Lacour, T. | bioRxiv Science

bioRxiv2023

Hypometabolism to survive the long polar night in the diatom Fragilariopsis cylindrus

Diatoms, the major eukaryotic phytoplankton in polar regions, are essential to sustain Arctic and Antarctic ecosystems. As such, it is fundamental to understand the physiological mechanisms and associated molecular basis of their resilience to the long polar night. Here, we report an integrative approach revealing that in prolonged darkness, diatom cells enter a state of quiescence associated 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. Our results further indicate that immediately following a return to light, diatom cells are able to use photoprotective mechanisms and rapidly resume photosynthesis. Cell division resumed rates similar to those before darkness. Our study demonstrates the remarkable robustness of polar diatoms to prolonged darkness at low temperatures. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=160 SRC="FIGDIR/small/524047v1_ufig1.gif" ALT="Figure 1"> View larger version (55K): org.highwire.dtl.DTLVardef@1da578corg.highwire.dtl.DTLVardef@1a9ac5borg.highwire.dtl.DTLVardef@729265org.highwire.dtl.DTLVardef@1e94c18_HPS_FORMAT_FIGEXP M_FIG C_FIG TeaserTo survive the long winter, polar diatoms slow down metabolism and express genes to assure survival following return to light.

ecology↗

bioRxiv2022

Combined in vivo and in situ genome-resolved metagenomics reveals novel symbiotic nitrogen fixing interactions between non-cyanobacterial diazotrophs and microalgae

BackgroundNon-cyanobacteria diazotrophs (NCDs) were shown to dominate in surface waters shifting the long-held paradigm of cyanobacteria dominance and raising fundamental questions on how these putative heterotrophic bacteria thrive in sunlit oceans. The absence of laboratory cultures of these bacteria significantly limits our ability to understand their behavior in natural environments and, consequently, their contribution to the marine nitrogen cycle. ResultsHere, we used a multidisciplinary approach and report an unprecedented finding in the diatom Phaeodactylum tricornutum (Pt) of NCDs in the phycosphere or the pelagic community sustaining its survival in the absence of bioavailable nitrogen. We sequenced the bacterial metacommunity associated with Pt and assembled several bacterial genomes, identifying multiple NCDs from the Rhizobiales order, including Bradyrhizobium, Mesorhizobium, Georhizobium and Methylobacterium. We demonstrated the nitrogen-fixing ability of PtNCDs through in silico identification of nitrogen fixation genes, or by using PCR, acetylene reduction, or 15N incorporation. We showed the wide occurrence of this type of interactions with the isolation of NCDs from other microalgae, their identification in the environment, and their predicted associations with photosynthetic microalgae. ConclusionsOur study underscores the importance of microalgae interactions with NCDs to permit and support nitrogen fixation. This work provides a unique model Pt-NCDs to study the ecology of this interaction advancing our understanding of the key drivers of global marine nitrogen fixation.

microbiology↗