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.

A decadal perspective on north water microbial eukaryotes as Arctic Ocean sentinels.

The North Water region, between Greenland and Ellesmere Island, with high populations of marine birds and mammals, is an Arctic icon. Due to climate related changes, seasonal patterns in water column primary production are changing but the implications for the planktonic microbial eukaryote communities that support the ecosystem are unknown. Here we report microbial community phenology in samples collected over 12 years (2005-2018) from July to October and analysed using high throughput 18S rRNA V4 amplicon sequencing. Community composition was tied to seasonality with summer communities more variable than distinct October communities. In summer, sentinel pan-Arctic species, including a diatom in the Chaetoceros socialis-gelidus complex and the picochlorophyte Micromonas polaris dominated phytoplankton and were summer specialists. In autumn, uncultured undescribed open water dinoflagellates were favored, and their ubiquity suggests they are sentinels of arctic autumn conditions. Despite the input of nutrients into surface waters, autumn chlorophyll concentrations remained low, refuting projected scenarios that longer ice-free seasons are synonymous with high autumn production and a diatom dominated bloom. Overall, the summer sentinel microbial taxa are persisting, and a subset oceanic dinoflagellate should be monitored for possible ecosystem shifts as later autumn ice formation becomes prevalent elsewhere.

PhaeoNet: A Holistic RNAseq-Based Portrait of Transcriptional Coordination in the Model Diatom Phaeodactylum tricornutum.

Transcriptional coordination is a fundamental component of prokaryotic and eukaryotic cell biology, underpinning the cell cycle, physiological transitions, and facilitating holistic responses to environmental stress, but its overall dynamics in eukaryotic algae remain poorly understood. Better understanding of transcriptional partitioning may provide key insights into the primary metabolism pathways of eukaryotic algae, which frequently depend on intricate metabolic associations between the chloroplasts and mitochondria that are not found in plants. Here, we exploit 187 publically available RNAseq datasets generated under varying nitrogen, iron and phosphate growth conditions to understand the co-regulatory principles underpinning transcription in the model diatom Phaeodactylum tricornutum. Using WGCNA (Weighted Gene Correlation Network Analysis), we identify 28 merged modules of co-expressed genes in the P. tricornutum genome, which show high connectivity and correlate well with previous microarray-based surveys of gene co-regulation in this species. We use combined functional, subcellular localization and evolutionary annotations to reveal the fundamental principles underpinning the transcriptional co-regulation of genes implicated in P. tricornutum chloroplast and mitochondrial metabolism, as well as the functions of diverse transcription factors underpinning this co-regulation. The resource is publically available as PhaeoNet, an advanced tool to understand diatom gene co-regulation.

Need for focus on microbial species following ice melt and changing freshwater regimes in a Janus Arctic Gateway.

Oceanic gateways are sensitive to climate driven processes. By connecting oceans, they have a global influence on marine biological production and biogeochemical cycles. The furthest north of these gateways is Nares Strait at the top of the North Water between Greenland and Ellesmere Island (Canada). This gateway is globally beneficial, first by supporting high local mammal and bird populations and second with the outflow of phosphate-rich Arctic waters fueling the North Atlantic spring bloom. Both sides of the North Water are hydrologically distinct with counter currents that make this Arctic portal a Janus gateway, after Janus, the Roman god of duality. We examined oceanographic properties and differences in phytoplankton and other protist communities from the eastern and western sides of the North Water (latitude 76.5°N) and found that species differed markedly due to salinity stratification regimes and local hydrography. Typical Arctic communities were associated with south flowing currents along the Canadian side, while potentially noxious Pseudo-nitzschia spp. were dominant on the Greenland side and associated with greater surface freshening from ice melt. This susceptibility of the Greenland side to Pseudo-nitzschia spp. blooms suggest that monitoring species responses to climate mediated changes is needed.

Seasonal patterns in Arctic prasinophytes and inferred ecology of Bathycoccus unveiled in an Arctic winter metagenome.

Prasinophytes occur in all oceans but rarely dominate phytoplankton populations. In contrast, a single ecotype of the prasinophyte Micromonas is frequently the most abundant photosynthetic taxon reported in the Arctic from summer through autumn. However, seasonal dynamics of prasinophytes outside of this period are little known. To address this, we analyzed high-throughput V4 18S rRNA amplicon data collected from November to July in the Amundsen Gulf Region, Beaufort Sea, Arctic. Surprisingly during polar sunset in November and December, we found a high proportion of reads from both DNA and RNA belonging to another prasinophyte, Bathycoccus. We then analyzed a metagenome from a December sample and the resulting Bathycoccus metagenome assembled genome (MAG) covered ~90% of the Bathycoccus Ban7 reference genome. In contrast, only ~20% of a reference Micromonas genome was found in the metagenome. Our phylogenetic analysis of marker genes placed the Arctic Bathycoccus in the B1 coastal clade. In addition, substitution rates of 129 coding DNA sequences were ~1.6% divergent between the Arctic MAG and coastal Chilean upwelling MAGs and 17.3% between it and a South East Atlantic open ocean MAG in the B2 Clade. The metagenomic analysis also revealed a winter viral community highly skewed toward viruses targeting Micromonas, with a much lower diversity of viruses targeting Bathycoccus. Overall a combination of Micromonas being relatively less able to maintain activity under dark winter conditions and viral suppression of Micromonas may have contributed to the success of Bathycoccus in the Amundsen Gulf during winter.

Establishment of primary cell culture from the temperate symbiotic cnidarian, Anemonia viridis.

The temperate symbiotic sea anemone Anemonia viridis, a member of the Cnidaria phylum, is a relevant experimental model to investigate the molecular and cellular events involved in the preservation or in the rupture of the symbiosis between the animal cells and their symbiotic microalgae, commonly named zooxanthellae. In order to increase research tools for this model, we developed a primary culture from A. viridis animal cells. By adapting enzymatic dissociation protocols, we isolated animal host cells from a whole tentacle in regeneration state. Each plating resulted in a heterogeneous primary culture consisted of free zooxanthellae and many regular, small rounded and adherent cells (of 3-5 µm diameter). Molecular analyses conducted on primary cultures, maintained for 2 weeks, confirmed a specific signature of A. viridis cells. Further serial dilutions and micromanipulation allowed us to obtain homogenous primary cultures of the small rounded cells, corresponding to A. viridis "epithelial-like cells". The maintenance and the propagation over a 4 weeks period of primary cells provide, for in vitro cnidarian studies, a preliminary step for further investigations on cnidarian cellular pathways notably in regard to symbiosis interactions.