Fragilariopsis Cylindrus Research Articles

Microzooplankton and phytoplankton of Ross Sea polynya areas and potential linkage among functional traits

The Ross Sea is characterized by a series of subsystems with different characteristics making it an extremely productive area. To understand whether species composition and functional traits of the plankton community can be used as biological tracers, we have analyzed the composition of phytoplankton and microzooplankton, and their potential relationships, in two different polynyas of the Ross Sea during the austral summer 2017. Sampling activities were carried out near Terra Nova Bay, between Cape Washington and the northern shore of the Drygalski Ice Tongue, and in the South-Central Ross Sea. We investigated the phytoplankton and microzooplankton structure using the phytoplankton body size classes and the tintinnids lorica oral diameter as functional traits, speculating on the relationship between the two plankton communities and their use as biological indicators in a changing Southern Ocean. Our data showed significant differences in terms of plankton composition and related functional traits between the two areas, suggesting the existence of distinct ecological dynamics despite the similar total carbon content. In Terra Nova Bay, heterotrophic dinoflagellates were the most abundant microzooplankton, in association with a large phytoplankton biomass mainly represented by diatoms and nano- and micro-phytoplankton. Tintinnids with large lorica oral diameters were abundant in Central Ross Sea, where phytoplankton was dominated by Phaeocystis antarctica and by the micro size class. Among microzooplankton, Protoperidinium defectum, P. applanatum and P. incertum were the most abundant dinoflagellates species, while Codonellopsis gaussi, C. gaussi forma cylindroconica, Laackmanniella prolongata and Cymatocylis drygalskii were the most abundant tintinnids. The phytoplankton was dominated by diatoms Pseudo-nitzschia subcurvata, Fragilariopsis cylindrus, F. curta and by the haptophyte P. antarctica. Our data indicate that beyond physical and chemical features defining distinct sectors of the Ross Sea, both species composition and functional traits of phytoplankton and microzooplankton represent a valid monitoring tool, especially with the ongoing global warming and its effects on Antarctic food webs.

Sea-ice melt determines seasonal phytoplankton dynamics and delimits the habitat of temperate Atlantic taxa as the Arctic Ocean atlantifies.

The Arctic Ocean is one of the regions where anthropogenic environmental change is progressing most rapidly and drastically. The impact of rising temperatures and decreasing sea ice on Arctic marine microbial communities is yet not well understood. Microbes form the basis of food webs in the Arctic Ocean, providing energy for larger organisms. Previous studies have shown that Atlantic taxa associated with low light are robust to more polar conditions. We compared to which extent sea ice melt influences light-associated phytoplankton dynamics and biodiversity over two years at two mooring locations in the Fram Strait. One mooring is deployed in pure Atlantic water, and the second in the intermittently ice-covered Marginal Ice Zone. Time-series analysis of amplicon sequence variants abundance over a 2-year period, allowed us to identify communities of co-occurring taxa that exhibit similar patterns throughout the annual cycle. We then examined how alterations in environmental conditions affect the prevalence of species. During high abundance periods of diatoms, polar phytoplankton populations dominated, while temperate taxa were weakly represented. Furthermore, we found that polar pelagic and ice-associated taxa, such as Fragilariopsis cylindrus and Melosira arctica, were more common in Atlantic conditions, while temperate taxa, such as Odontella aurita and Proboscia alata, were less abundant under polar conditions. This suggests that sea ice melt may act as a barrier to the northward expansion of temperate phytoplankton, preventing their dominance in regions still strongly influenced by polar conditions. Our findings highlight the complex interactions between sea ice melt, phytoplankton dynamics, and biodiversity in the Arctic.

