast, intermediate-sized diatoms (11.5 µm diameter) displayed fastest net growth under low light, subsequently dominating the phytoplankton community. Low light was a key factor limiting accumulation and peak phytoplankton biomass, except one incubation displaying relatively high accumulation rates under low light. The 3-week low-light period prior to experimentation likely allowed adaptation to maximize achievable growth and seems a strong determinant of whether the different natural Antarctic phytoplankton populations sustain, thrive or decline. Our study provides improved insight into how light intensity modulates the net response of key Antarctic phytoplankton, both between and within taxonomic groups.

Photoacclimation of the polar diatom Chaetoceros neogracilis at low temperature

Polar microalgae face two major challenges: 1- growing at temperatures (-1.7 to 5°C) that limit enzyme kinetics; and 2- surviving and exploiting a wide range of irradiance. The objective of this study is to understand the adaptation of an Arctic diatom to its environment by studying its ability to acclimate to changes in light and temperature. We acclimated the polar diatom Chaetoceros neogracilis to various light levels at two different temperatures and studied its growth and photosynthetic properties using semi-continuous cultures. Rubisco content was high, to compensate for low catalytic rates, but did not change detectably with growth temperature. Contrary to what is observed in temperate species, in C. neogracilis, carbon fixation rate (20 min 14C incorporation) equaled net growth rate (μ) suggesting very low or very rapid (<20 min) re-oxidation of the newly fixed carbon. The comparison of saturation irradiances for electron transport, oxygen net production and carbon fixation revealed alternative electron pathways that could provide energy and reducing power to the cell without consuming organic carbon which is a very limiting product at low temperatures. High protein contents, low re-oxidation of newly fixed carbon and the use of electron pathways alternative to carbon fixation may be important characteristics allowing efficient growth under those extreme environmental conditions.

Molecular Cloning and Expression Analysis of the Cryptochrome Gene CiPlant-CRY1 in Antarctic Ice Alga Chlamydomonas sp. ICE-L

Cryptochrome (CRY) is a kind of flavin-binding protein that can sense blue light and near-ultraviolet light, and participates in the light response of organisms and the regulation of the circadian clock. The complete open reading frame (ORF) of CiPlant-CRY1 (GenBank ID OM389130.1), encoding one kind of CRY, was cloned from the Antarctic ice alga Chlamydomonas sp. ICE-L. The quantitative real-time PCR study showed that the expression level of the CiPlant-CRY1 gene was the highest at 5 °C and salinity of 32‰. CiPlant-CRY1 was positively regulated by blue or yellow light, suggesting that it is involved in the establishment of photomorphology. The CiPlant-CRY1 gene can respond to polar day and polar night, indicating its expression is regulated by circadian rhythm. The expression level of CiPlant-CRY1 was most affected by UVB irradiation, which may be related to the adaptation of ice algae to a strong ultraviolet radiation environment. Moreover, the recombinant protein of CiPlant-CRY1 was expressed by prokaryotic expression. This study may be important for exploring the light-induced rhythm regulation of Antarctic ice algae in the polar marine environment.

Potential for the Production of Carotenoids of Interest in the Polar Diatom Fragilariopsis cylindrus

Carotenoid xanthophyll pigments are receiving growing interest in various industrial fields due to their broad and diverse bioactive and health beneficial properties. Fucoxanthin (Fx) and the inter-convertible couple diadinoxanthin–diatoxanthin (Ddx+Dtx) are acknowledged as some of the most promising xanthophylls; they are mainly synthesized by diatoms (Bacillariophyta). While temperate strains of diatoms have been widely investigated, recent years showed a growing interest in using polar strains, which are better adapted to the natural growth conditions of Nordic countries. The aim of the present study was to explore the potential of the polar diatom Fragilariopsis cylindrus in producing Fx and Ddx+Dtx by means of the manipulation of the growth light climate (daylength, light intensity and spectrum) and temperature. We further compared its best capacity to the strongest xanthophyll production levels reported for temperate counterparts grown under comparable conditions. In our hands, the best growing conditions for F. cylindrus were a semi-continuous growth at 7 °C and under a 12 h light:12 h dark photoperiod of monochromatic blue light (445 nm) at a PUR of 11.7 μmol photons m⁻² s⁻¹. This allowed the highest Fx productivity of 43.80 µg L⁻¹ day⁻¹ and the highest Fx yield of 7.53 µg Wh⁻¹, more than two times higher than under ‘white’ light. For Ddx+Dtx, the highest productivity (4.55 µg L⁻¹ day⁻¹) was reached under the same conditions of ‘white light’ and at 0 °C. Our results show that F. cylindrus, and potentially other polar diatom strains, are very well suited for Fx and Ddx+Dtx production under conditions of low temperature and light intensity, reaching similar productivity levels as model temperate counterparts such as Phaeodactylum tricornutum. The present work supports the possibility of using polar diatoms as an efficient cold and low light-adapted bioresource for xanthophyll pigments, especially usable in Nordic countries.

