Gross and net production during the spring bloom along the Western Antarctic Peninsula

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.

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.

Hardy S, Falciatore A, Lavaud J. Genome-Scale Metabolic Reconstruction and in Silico Perturbation Analysis of the Polar Diatom Fragilariopsis cylindrus Predicts High Metabolic Robustness

Diatoms are major primary producers in polar environments where they can actively grow under extremely variable conditions. Integrative modeling using a genome-scale model (GSM) is a powerful approach to decipher the complex interactions between components of diatom metabolism and can provide insights into metabolic mechanisms underlying their evolutionary success in polar ecosystems. We developed the first GSM for a polar diatom, Fragilariopsis cylindrus, which enabled us to study its metabolic robustness using sensitivity analysis. We find that the predicted growth rate was robust to changes in all model parameters (i.e., cell biochemical composition) except the carbon uptake rate. Constraints on total cellular carbon buffer the effect of changes in the input parameters on reaction fluxes and growth rate. We also show that single reaction deletion of 20% to 32% of active (nonzero flux) reactions and single gene deletion of 44% to 55% of genes associated with active reactions affected the growth rate, as well as the production fluxes of total protein, lipid, carbohydrate, DNA, RNA, and pigments by less than 1%, which was due to the activation of compensatory reactions (e.g., analogous enzymes and alternative pathways) with more highly connected metabolites involved in the reactions that were robust to deletion. Interestingly, including highly divergent alleles unique for F. cylindrus increased its metabolic robustness to cellular perturbations even more. Overall, our results underscore the high robustness of metabolism in F. cylindrus, a feature that likely helps to maintain cell homeostasis under polar conditions.

Benthic primary production and respiration of shallow rocky habitats: a case study from South Bay (Doumer Island, Western Antarctic Peninsula)

Rocky benthic communities are common in Antarctic coastal habitats; yet little is known about their carbon turnover rates. Here, we performed a broad survey of shallow ( < 65 m depth) rocky ice-scoured habitats of South Bay (Doumer Island, Western Antarctic Peninsula), combining (i) biodiversity assessments from benthic imaging, and (ii) in situ benthic dissolved oxygen (O₂) exchange rates quantified by the aquatic eddy covariance technique. The 18 study sites revealed a gradual transition from macroalgae and coralline-dominated communities at ice-impacted depths (15–25 m; zone I) to large suspension feeders (e.g., sponges, bivalves) at depth zone II (25–40 m) and extensive suspension feeders at the deepest study location (zone III; 40–65 m). Gross primary production (GPP) in zone I was up to 70 mmol O₂ m⁻² d⁻¹ and dark ecosystem respiration (ER) ranged from 15 to 90 mmol m⁻² d⁻¹. Zone II exhibited reduced GPP (average 1.1 mmol m⁻² d⁻¹) and ER rates from 6 to 36 mmol m⁻² d⁻¹, whereas aphotic zone III exhibited ER between 1 and 6 mmol m⁻² d⁻¹. Benthic ER exceeded GPP at all study sites, with daily net ecosystem metabolism (NEM) ranging from − 22 mmol m⁻² d⁻¹ at the shallow sites to − 4 mmol m⁻² d⁻¹ at 60 m. Similar NEM dynamics have been observed for hard-substrate Arctic habitats at comparable depths. Despite relatively high GPP during summer, coastal rocky habitats appear net heterotrophic. This is likely due to active drawdown of organic material by suspension-feeding communities that are key for biogeochemical and ecological functioning of high-latitude coastal ecosystems.