Major Diatom Microfossils from the Arctic Region: A Review

Abstract Diatoms-the unicellular, eukaryotic, photosynthetic phytoplankton belonging to Class Bacillariophyceae are an excellent tool to understand past ocean surface conditions, especially in high latitude regions. Along with sea surface temperature and sea ice, which they are primarily sensitive to, diatom assemblages could potentially be used to record runoff from melting glaciers and atmospheric teleconnections such as North Atlantic Oscillations (NAO) in Arctic fjords. However, understanding the ecology and geographical distribution of diatoms is extremely important before they can be used for palaeoceanographic reconstructions. In this study, four significant Arctic diatoms namely Bacterosira bathyomphala, Fragilariopsis cylindrus, Fragilaripsis oceanica and Thalassiosira antarctica var. borealis resting spores (R.S.) have been reviewed to understand their current status in their use for palaeoceanographic reconstructions. These four diatoms have been selected as they represent a typical Arctic environment. Numerous studies in the past thirty years had classified these diatoms into different assemblage groups, indicating specific ocean surface conditions. Here, we review the previous studies and to present the up-to-date classification of these species. We also compared the change in abundance of these four species through mid to late Holocene and modern period to their respective sea surface temperature and sea ice reconstructions, to show the robustness of their usage in palaeoceanographic reconstructions. The findings of this study will help to understand the up-to-date ecological characteristics of Arctic diatoms used in palaeoceanographic as well as palaeoecological reconstructions and the gap areas in diatom research such as a requirement for pan Arctic Sea ice diatom dataset.

Impacts of crude oil on Arctic sea-ice diatoms modified by irradiance

Anthropogenic climate change is reducing ice and snow thickness in the Arctic. The loss of summer sea ice has led to increased access to Arctic waters and the development of marine resources, which raises the risk of oil spills. Thinning ice and snow also increases irradiance in the upper ocean which is predicted to increase primary productivity, disfavoring shade-adapted sea-ice algae while benefitting phytoplankton and cryopelagic taxa. Studies have confirmed the lethality of crude oil and its distillates to Arctic phytoplankton; less well-constrained are the sublethal impacts to sea-ice algae in combination with other drivers. This study investigates the combination of two drivers, crude oil exposure and irradiance, on the growth rate and maximum cell concentration of four sea-ice diatoms (Attheya septentrionalis, Fragilariopsis cylindrus, and two strains of Synedropsis hyperborea) isolated from landfast sea ice near Utqiaġvik, Alaska. Crude oil inhibition of growth was complex and dependent on species and irradiance level. A. septentrionalis was generally tolerant to crude oil exposure, but toxicity was enhanced at the highest irradiance. The cryopelagic taxon, F. cylindrus, exhibited strong growth inhibition at TPH concentrations greater than approximately 6 mg L−1. Growth rates of S. hyperborea strains were stimulated at low concentrations of oil at all light levels. A simple numerical model was used to simulate an oil spill under varying snow depths to follow composition of a mock community comprised of these four isolates across a spring season. Results highlight that the reduction of algal biomass accumulation and the community composition change following a crude oil spill are more severe in a simulated low-snow spring, due to the relative sensitivity of F. cylindrus. We show that a brighter Arctic, which is predicted to increase the relative importance of cryopelagic taxa like F. cylindrus, may render the Arctic ecosystem more vulnerable to crude oil spills.

Hypometabolism to survive the long polar night and subsequent successful return to light in the diatom Fragilariopsis cylindrus.

Diatoms, the main eukaryotic phytoplankton of the polar marine regions, are essential for the maintenance of food chains specific to Arctic and Antarctic ecosystems, and are experiencing major disturbances under current climate change. As such, it is fundamental to understand the physiological mechanisms and associated molecular basis of their endurance during the long polar night. Here, using the polar diatom Fragilariopsis cylindrus, we report an integrative analysis combining transcriptomic, microscopic and biochemical approaches to shed light on the strategies used to survive the polar night. We reveal that in prolonged darkness, diatom cells enter a state of quiescence with reduced metabolic and transcriptional activity, during which no cell division occurs. We propose that minimal energy is provided by respiration and degradation of protein, carbohydrate and lipid stores and that homeostasis is maintained by autophagy in prolonged darkness. We also report internal structural changes that manifest the morphological acclimation of cells to darkness, including the appearance of a large vacuole. Our results further show that immediately following a return to light, diatom cells are able to use photoprotective mechanisms and rapidly resume photosynthesis, demonstrating the remarkable robustness of polar diatoms to prolonged darkness at low temperature.

Plastid-localized xanthorhodopsin increases diatom biomass and ecosystem productivity in iron-limited surface oceans.