Diversity of Microbial Eukaryotes Along the West Antarctic Peninsula in Austral Spring

During a cruise from October to November 2019, along the West Antarctic Peninsula, between 64.32 and 68.37°S, we assessed the diversity and composition of the active microbial eukaryotic community within three size fractions: micro- (> 20 μm), nano- (20-5 μm), and pico-size fractions (5-0.2 μm). The communities and the environmental parameters displayed latitudinal gradients, and we observed a strong similarity in the microbial eukaryotic communities as well as the environmental parameters between the sub-surface and the deep chlorophyll maximum (DCM) depths. Chlorophyll concentrations were low, and the mixed layer was shallow for most of the 17 stations sampled. The richness of the microplankton was higher in Marguerite Bay (our southernmost stations), compared to more northern stations, while the diversity for the nano- and pico-plankton was relatively stable across latitude. The microplankton communities were dominated by autotrophs, mostly diatoms, while mixotrophs (phototrophs-consuming bacteria and kleptoplastidic ciliates, mostly alveolates, and cryptophytes) were the most abundant and active members of the nano- and picoplankton communities. While phototrophy was the dominant trophic mode, heterotrophy (mixotrophy, phagotrophy, and parasitism) tended to increase southward. The samples from Marguerite Bay showed a distinct community with a high diversity of nanoplankton predators, including spirotrich ciliates, and dinoflagellates, while cryptophytes were observed elsewhere. Some lineages were significantly related-either positively or negatively-to ice coverage (e.g., positive for Pelagophyceae, negative for Spirotrichea) and temperature (e.g., positive for Cryptophyceae, negative for Spirotrichea). This suggests that climate changes will have a strong impact on the microbial eukaryotic community.

The marine carbonate system along the northern Antarctic Peninsula: current knowledge and future perspectives

Among the regions of the Southern Ocean, the northern Antarctic Peninsula (NAP) has emerged as a hotspot of climate change investigation. Nonetheless, studies have indicated issues and knowledge gaps that must be addressed to expand the understanding of the carbonate system in the region. Therefore, we focused on identifying current knowledge about sea-air CO2 fluxes (FCO2), anthropogenic carbon (Cant) and ocean acidification along NAP and provide a better comprehension of the key physical processes controlling the carbonate system. Regarding physical dynamics, we discuss the role of water masses formation, climate modes, upwelling and intrusions of Circumpolar Deep Water, and mesoscale processes. For FCO2, we show that the summer season corresponds to a strong sink in coastal areas, leading to CO2 uptake that is greater than or equal to that of the open ocean. We highlight that the prevalence of summer studies prevents comprehending processes occurring throughout the year and the net annual CO2 balance in the region. Thus, temporal investigations are necessary to determine natural environmental fluctuations and to distinguish natural variability from anthropogenically driven changes. We emphasize the importance of more studies regarding Cant uptake rate, accumulation, and export to global oceans.