Rubisco carboxylation kinetics and inorganic carbon utilization in polar versus cold-temperate seaweeds

Despite the high productivity and ecological importance of seaweeds in polar coastal regions, little is known about their carbon utilization mechanisms, especially the kinetics of the CO2-fixing enzyme Rubisco. We analyzed Rubisco carboxylation kinetics at 4 °C and 25 °C in 12 diverse polar seaweed species (including cold-temperate populations of the same species) and the relationship with their ability to use bicarbonate, by using 13C isotope discrimination and pH drift experiments. We observed a large variation in Rubisco carboxylation kinetics among the selected species, although no correlation was found between either the Michaelis-Menten constant for CO2 (Kc) or Rubisco content per total soluble protein ([Rubisco]/[TSP]) and the ability to use bicarbonate for non-green seaweeds. This study reports intraspecific Rubisco cold adaptation by means of either higher Rubisco carboxylation turnover rate (kcatc) and carboxylase efficiency (kcatc/Kc) at 4 °C or higher [Rubisco]/[TSP] in some of the analyzed species. Our data point to a widespread ability for photosynthetic bicarbonate usage among polar seaweeds, despite the higher affinity of Rubisco for CO2 and higher dissolved CO2 concentration in cold seawater. Moreover, the reported catalytic variation within form ID Rubisco might avert the canonical trade-off previously observed between Kc and kcatc for plant Rubiscos.

Electron balancing under different sink conditions reveals positive effects on photon efficiency and metabolic activity of Synechocystis sp. PCC 6803

BACKGROUND

Cyanobacteria are ideal model organisms to exploit photosynthetically derived electrons or fixed carbon for the biotechnological synthesis of high value compounds and energy carriers. Much effort is spent on the rational design of heterologous pathways to produce value-added chemicals. Much less focus is drawn on the basic physiological responses and potentials of phototrophs to deal with natural or artificial electron and carbon sinks. However, an understanding of how electron sinks influence or regulate cellular physiology is essential for the efficient application of phototrophic organisms in an industrial setting, i.e., to achieve high productivities and product yields.

RESULTS

The physiological responses of the cyanobacterium Synechocystis sp. PCC 6803 to electron sink variation were investigated in a systematic and quantitative manner. A variation in electron demand was achieved by providing two N sources with different degrees of reduction. By additionally varying light and CO₂ availabilities, steady state conditions with strongly differing source-sink ratios were established. Balancing absorbed photons and electrons used for different metabolic processes revealed physiological responses to sink/source ratio variation. Surprisingly, an additional electron sink under light and thus energy limitation was found not to hamper growth, but was compensated by improved photosynthetic efficiency and activity. In the absence of carbon and light limitation, an increase in electron demand even stimulated carbon assimilation and growth.

CONCLUSION

The metabolism of Synechocystis sp. PCC 6803 is highly flexible regarding the compensation of additional electron demands. Under light limitation, photosynthesis obviously does not necessarily run at its maximal capacity, possibly for the sake of robustness. Increased electron demands can even boost photosynthetic activity and growth.

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high natural solar radiation

Impacts of rising atmospheric CO2 concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii, the effects of moderate and high natural solar radiation combined with either ambient or future pCO2 on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO2 concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO2 in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (Fv /Fm ) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom-forming Antarctic coastal diatoms might increase carbon contents under future pCO2 conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO2 sequestration potential of diatom communities in the future coastal Southern Ocean.

Recurrent seascape units identify key ecological processes along the western Antarctic Peninsula

The western Antarctic Peninsula (WAP) is a bellwether of global climate change and natural laboratory for identifying interactions between climate and ecosystems. The Palmer Long-Term Ecological Research (LTER) project has collected data on key ecological and environmental processes along the WAP since 1993. To better understand how key ecological parameters are changing across space and time, we developed a novel seascape classification approach based on in situ temperature, salinity, chlorophyll a, nitrate + nitrite, phosphate, and silicate. We anticipate that this approach will be broadly applicable to other geographical areas. Through the application of self-organizing maps (SOMs), we identified eight recurrent seascape units (SUs) in these data. These SUs have strong fidelity to known regional water masses but with an additional layer of biogeochemical detail, allowing us to identify multiple distinct nutrient profiles in several water masses. To identify the temporal and spatial distribution of these SUs, we mapped them across the Palmer LTER sampling grid via objective mapping of the original parameters. Analysis of the abundance and distribution of SUs since 1993 suggests two year types characterized by the partitioning of chlorophyll a into SUs with different spatial characteristics. By developing generalized linear models for correlated, time-lagged external drivers, we conclude that early spring sea ice conditions exert a strong influence on the distribution of chlorophyll a and nutrients along the WAP, but not necessarily the total chlorophyll a inventory. Because the distribution and density of phytoplankton biomass can have an impact on biomass transfer to the upper trophic levels, these results highlight anticipated links between the WAP marine ecosystem and climate.