Microbial rhodopsins are photoreceptor proteins that convert light into biological signals or energy. Proteins of the xanthorhodopsin family are common in eukaryotic photosynthetic plankton including diatoms. However, their biological role in these organisms remains elusive. Here we report on a xanthorhodopsin variant (FcR1) isolated from the polar diatom Fragilariopsis cylindrus. Applying a combination of biophysical, biochemical and reverse genetics approaches, we demonstrate that FcR1 is a plastid-localized proton pump which binds the chromophore retinal and is activated by green light. Enhanced growth of a Thalassiora pseudonana gain-of-function mutant expressing FcR1 under iron limitation shows that the xanthorhodopsin proton pump supports growth when chlorophyll-based photosynthesis is iron-limited. The abundance of xanthorhodopsin transcripts in natural diatom communities of the surface oceans is anticorrelated with the availability of dissolved iron. Thus, we propose that these proton pumps convey a fitness advantage in regions where phytoplankton growth is limited by the availability of dissolved iron.

Effect of dissolved iron (II) and temperature on growth of the Southern Ocean phytoplankton species Fragilariopsis cylindrus and Phaeocystis antarctica

Low bioavailability of the vital element iron (Fe) limits primary production in large regions of the Southern Ocean, thus impacting phytoplankton community structures. Primary productivity seems to be particularly sensitive to the reduced form of iron (Fe(II)), which is thought to be the most readily bioavailable redox form of Fe in the ocean. Here, we investigated the impact of temperature (3 °C, 5 °C and 7 °C) and Fe(II) additions (+ 5 nM) on growth of two Southern Ocean phytoplankton species Fragilariopsis cylindrus and Phaeocystis antarctica in coastal and open ocean water. At all tested temperatures, growth rates of P. antarctica were significantly higher with added iron, compared to the treatments without added iron in both waters. Temperature only had a significant effect on the growth rate of this species when it was raised to 7 °C in all treatments. For F. cylindrus, growth rates only significantly increased with iron addition at 7 °C in both water types. Temperature did not affect the growth rate of F. cylindrus except for a significant reduction without iron addition at 7 °C in coastal water. These results highlight the complex interactions between Fe bioavailability and temperature on Southern Ocean phytoplankton growth. Thus, certain Southern Ocean phytoplankton species may have higher growth rates in regions of the ocean that will warm the most and possibly experience greater Fe supply under future climate conditions, such as coastal regions. This may result in changes in phytoplankton community structures with implications for carbon sequestration efficiency under future climate conditions.

Interrelated influence of iron, light, and CO2 on carbon fixation in a Southern Ocean diatom

AbstractThe productivity of diatoms in the Southern Ocean plays a key role in the transfer of carbon (C) from the atmosphere to the ocean's interior, which impacts climate. However, diatom growth in the Southern Ocean is limited by several environmental factors including iron (Fe) and light. Ongoing increases in ocean CO2 concentrations may increase diatom carbon fixation, but it is uncertain how this will interact with extant Fe and light limitation. Here we grew the Southern Ocean diatom Fragilariopsis cylindrus under a matrix of growth sufficient and limiting Fe and light levels and current and elevated CO2 concentrations, and found that a decrease in Fe concentration at high light, or in light intensity at high Fe, caused a similar 28–35% decrease in growth rate. Combined low Fe and low light caused a much larger (71–75%) decrease in growth rate than occurred with low Fe or low light alone, indicating Fe and light co‐limitation. At a given concentration of bioavailable dissolved inorganic Fe (Fe′), increasing pCO2 from 400 to 750 μatm had no significant effect on growth or C‐fixation rates under any Fe and light conditions. These results suggest that unlike previous measurements in Fe‐ and light‐limited temperate diatoms, increased CO2 should have little effect on C‐fixation rates in Southern Ocean diatoms. The different physiological responses of cold‐water and temperate diatoms to the changing environment warrant further investigation for understanding and predicting changes in the efficiency of the biological carbon pump and the associated potential feedback to the climate change.

The diatom Fragilariopsis cylindrus: A model alga to understand cold-adapted life.

Diatoms are significant primary producers especially in cold, turbulent, and nutrient-rich surface oceans. Hence, they are abundant in polar oceans, but also underpin most of the polar food webs and related biogeochemical cycles. The cold-adapted pennate diatom Fragilariopsis cylindrus is considered a keystone species in polar oceans and sea ice because it can thrive under different environmental conditions if temperatures are low. In this perspective paper, we provide insights into the latest molecular work that has been done on F. cylindrus and discuss its role as a model alga to understand cold-adapted life.