Biomolecular Composition of Sea Ice Microalgae and Its Influence on Marine Biogeochemical Cycling and Carbon Transfer through Polar Marine Food Webs

Microalgae growing on the underside of sea ice are key primary producers in polar marine environments. Their nutritional status, determined by their macromolecular composition, contributes to the region’s biochemistry and the unique temporal and spatial characteristics of their growth makes them essential for sustaining polar marine food webs. Here, we review the plasticity and taxonomic diversity of sea ice microalgae macromolecular composition, with a focus on how different environmental conditions influence macromolecular production and partitioning within cells and communities. The advantages and disadvantages of methodologies for assessing macromolecular composition are presented, including techniques that provide high throughput, whole macromolecular profile and/or species-specific resolution, which are particularly recommended for future studies. The directions of environmentally driven macromolecular changes are discussed, alongside anticipated consequences on nutrients supplied to the polar marine ecosystem. Given that polar regions are facing accelerated rates of environmental change, it is argued that a climate change signature will become evident in the biochemical composition of sea ice microalgal communities, highlighting the need for further research to understand the synergistic effects of multiple environmental stressors. The importance of sea ice microalgae as primary producers in polar marine ecosystems means that ongoing research into climate-change driven macromolecular phenotyping is critical to understanding the implications for the regions biochemical cycling and carbon transfer.

Impacts of global warming on marine microbial communities

Global warming in ocean ecosystems alters temperature, acidification, oxygen content, circulation, stratification, and nutrient inputs. Microorganisms play a dominant role in global biogeochemical cycles crucial for a planet's sustainability. Since microbial communities are highly dependent on the temperature factor, fluctuations in the same will lead to adverse effects on the microbial community organization. Throughout the Ocean, increase in evaporation rates causes the surface mixed layer to become shallower. This intensified stratification inhibits vertical transport of nutrient supplies. Such density driven processes will decrease oxygen solubility in surface waters leading to significant decrease of oxygen from future Ocean. Metabolism and diversity of microbes along with ocean biogeochemistry will be at great risk due to global warming and its related effects. As a response to the changes in temperature, alteration in the distribution of phytoplankta communities is observed all over the planet, creating changes in the primary production of the ocean causing massive impact on the biosphere. Marine microbial communities try to adapt to the changing ocean environmental conditions by responding with biogeographic range shifts, community structure modifications, and adaptive evolution. Persistence of this climate change on ocean ecosystems, in future, will pose serious threat to the metabolism and distribution of marine microbes leading to fluctuations in the biogeochemical cycles thereby affecting the overall ecosystem functioning. Genomics plays an important role in marine microbial research by providing tools to study the association between environment and organisms. The ecological and genomic perspectives of marine microbes are being investigated to design effective models to understand their physiology and evolution in a changing ocean. Mesocosm/microcosm experimental studies and field studies are in the need of the hour to evaluate the impact of climate shifts on microbial genesis.

Ocean warming and freshening effects on lipid metabolism in coastal Antarctic phytoplankton assemblages dominated by sub-Antarctic species

Marine phytoplankton can utilize different strategies to cope with ocean warming and freshening from glacial melting in polar regions, which are disproportionally impacted by global warming. In the present study, we investigated the individual and combined effects of a 4 °C increase in seawater temperature (T⁺) and a 4 psu decrease in salinity (S^-) from ambient values on biomass, nutrient use, fatty acid composition and lipid damage biochemistry of natural phytoplankton assemblages from Potter Cove (25 de Mayo/King George Island, Antarctica). Experiments were conducted by exposing the assemblages to four treatments during a 7-day incubation period using microcosm located along shore from January 23 to 31, 2016. The N:P ratio decreased in all treatments from day 4 onwards, but especially under high temperature (T⁺). Lipid damage was mainly detected under S⁰T⁺ and S^-T⁺ conditions, and it decreased when the production of the antioxidant α-tocopherol increased. This antioxidant protection resulted in a build-up of phytoplankton biomass, especially at T⁺. Under the combined effect of both stressors (S^-T⁺), the concentration of ω3 fatty acids increased, potentially leading to higher-quality FA composition. These results, which were related to the dominance of sub-Antarctic species in phytoplankton assemblages, contribute to the understanding of the potential consequences of ocean warming and increase seawater freshening on the trophic webs of the Southern Ocean.