Specific eukaryotic plankton are good predictors of net community production in the Western Antarctic Peninsula

Despite our current realization of the tremendous diversity that exists in plankton communities, we have little understanding of how this biodiversity influences the biological carbon pump other than broad paradigms such as diatoms contributing disproportionally to carbon export. Here we combine high-resolution underway O₂/Ar, which provides an estimate of net community production, with high-throughput 18 S ribosomal DNA sequencing to elucidate the relationship between eukaryotic plankton community structure and carbon export potential at the Western Antarctica Peninsula (WAP), a region which has experienced rapid warming and ecosystem changes. Our results show that in a diverse plankton system comprised of ~464 operational taxonomic units (OTUs) with at least 97% 18 S identity, as few as two or three key OTUs, i.e. large diatoms, Phaeocystis, and mixotrophic/phagotrophic dinoflagellates, can explain a large majority of the spatial variability in the carbon export potential (76-92%). Moreover, we find based on a community co-occurrence network analysis that ecosystems with lower export potential have more tightly coupled communities. Our results indicate that defining plankton communities at a deeper taxonomic resolution than by functional groups and accounting for the differences in size and coupling between groups can substantially improve organic carbon flux predictions.

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

The Arctic Ocean is a region particularly prone to ongoing ocean acidification (OA) and climate-driven changes. The influence of these changes on Arctic phytoplankton assemblages, however, remains poorly understood. In order to understand how OA and enhanced irradiances (e.g., resulting from sea–ice retreat) will alter the species composition, primary production, and eco-physiology of Arctic phytoplankton, we conducted an incubation experiment with an assemblage from Baffin Bay (71°N, 68°W) under different carbonate chemistry and irradiance regimes. Seawater was collected from just below the deep Chl a maximum, and the resident phytoplankton were exposed to 380 and 1000 µatm pCO₂ at both 15 and 35% incident irradiance. On-deck incubations, in which temperatures were 6 °C above in situ conditions, were monitored for phytoplankton growth, biomass stoichiometry, net primary production, photo-physiology, and taxonomic composition. During the 8-day experiment, taxonomic diversity decreased and the diatom Chaetoceros socialis became increasingly dominant irrespective of light or CO₂ levels. We found no statistically significant effects from either higher CO₂ or light on physiological properties of phytoplankton during the experiment. We did, however, observe an initial 2-day stress response in all treatments, and slight photo-physiological responses to higher CO₂ and light during the first five days of the incubation. Our results thus indicate high resistance of Arctic phytoplankton to OA and enhanced irradiance levels, challenging the commonly predicted stimulatory effects of enhanced CO₂ and light availability for primary production.

Evolutionarily distinct strategies for the acquisition of inorganic carbon from seawater in marine diatoms

The acquisition of dissolved inorganic carbon (DIC) in CO2-limited seawater is a central issue to understand in marine primary production. We previously demonstrated the occurrence of direct HCO3- uptake by solute carrier (SLC) 4 transporters in a diatom, a major marine primary producer. Homologs of SLC are found in both centric and pennate marine diatoms, suggesting that SLC transporters are generally conserved. Here, the generality of SLC-mediated DIC uptake in diatoms was examined using an SLC inhibitor, diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), and an inhibitor of external carbonic anhydrase, acetazolamide. DIDS suppressed high-DIC-affinity photosynthesis in the pennate diatom Phaeodactylum tricornutum and the centric diatom Chaetoceros muelleri, but there was no effect on either the pennate Cylindrotheca fusiformis or the centric Thalassiosira pseudonana. Interestingly, the DIC affinity of DIDS-insensitive strains was sensitive to treatment with up to 100 μM acetazolamide, displaying a 2-4-fold increase in K0.5[DIC]. In contrast, acetazolamide did not affect the DIDS-sensitive group. These results indicate the occurrence of two distinct strategies for DIC uptake-one primarily facilitated by SLC and the other being passive CO2 entry facilitated by external carbonic anhydrase. The phylogenetic independence of these strategies suggests that environmental demands drove the evolution of distinct DIC uptake mechanisms in diatoms.