Ice nucleating properties of the sea ice diatom Fragilariopsis cylindrus and its exudates

Abstract. In this study, we investigated the ice nucleation activity of the Antarctic sea ice diatom Fragilariopsis cylindrus. Diatoms are the main primary producers of organic carbon in the Southern Ocean, and the Antarctic sea ice diatom F. cylindrus is one of the predominant species. This psychrophilic diatom is abundant in open waters and within sea ice. It has developed several mechanisms to cope with the extreme conditions of its environment, for example, the production of ice-binding proteins (IBPs) and extracellular polymeric substances known to alter the structure of ice. Here, we investigated the ice nucleation activity of F. cylindrus using a microfluidic device containing individual sub-nanolitre (∼90 µm) droplet samples. The experimental method and a newly implemented Poisson-statistics-based data evaluation procedure applicable to samples with low ice nucleating particle concentrations were validated by comparative ice nucleation experiments with well-investigated bacterial samples from Pseudomonas syringae (Snomax®). The experiments reveal an increase of up to 7.2 ∘C in the ice nucleation temperatures for seawater containing F. cylindrus diatoms when compared to pure seawater. Moreover, F. cylindrus fragments also show ice nucleation activity, while experiments with the F. cylindrus ice-binding protein (fcIBP) show no significant ice nucleation activity. A comparison with experimental results from other diatoms suggests a universal behaviour of polar sea ice diatoms, and we provide a diatom-mass-based parameterization of their ice nucleation activity for use in models.

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.

Winter microalgal communities of the southern Sea of Okhotsk: A comparison of sea ice, coastal, and basinal seawater

The Sea of Okhotsk is a marginal sea of the western North Pacific Ocean and a key region for the seawater circulation of the North Pacific. In winter, a large amount of sea ice forms over the northern shelf and off the eastern Sakhalin coast. The sea ice is transported to the southern part of the sea by the fresh and cold East Sakhalin Current (ESC). Microalgae are the main players in the primary production of the sea ice ecosystems. We investigated the winter microalgal communities of the sea ice ecosystem in the southern Sea of Okhotsk, including the communities living in sea ice and seawater using microscopy and DNA barcoding. The results showed that diatoms were the major component in the microalgal communities. Sea ice algal communities were dominated by Thalassiosira spp., Porosira glacialis, and Fragilariopsis cylindrus, while other typical sea ice pennate diatoms were rarely found. The seawater stations were separated into two groups by hydrological conditions: coastal stations strongly affected by ESC; basinal stations influenced by the deep mixing and the basinal gyre. Small-sized diatoms and naked nanoflagellates were more abundant at the ESC-influenced stations that had higher concentrations of dissolved Fe (DFe) and ammonia (NH4). In particular, spores of Chaetoceros dominated the ESC-influenced seawater. Representative algal taxa specific to the basinal stations, including the diatom Shionodiscus spp., the haptophyte Coccolithus pelagicus, and the dinoflagellate Prorocentrum minimum, were either typical for the basinal gyres or the remnants of the autumn algal blooms. The results suggested that the winter phytoplanktonic communities of the southern Sea of Okhotsk were dependent almost entirely on whether they were within the influence of the ESC. Ice algae were far more abundant than phytoplankton, indicating their major role in the seeding of the spring bloom.

Molecular Insights into Lipoxygenases in Diatoms Based on Structure Prediction: a Pioneering Study on Lipoxygenases Found in Pseudo-nitzschia arenysensis and Fragilariopsis cylindrus.

Diatoms produce a variety of oxylipins which are oxygenated polyunsaturated fatty acids and are involved in chemical defense and intercellular communication, among other roles. Although the chemistry of diatom oxylipins has long been studied, the enzymes involved in their production, in particular lipoxygenase (LOX), which catalyzes the initial reaction of the synthesis, have not been discovered in diatom genomes. Recently, diatom LOXs were found in two species, Pseudo-nitzschia arenysensis (PaLOX) and Fragilariopsis cylindrus (FcLOX); however, the enzymology of these LOXs is largely unknown. In this review article, we discuss the potential functions of the diatom LOXs based on previously reported structures of LOXs derived from various organisms other than diatoms. Since the structures of PaLOX and FcLOX have not yet been solved, we discussed their functions, such as regio- and stereospecificities, on the basis of their structures predicted using a computational tool based on deep learning technology. Both diatom LOXs were predicted to conserve common core domains with relatively wide substrate-binding pockets. The stereo-determinant residues in PaLOX and FcLOX suggest S specificity. We assume that the highly conserved common core domain can be a clue to reveal unidentified lox genes from the accumulated diatom genome information with the aid of high-throughput structure prediction tools and structure-based alignment tools in the near future.