High latitude Southern Ocean phytoplankton have distinctive bio-optical properties

Studying the biogeochemistry of the Southern Ocean using remote sensing relies on accurate interpretation of ocean colour through bio-optical and biogeochemical relationships between quantities and properties of interest. During the Antarctic Circumnavigation Expedition of the 2016/2017 Austral Summer, we collected a spatially comprehensive dataset of phytoplankton pigment concentrations, particulate absorption and particle size distribution and compared simple bio-optical and particle property relationships as a function of chlorophyll a. Similar to previous studies we find that the chlorophyll-specific phytoplankton absorption coefficient is significantly lower than in other oceans at comparable chlorophyll concentrations. This appears to be driven in part by lower concentrations of accessory pigments per unit chlorophyll a as well as increased pigment packaging due to relatively larger sized phytoplankton at low chlorophyll a than is typically observed in other oceans. We find that the contribution of microphytoplankton (>20 µm size) to chlorophyll a estimates of phytoplankton biomass is significantly higher than expected for the given chlorophyll a concentration, especially in higher latitudes south of the Southern Antarctic Circumpolar Current Front. Phytoplankton pigments are more packaged in larger cells, which resulted in a flattening of phytoplankton spectra as measured in these samples when compared to other ocean regions with similar chlorophyll a concentration. Additionally, we find that at high latitude locations in the Southern Ocean, pheopigment concentrations can exceed mono-vinyl chlorophyll a concentrations. Finally, we observed very different relationships between particle volume and chlorophyll a concentrations in high and low latitude Southern Ocean waters, driven by differences in phytoplankton community composition and acclimation to environmental conditions and varying contribution of non-algal particles to the particulate matter. Our data confirm that, as previously suggested, the relationships between bio-optical properties and chlorophyll a in the Southern Ocean are different to other oceans. In addition, distinct bio-optical properties were evident between high and low latitude regions of the Southern Ocean basin. Here we provide a region-specific set of power law functions describing the phytoplankton absorption spectrum as a function of chlorophyll a.

Evidence for the Impact of Climate Change on Primary Producers in the Southern Ocean

Within the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO), this paper brings together analyses of recent trends in phytoplankton biomass, primary production and irradiance at the base of the mixed layer in the Southern Ocean and summarises future projections. Satellite observations suggest that phytoplankton biomass in the mixed-layer has increased over the last 20 years in most (but not all) parts of the Southern Ocean, whereas primary production at the base of the mixed-layer has likely decreased over the same period. Different satellite models of primary production (Vertically Generalised versus Carbon Based Production Models) give different patterns and directions of recent change in net primary production (NPP). At present, the satellite record is not long enough to distinguish between trends and climate-related cycles in primary production. Over the next 100 years, Earth system models project increasing NPP in the water column in the MEASO northern and Antarctic zones but decreases in the Subantarctic zone. Low confidence in these projections arises from: (1) the difficulty in mapping supply mechanisms for key nutrients (silicate, iron); and (2) understanding the effects of multiple stressors (including irradiance, nutrients, temperature, pCO2, pH, grazing) on different species of Antarctic phytoplankton. Notwithstanding these uncertainties, there are likely to be changes to the seasonal patterns of production and the microbial community present over the next 50–100 years and these changes will have ecological consequences across Southern Ocean food-webs, especially on key species such as Antarctic krill and silverfish.

Impact of climate change on the primary production and related biogeochemical cycles in the coastal and sea ice zone of the Southern Ocean

Climate change in the Southern Hemisphere has exerted impact on the primary production in the Southern Ocean (SO). Using a recently released reanalysis dataset on global biogeochemistry, a comprehensive analysis was conducted on the complex biogeochemical seasonal cycle and the impact of climate change with a focus in areas within the meridional excursion of the sea ice boundary-coastal and continental shelf zone (CCSZ) and seasonal sea ice zone (SIZ). The seasonal cycles of primary production and related nutrients are closely linked with the seasonal changes in sea ice and sea surface temperatures. As sea ice retreats and allows energy and gas exchange across the sea surface, phytoplankton growth is initiated, consuming accumulated nutrients within the shallow depth of ~40 m. The seasonal evolutions of physical, biological and chemical variables show both spatial and temporal consistency with each other. Climate change has altered the timing and amplitude of the seasonal cycle. While primary production has generally increased along with an intensified uptake of CO₂, some areas show a reduction in production (e.g., Prydz Bay, eastern Indian Ocean). In the CCSZ, increased iron utilization and light availability allowed production to be increased. However, the mechanism by which these factors are altered varies from one location to another, including changes in sea ice cover, surface stratification, and downwelling/upwelling. In the SIZ, where iron is generally a limiting factor, iron supply is a key driver of changes in primary production regardless of other nutrients. There is a clear influence of climatic change on the biogeochemical cycle although the signal is still weak.

