Low Fe Availability for Photosynthesis of Sea-Ice Algae: Ex situ Incubation of the Ice Diatom Fragilariopsis cylindrus in Low-Fe Sea Ice Using an Ice Tank

Sea-ice algae play a crucial role in the ecology and biogeochemistry of sea-ice zones. They not only comprise the base of sea-ice ecosystems, but also seed populations of extensive ice-edge blooms during ice melt. Ice algae must rapidly acclimate to dynamic light environments, from the low light under sea ice to high light within open waters. Recently, iron (Fe) deficiency has been reported for diatoms in eastern Antarctic pack ice. Low Fe availability reduces photosynthetic plasticity, leading to reduced ice-algal primary production. We developed a low-Fe ice tank to manipulate Fe availability in sea ice. Over 20 days in the ice tank, the Antarctic ice diatom Fragilariopsis cylindrus was incubated in artificial low-Fe sea ice ([total Fe] = 20 nM) in high light (HL) and low light (LL) conditions. Melted ice was also exposed to intense light to simulate light conditions typical for melting ice in situ. When diatoms were frozen in, the maximum photochemical quantum efficiency of photosystem II (PSII), Fv/Fm, was suppressed by freezing stress. However, the diatoms maintained photosynthetic capability throughout the ice periods with a stable Fv/Fm value and increased photoprotection through non-photochemical quenching (NPQ) via photoprotective xanthophyll cycling (XC) and increased photoprotective carotenoid levels compared to pre-freeze-up. Photoprotection was more pronounced in the HL treatment due to greater light stress. However, the functional absorption cross section of PSII, σPSII, in F. cylindrus consistently increased after freezing, especially in the LL treatment (σPSII > 10 nm2 PSII–1). Our study is the first to report such a large σPSII in ice diatoms at low Fe conditions. When the melted sea ice was exposed to high light, Fv/Fm was suppressed. NPQ and XC were slightly upregulated, but not to values normally observed when Fe is not limiting, which indicates reduced photosynthetic flexibility to adapt to environmental changes during ice melt under low Fe conditions. Although ice algae can optimize their photosynthesis to sea-ice environments, chronic Fe starvation led to less flexibility of photoacclimation, particularly in low light conditions. This may have detrimental consequences for ice algal production and trophic interactions in sea-ice ecosystems if the recent reduction in sea-ice extent continues.

An assessment of diatom assemblages in the Sea of Okhotsk as a proxy for sea-ice cover

Knowledge of past variations in sea-ice extent is crucial for understanding the relationship between climate change and changes in sea ice. Diatom assemblages could be applied as a proxy for paleo-sea-ice extent; this requires accurate information on the modern species that are indicative of sea ice. Scanning electron microscope observations were performed on modern diatom assemblages in sea ice, sinking particles, and surface sediments in the Sea of Okhotsk. A sea-ice sample was collected in the southwestern Sea of Okhotsk near Hokkaido island in February 2013. Fragilariopsis cylindrus was the dominant diatom species in the sea-ice sample, accounting for 87 % of the total diatom assemblage. Time-series sediment traps were deployed during 1998–2000 at two stations, M4 and M6, off Sakhalin island. Total diatom fluxes ranged from 105 to 108 valves m−2 d−1 with noticeable seasonality. During the sea-ice covering period, the total diatom flux decreased by 1 or 2 orders of magnitude. The highest diatom fluxes were observed in spring and summer. The diatom species composition in sinking particles also showed pronounced seasonal changes. During summer and fall, the Shionodiscus trifultus group and Neodenticula seminae were the major diatom taxa. During the sea-ice covering period, Fragilariopsis cylindrus and Bacterosira bathyomphala resting spores were abundant. Both the sea-ice-related species showed similar flux patterns except for the spring bloom after sea-ice retreat: F. cylindrus fluxes exhibited pronounced spring bloom peaks of 108 valves m−2 d−1; in contrast, the fluxes of Bacterosira bathyomphala resting spores during the spring bloom were 1 order of magnitude lower than those of F. cylindrus. Surface-sediment core XP98-MC4 was obtained near station M6 sediment-trap site off Sakhalin island. The relative abundance of Fragilariopsis cylindrus in the surface-sediment diatom assemblage was only 6.4 %, markedly lower than that in the sediment-trap samples (43.4 %). In the surface sediment, the relative abundances of diatom taxa with heavily silicified valves such as B. bathyomphala resting spores, Shionodiscus variantius, and Thalassionema nitzschioides were greater than their relative abundances in sinking particles.

An annual cycle of diatom succession in two contrasting Greenlandic fjords: from simple sea-ice indicators to varied seasonal strategists

<p>Understanding environmental factors affecting diatom species composition at seasonal resolution can contribute to the improvement of paleo sea-ice reconstruction. We recorded diatom species succession over one full year (May 2017‒May 2018) using automated sediment traps installed in two contrasting Greenlandic fjords: seasonally ice-covered Young Sound in high arctic NE Greenland and nearly sea-ice free Godthåbsfjord in subarctic SW Greenland. The two study sites had distinct seasonal regimes in terms of both sediment and diatom fluxes. In Young Sound, diatom fluxes peaked during the ice-melt in June–July (max. 880×10<sup>6</sup> valves m<sup>-2 </sup>d<sup>-1</sup>), but were very low (0.11­–12.7×10<sup>6</sup> valves m<sup>-2 </sup>d<sup>-1</sup>) for the rest of the year. The pattern was very different in Godthåbsfjord, where diatom fluxes were more stable throughout the year and at maximum 320×10<sup>6</sup> valves m<sup>-2 </sup>d<sup>-1</sup> in summer. A total of 60 diatom taxa were present in Young Sound and 50 in Godthåbsfjord, with 19 and 22 sympagic or pelagic species, respectively. The diatom assemblage in Young Sound is strongly dominated by the pennate sea-ice species<em> Fragilariopsis oceanica</em>, <em>Fragilariopsis reginae-jahniae</em> and <em>Fossula arctica</em>, which exhibited pulse-like deposition in the trap during and after the ice melt. In Godthåbsfjord, the fluxes were dominated by resting spores of centric Chaetoceros, while the rest of the assemblage was characterized by the cold-water indicator species <em>Detonula confervacea</em> spore, <em>Fragilariopsis cylindrus</em> and <em>Thalassiosira antarctica</em> var. <em>borealis</em> spore accompanied by some warmer-water species. Some sea-ice indicator species were also observed in Godthåbsfjord, but at very low counts and throughout the year, likely transported from the inner fjord, which experiences seasonal sea-ice coverage. We show that <em>F. oceanica</em>, <em>F. reginae-jahniae</em> and <em>F. arctica</em> exhibit similar seasonal behaviour and are clearly linked to sea ice. On the other hand,<em> Fragilariopsis cylindrus</em> seems to have a more flexible niche, and is not an unequivocal ice indicator. Similarly, <em>Pauliella taeniata</em> has a differing niche, and does not favour our study locations probably due to its preference for lower salinities. We underscore the importance of taking into account ecological and seasonal preferences of the individual diatom species when reconstructing past sea-ice conditions qualitatively or quantitatively.</p>

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.