The possible fates of Fragilariopsis cylindrus (polar diatom) cells exposed to prolonged darkness.

At high latitudes, the polar night poses a great challenge to photosynthetic organisms that must survive up to six months without light. Numerous studies have already shed light on the physiological changes involved in the acclimation of microalgae to prolonged darkness and subsequent re-illumination. However, these studies have never considered inter-individual variability because they have mainly been conducted with bulk measurements. On the other hand, such long periods are likely to impact within-population selection processes. In this study, we hypothesized that distinct subpopulations with specific traits may emerge during acclimation of a population of diatoms to darkness. We addressed this hypothesis using flow cytometry (FCM), which allow to individually characterize large numbers of cells. The ecologically dominant polar pennate diatom Fragilariopsis cylindrus was subjected to three dark acclimation (DA) experiments of one, three, and five months duration, during which all cultures showed signs of recovery once light became available again. Our results suggest that darkness survival of F. cylindrus relies on reduction of metabolic activity and consumption of carbon reserves. In addition, FCM allowed us to record three different causes of death, each shared by significant numbers of individuals. The first rendered cells were unable to survive the stress caused by the return to light, probably due to a lack of sufficient photoprotective defenses. The other two were observed in two subpopulations of cells whose physiological state deviated from the original population. The data suggest that starvation and failure to maintain dormancy were the cause of cell mortality in these two subpopulations.

Structural Identification of Metalloproteomes in Marine Diatoms, an Efficient Algae Model in Toxic Metals Bioremediation.

The biosorption of pollutants using microbial organisms has received growing interest in the last decades. Diatoms, the most dominant group of phytoplankton in oceans, are (i) pollution tolerant species, (ii) excellent biological indicators of water quality, and (iii) efficient models in assimilation and detoxification of toxic metal ions. Published research articles connecting proteomics with the capacity of diatoms for toxic metal removal are very limited. In this work, we employed a structural based systematic approach to predict and analyze the metalloproteome of six species of marine diatoms: Thalassiosira pseudonana, Phaeodactylum tricornutum, Fragilariopsis cylindrus, Thalassiosira oceanica, Fistulifera solaris, and Pseudo-nitzschia multistriata. The results indicate that the metalloproteome constitutes a significant proportion (~13%) of the total diatom proteome for all species investigated, and the proteins binding non-essential metals (Cd, Hg, Pb, Cr, As, and Ba) are significantly more than those identified for essential metals (Zn, Cu, Fe, Ca, Mg, Mn, Co, and Ni). These findings are most likely related to the well-known toxic metal tolerance of diatoms. In this study, metalloproteomes that may be involved in metabolic processes and in the mechanisms of bioaccumulation and detoxification of toxic metals of diatoms after exposure to toxic metals were identified and described.

Diploid genomic architecture of Nitzschia inconspicua, an elite biomass production diatom

A near-complete diploid nuclear genome and accompanying circular mitochondrial and chloroplast genomes have been assembled from the elite commercial diatom species Nitzschia inconspicua. The 50 Mbp haploid size of the nuclear genome is nearly double that of model diatom Phaeodactylum tricornutum, but 30% smaller than closer relative Fragilariopsis cylindrus. Diploid assembly, which was facilitated by low levels of allelic heterozygosity (2.7%), included 14 candidate chromosome pairs composed of long, syntenic contigs, covering 93% of the total assembly. Telomeric ends were capped with an unusual 12-mer, G-rich, degenerate repeat sequence. Predicted proteins were highly enriched in strain-specific marker domains associated with cell-surface adhesion, biofilm formation, and raphe system gliding motility. Expanded species-specific families of carbonic anhydrases suggest potential enhancement of carbon concentration efficiency, and duplicated glycolysis and fatty acid synthesis pathways across cytosolic and organellar compartments may enhance peak metabolic output, contributing to competitive success over other organisms in mixed cultures. The N. inconspicua genome delivers a robust new reference for future functional and transcriptomic studies to illuminate the physiology of benthic pennate diatoms and harness their unique adaptations to support commercial algae biomass and bioproduct production.