Antarctic ecosystems in transition - life between stresses and opportunities

Important findings from the second decade of the 21st century on the impact of environmental change on biological processes in the Antarctic were synthesised by 26 international experts. Ten key messages emerged that have stakeholder-relevance and/or a high impact for the scientific community. They address (i) altered biogeochemical cycles, (ii) ocean acidification, (iii) climate change hotspots, (iv) unexpected dynamism in seabed-dwelling populations, (v) spatial range shifts, (vi) adaptation and thermal resilience, (vii) sea ice related biological fluctuations, (viii) pollution, (ix) endangered terrestrial endemism and (x) the discovery of unknown habitats. Most Antarctic biotas are exposed to multiple stresses and considered vulnerable to environmental change due to narrow tolerance ranges, rapid change, projected circumpolar impacts, low potential for timely genetic adaptation, and migration barriers. Important ecosystem functions, such as primary production and energy transfer between trophic levels, have already changed, and biodiversity patterns have shifted. A confidence assessment of the degree of 'scientific understanding' revealed an intermediate level for most of the more detailed sub-messages, indicating that process-oriented research has been successful in the past decade. Additional efforts are necessary, however, to achieve the level of robustness in scientific knowledge that is required to inform protection measures of the unique Antarctic terrestrial and marine ecosystems, and their contributions to global biodiversity and ecosystem services.

Rapid Manipulation in Irradiance Induces Oxidative Free-Radical Release in a Fast-Ice Algal Community (McMurdo Sound, Antarctica)

Sea ice supports a unique assemblage of microorganisms that underpin Antarctic coastal food-webs, but reduced ice thickness coupled with increased snow cover will modify energy flow and could lead to photodamage in ice-associated microalgae. In this study, microsensors were used to examine the influence of rapid shifts in irradiance on extracellular oxidative free radicals produced by sea-ice algae. Bottom-ice algal communities were exposed to one of three levels of incident light for 10 days: low (0.5 μmol photons m^-2 s^-1, 30 cm snow cover), mid-range (5 μmol photons m^-2 s^-1, 10 cm snow), or high light (13 μmol photons m^-2 s^-1, no snow). After 10 days, the snow cover was reversed (either removed or added), resulting in a rapid change in irradiance at the ice-water interface. In treatments acclimated to low light, the subsequent exposure to high irradiance resulted in a ~400× increase in the production of hydrogen peroxide (H₂O₂) and a 10× increase in nitric oxide (NO) concentration after 24 h. The observed increase in oxidative free radicals also resulted in significant changes in photosynthetic electron flow, RNA-oxidative damage, and community structural dynamics. In contrast, there was no significant response in sea-ice algae acclimated to high light and then exposed to a significantly lower irradiance at either 24 or 72 h. Our results demonstrate that microsensors can be used to track real-time in-situ stress in sea-ice microbial communities. Extrapolating to ecologically relevant spatiotemporal scales remains a significant challenge, but this approach offers a fundamentally enhanced level of resolution for quantifying the microbial response to global change.

It's what's inside that matters: physiological adaptations of high-latitude marine microalgae to environmental change

Marine microalgae within seawater and sea ice fuel high-latitude ecosystems and drive biogeochemical cycles through the fixation and export of carbon, uptake of nutrients, and production and release of oxygen and organic compounds. High-latitude marine environments are characterized by cold temperatures, dark winters and a strong seasonal cycle. Within this environment a number of diverse and dynamic habitats exist, particularly in association with the formation and melt of sea ice, with distinct microalgal communities that transition with the season. Algal physiology is a crucial component, both responding to the dynamic environment and in turn influencing its immediate physicochemical environment. As high-latitude oceans shift into new climate regimes the analysis of seasonal responses may provide insights into how microalgae will respond to long-term environmental change. This review discusses recent developments in our understanding of how the physiology of high-latitude marine microalgae is regulated over a polar seasonal cycle, with a focus on ice-associated (sympagic) algae. In particular, physiologies that impact larger scale processes will be explored, with an aim to improve our understanding of current and future ecosystems and biogeochemical cycles.