Continuous daylight in the high-Arctic summer supports high plankton respiration rates compared to those supported in the dark

Plankton respiration rate is a major component of global CO₂ production and is forecasted to increase rapidly in the Arctic with warming. Yet, existing assessments in the Arctic evaluated plankton respiration in the dark. Evidence that plankton respiration may be stimulated in the light is particularly relevant for the high Arctic where plankton communities experience continuous daylight in spring and summer. Here we demonstrate that plankton community respiration evaluated under the continuous daylight conditions present in situ, tends to be higher than that evaluated in the dark. The ratio between community respiration measured in the light (R_light) and in the dark (R_dark) increased as the 2/3 power of R_light so that the R_light:R_dark ratio increased from an average value of 1.37 at the median R_light measured here (3.62 µmol O₂ L^-1 d^-1) to an average value of 17.56 at the highest R_light measured here (15.8 µmol O₂ L^-1 d^-1). The role of respiratory processes as a source of CO₂ in the Arctic has, therefore, been underestimated and is far more important than previously believed, particularly in the late spring, with 24 h photoperiods, when community respiration rates are highest.

Evolutionary genomics of the cold-adapted diatom Fragilariopsis cylindrus

The Southern Ocean houses a diverse and productive community of organisms. Unicellular eukaryotic diatoms are the main primary producers in this environment, where photosynthesis is limited by low concentrations of dissolved iron and large seasonal fluctuations in light, temperature and the extent of sea ice. How diatoms have adapted to this extreme environment is largely unknown. Here we present insights into the genome evolution of a cold-adapted diatom from the Southern Ocean, Fragilariopsis cylindrus, based on a comparison with temperate diatoms. We find that approximately 24.7 per cent of the diploid F. cylindrus genome consists of genetic loci with alleles that are highly divergent (15.1 megabases of the total genome size of 61.1 megabases). These divergent alleles were differentially expressed across environmental conditions, including darkness, low iron, freezing, elevated temperature and increased CO₂. Alleles with the largest ratio of non-synonymous to synonymous nucleotide substitutions also show the most pronounced condition-dependent expression, suggesting a correlation between diversifying selection and allelic differentiation. Divergent alleles may be involved in adaptation to environmental fluctuations in the Southern Ocean.

Southern Ocean phytoplankton physiology in a changing climate

The Southern Ocean (SO) is a major sink for anthropogenic atmospheric carbon dioxide (CO₂), potentially harbouring even greater potential for additional sequestration of CO₂ through enhanced phytoplankton productivity. In the SO, primary productivity is primarily driven by bottom up processes (physical and chemical conditions) which are spatially and temporally heterogeneous. Due to a paucity of trace metals (such as iron) and high variability in light, much of the SO is characterised by an ecological paradox of high macronutrient concentrations yet uncharacteristically low chlorophyll concentrations. It is expected that with increased anthropogenic CO₂ emissions and the coincident warming, the major physical and chemical process that govern the SO will alter, influencing the biological capacity and functioning of the ecosystem. This review focuses on the SO primary producers and the bottom up processes that underpin their health and productivity. It looks at the major physico-chemical drivers of change in the SO, and based on current physiological knowledge, explores how these changes will likely manifest in phytoplankton, specifically, what are the physiological changes and floristic shifts that are likely to ensue and how this may translate into changes in the carbon sink capacity, net primary productivity and functionality of the SO.