Rapid changes in spectral composition after darkness influences nitric oxide, glucose and hydrogen peroxide production in the Antarctic diatom Fragilariopsis cylindrus

Ice-associated phototrophic taxa contribute significantly to Antarctic primary production and are crucial to ecosystem stability in the Southern Ocean. The quantity and quality of light required for photosynthesis is the single most influential driver of ice-associated algal communities. While the presence of ice and snow greatly reduces the irradiance reaching the ice-water interface, it is also the spectral quality that influences the phototrophy of ice-associated microalgae communities. Here we test the capability of three electrochemical microsensors to detect photosynthetically derived “stress” metabolites produced by the Antarctic diatom F. cylindrus. Following a period of dark incubation, this photo-physiological response differed with respect to the intensity and spectral quality of light during re-illumination. Exposure to blue light resulted in impairment in photosynthetic efficiency of PSII (Fv/Fm) and resulted in the production of nitric oxide (NO), hydrogen peroxide (H2O2) and glucose exudation. A similar trend in metabolite production was observed when subjected to white light, but not during red or green illumination. These results indicate that rapid exposure to light and variation in spectral composition can cause significant stress that can be quantified using H2O2, NO and glucose microsensors. This metabolic overflow was triggered by the disruption of normal photosynthetic electron flow and it is proposed that the detection of extracellular metabolites can be directly attributed to intracellular activity.

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 diatomFragilariopsis cylindruswas 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 icein 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 stableFv/Fmvalue 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, inF. cylindrusconsistently increased after freezing, especially in the LL treatment (σPSII> 10 nm2PSII–1). Our study is the first to report such a large σPSIIin ice diatoms at low Fe conditions. When the melted sea ice was exposed to high light,Fv/Fmwas 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.

Genome-wide analysis of allele-specific expression of genes in the model diatom Phaeodactylum tricornutum

Recent advances in next generation sequencing technologies have allowed the discovery of widespread autosomal allele-specific expression (aASE) in mammals and plants with potential phenotypic effects. Extensive numbers of genes with allele-specific expression have been described in the diatom Fragilariopsis cylindrus in association with adaptation to external cues, as well as in Fistulifera solaris in the context of natural hybridization. However, the role of aASE and its extent in diatoms remain elusive. In this study, we investigate allele-specific expression in the model diatom Phaeodactylum tricornutum by the re-analysis of previously published whole genome RNA sequencing data and polymorphism calling. We found that 22% of P. tricornutum genes show moderate bias in allelic expression while 1% show nearly complete monoallelic expression. Biallelic expression associates with genes encoding components of protein metabolism while moderately biased genes associate with functions in catabolism and protein transport. We validated candidate genes by pyrosequencing and found that moderate biases in allelic expression were less stable than monoallelically expressed genes that showed consistent bias upon experimental validations at the population level and in subcloning experiments. Our approach provides the basis for the analysis of aASE in P. tricornutum and could be routinely implemented to test for variations in allele expression under different environmental conditions.

Large Diversity in Nitrogen- and Sulfur-Containing Compatible Solute Profiles in Polar and Temperate Diatoms.

Intense bottom-ice algal blooms, often dominated by diatoms, are an important source of food for grazers, organic matter for export during sea ice melt, and dissolved organic carbon. Sea-ice diatoms have a number of adaptations, including accumulation of compatible solutes, that allows them to inhabit this highly variable environment characterized by extremes in temperature, salinity, and light. In addition to protecting them from extreme conditions, these compounds present a labile, nutrient-rich source of organic matter, and include precursors to climate active compounds (e.g., dimethyl sulfide [DMS]), which are likely regulated with environmental change. Here, intracellular concentrations of 45 metabolites were quantified in three sea-ice diatom species and were compared to two temperate diatom species, with a focus on compatible solutes and free amino acid pools. There was a large diversity of metabolite concentrations between diatoms with no clear pattern identifiable for sea-ice species. Concentrations of some compatible solutes (isethionic acid, homarine) approached 1 M in the sea-ice diatoms, Fragilariopsis cylindrus and Navicula cf. perminuta, but not in the larger sea-ice diatom, Nitzschia lecointei or in the temperate diatom species. The differential use of compatible solutes in sea-ice diatoms suggests different adaptive strategies and highlights which small organic compounds may be important in polar biogeochemical cycles.