The hemispheric contrast in cloud microphysical properties constrains aerosol forcing

The change in planetary albedo due to aerosol-cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth's climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol-cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm-3 and 24 cm-3 By extension, the radiative forcing since 1850 from aerosol-cloud interactions is constrained to be -1.2 W⋅m-2 to -0.6 W⋅m-2 The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol-cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models.

Diversity in Xanthophyll Cycle Pigments Content and Related Nonphotochemical Quenching (NPQ) Among Microalgae: Implications for Growth Strategy and Ecology

Xanthophyll cycle-related nonphotochemical quenching (NPQ), which is present in most photoautotrophs, allows dissipation of excess light energy. Xanthophyll cycle-related NPQ depends principally on xanthophyll cycle pigments composition and their effective involvement in NPQ. Xanthophyll cycle-related NPQ is tightly controlled by environmental conditions in a species-/strain-specific manner. These features are especially relevant in microalgae living in a complex and highly variable environment. The goal of this study was to perform a comparative assessment of NPQ ecophysiologies across microalgal taxa in order to underline the specific involvement of NPQ in growth adaptations and strategies. We used both published results and data acquired in our laboratory to understand the relationships between growth conditions (irradiance, temperature, and nutrient availability), xanthophyll cycle composition, and xanthophyll cycle pigments quenching efficiency in microalgae from various taxa. We found that in diadinoxanthin-containing species, the xanthophyll cycle pigment pool is controlled by energy pressure in all species. At any given energy pressure, however, the diatoxanthin content is higher in diatoms than in other diadinoxanthin-containing species. XC pigments quenching efficiency is species-specific and decreases with acclimation to higher irradiances. We found a clear link between the natural light environment of species/ecotypes and quenching efficiency amplitude. The presence of diatoxanthin or zeaxanthin at steady state in all species examined at moderate and high irradiances suggests that cells maintain a light-harvesting capacity in excess to cope with potential decrease in light intensity.

Characterizing spatial distribution of chlorophyll a in the Southern Ocean on a circumpolar cruise in summer

The spatial variation of chlorophyll a in the Southern Ocean (SO) was of great significance. Sea surface chlorophyll a concentrations was measured by Ferry Box monitoring system on the Chinese polar research vessel Xue Long, which circumnavigated the Antarctic continent in a clockwise direction during the austral summer 2013-2014 (November 2013-April 2014). The concentrations of chlorophyll a indicated a relatively uniform distribution of 0.049-11.647 mg m^-3 (mean 0. 869 mg m^-3, n = 152,751). The highest chlorophyll a concentrations (mean 1.847 mg m^-3) was found in the Ross sea (RS). In addition, six high-chlorophyll a hot spots were recognized. Analysis revealed that phytoplankton bloom could be controlled by multiple factors in different regions, and the chlorophyll a bloom is attributed to the combined effect of surface and subsurface processes such as, continental shelf, sea ice melting, Circumpolar Deep Water (CDW) upwelling, suitabletemperature, and nutrient injection from subsurface to the surface. The topographic effects, sea ice melting and CDW upwelling may play a major role in controlling primary productivity in the SO. Among of all, CDW upwelling may be the most important role improving primary productivity. This study presented the phytoplankton distribution patterns and the relation with potential growth-controlling factors in the SO, which will provide more insight in the mechanisms that control global warming to reduce global CO₂ the atmosphere into the ocean interior.

Effects of temperature and salinity on respiratory losses and the ratio of photosynthesis to respiration in representative Antarctic phytoplankton species

The Southern Ocean (SO) is a net sink for atmospheric CO₂ whereby the photosynthetic activity of phytoplankton and sequestration of organic carbon (biological pump) plays an important role. Global climate change will tremendously influence the dynamics of environmental conditions for the phytoplankton community, and the phytoplankton will have to acclimate to a combination of changes of e.g. water temperature, salinity, pH, and nutrient supply. The efficiency of the biological pump is not only determined by the photosynthetic activity but also by the extent of respiratory carbon losses of phytoplankton cells. Thus, the present study investigated the effect of different temperature and salinity combinations on the ratio of gross photosynthesis to respiration (rGP/R) in two representative phytoplankton species of the SO. In the comparison of phytoplankton grown at 1 and 4°C the rGP/R decreased from 11.5 to 7.7 in Chaetoceros sp., from 9.1 to 3.2 in Phaeocystis antarctica strain 109, and from 12.4 to 7.0 in P. antarctica strain 764, respectively. The decrease of rGP/R was primarily dependent on temperature whereas salinity was only of minor importance. Moreover, the different rGP/R at 1 and 4°C were caused by changes of temperature-dependent respiration rates but were independent of changes of photosynthetic rates. For further interpretation, net primary production (NPP) was calculated for different seasonal conditions in the SO with specific combinations of irradiance, temperature, and salinity. Whereas, maximum photosynthetic rates significantly correlated with calculated NPP under experimental ‘Spring’, ‘Summer’, and ‘Autumn’ conditions, there was no correlation between rGP/R and the respective values of NPP. The study revealed species-specific differences in the acclimation to temperature and salinity changes that could be linked to their different original habitats.

Quantitative macromolecular patterns in phytoplankton communities resolved at the taxonomical level by single-cell Synchrotron FTIR-spectroscopy

BACKGROUND

Technical limitations regarding bulk analysis of phytoplankton biomass limit our comprehension of carbon fluxes in natural populations and, therefore, of carbon, nutrients and energy cycling in aquatic ecosystems. In this study, we took advantage of Synchrotron FTIR micro-spectroscopy and the partial least square regression (PLSr) algorithm to simultaneously quantify the protein, lipid and carbohydrate content at the single-cell level in a mock phytoplankton community (composed by a cyanobacterium, a green-alga and a diatom) grown at two temperatures (15 °C and 25 °C).

RESULTS

The PLSr models generated to quantify cell macromolecules presented high quality fit (R² ≥ 0.90) and low error of prediction (RMSEP 2-6% of dry weight). The regression coefficients revealed that the prediction of each macromolecule was not exclusively dependent on spectral features corresponding to that compound, but rather on all major macromolecular pools, reflecting adjustments in the overall cell carbon balance. The single-cell analysis, studied by means of Kernel density estimators, showed that the modes of density distribution of macromolecules were different at 15 °C and 25 °C. However, a substantial proportion of cells was biochemically identical at the two temperatures because of population heterogeneity.

CONCLUSIONS

The spectroscopic approach presented in this study allows the quantification of macromolecules in single phytoplankton cells. This method showed that population heterogeneity most likely ensures a backup of non-acclimated cells that may rapidly exploit new favourable niches. This finding may have important consequences for the ecology of phytoplankton populations and shows that the "average cell" concept might substantially limit our comprehension of population dynamics and biogeochemical cycles in aquatic ecosystems.

Horizontal and vertical food web structure drives trace element trophic transfer in Terra Nova Bay, Antarctica

Despite a vast amount of literature has focused on trace element (TE) contamination in Antarctica during the last decades, the assessment of the main pathways driving TE transfer to the biota is still an overlooked issue. This limits the ability to predict how variations in sea-ice dynamics and productivity due to climate change will affect TE allocation in the food web. Here, food web structure of Tethys Bay (Terra Nova Bay, Ross Sea, Antarctica) was first characterised by analysing carbon and nitrogen stable isotopes (δ¹³C, δ¹⁵N) in organic matter sources (sediment and planktonic, benthic and sympagic primary producers) and consumers (zooplankton, benthic invertebrates, fish and birds). Diet and trophic position were also characterised using Bayesian mixing models. Then, relationships between stable isotopes, diet and TEs (Cd, Cr, Co, Cu, Hg, Ni, Pb and V) were assessed in order to evaluate if and how horizontal (organic matter pathways) and vertical (trophic position) food web features influence TE transfer to the biota. Regressions between log[TE] and δ¹³C revealed that the sympagic pathway drives accumulation of V in primary consumers and Cd and Hg in secondary consumers, and that a coupled benthic/pelagic pathway drives Pb transfer to all consumers. Regressions between log[TE] and δ¹⁵N showed that only Hg biomagnifies across trophic levels, while all the others TEs showed a biodilution pattern, consistent with patterns observed in temperate food webs. Although the Cd behavior needs further investigations, the present findings provide new insights about the role of basal sources in the transfer of TEs in polar systems. This is especially important nowadays in light of the forecasted trophic changes potentially resulting from climate change-induced modification of sea-ice dynamics.

Phytoplankton as Key Mediators of the Biological Carbon Pump: Their Responses to a Changing Climate

The world’s oceans are a major sink for atmospheric carbon dioxide (CO2). The biological carbon pump plays a vital role in the net transfer of CO2 from the atmosphere to the oceans and then to the sediments, subsequently maintaining atmospheric CO2 at significantly lower levels than would be the case if it did not exist. The efficiency of the biological pump is a function of phytoplankton physiology and community structure, which are in turn governed by the physical and chemical conditions of the ocean. However, only a few studies have focused on the importance of phytoplankton community structure to the biological pump. Because global change is expected to influence carbon and nutrient availability, temperature and light (via stratification), an improved understanding of how phytoplankton community size structure will respond in the future is required to gain insight into the biological pump and the ability of the ocean to act as a long-term sink for atmospheric CO2. This review article aims to explore the potential impacts of predicted changes in global temperature and the carbonate system on phytoplankton cell size, species and elemental composition, so as to shed light on the ability of the biological pump to sequester carbon in the future ocean.

Thermal windows and metabolic performance curves in a developing Antarctic fish

For ectotherms, temperature modifies the rate of physiological function across a temperature tolerance window depending on thermal history, ontogeny, and evolutionary history. Some adult Antarctic fishes, with comparatively narrow thermal windows, exhibit thermal plasticity in standard metabolic rate; however, little is known about the shape or breadth of thermal performance curves of earlier life stages of Antarctic fishes. We tested the effects of acute warming (- 1 to 8 °C) and temperature acclimation (2 weeks at - 1, 2, 4 °C) on survival and standard metabolic rate in early embryos of the dragonfish Gymnodraco acuticeps from McMurdo Sound, Ross Island, Antarctica. Contrary to predictions, embryos acclimated to warmer temperatures did not experience greater mortality and nearly all embryos survived acute warming to 8 °C. Metabolic performance curve height and shape were both significantly altered after 2 weeks of development at - 1 °C, with further increase in curve height, but not alteration of shape, with warm temperature acclimation. Overall metabolic rate temperature sensitivity (Q ₁₀) from - 1 to 8 °C varied from 2.6 to 3.6, with the greatest thermal sensitivity exhibited by embryos at earlier developmental stages. Interclutch variation in metabolic rates, mass, and development of simultaneously collected embryos was also documented. Taken together, metabolic performance curves provide insight into the costs of early development under warming temperatures, with the potential for thermal sensitivity to be modified by dragonfish phenology and magnitude of seasonal changes in temperature.

Title: Freshwater phytoplankton responses to global warming

Global warming alters species composition and function of freshwater ecosystems. However, the impact of temperature on primary productivity is not sufficiently understood and water quality models need to be improved in order to assess the quantitative and qualitative changes of aquatic communities. On the basis of experimental data, we demonstrate that the commonly used photosynthetic and water chemistry parameters alone are not sufficient for modeling phytoplankton growth under changing temperature regimes. We present some new aspects of the acclimation process with respect to temperature and how contrasting responses may be explained by a more complete physiological knowledge of the energy flow from photons to new biomass. We further suggest including additional bio-markers/traits for algal growth such as carbon allocation patterns to increase the explanatory power of such models. Although carbon allocation patterns are promising and functional cellular traits for growth prediction under different nutrient and light conditions, their predictive power still waits to be tested with respect to temperature. A great challenge for the near future will be the prediction of primary production efficiencies under the global change scenario using a uniform model for phytoplankton assemblages.