Combinedin vivoandin situgenome-resolved metagenomics reveals novel nitrogen fixing interactions between non-cyanobacterial diazotrophs and microalgae

Abstract

Non-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. Here, we report an unprecedented finding in the widely used model diatom Phaeodactylum triconrnutum (Pt) of NCDs sustaining diatom cells in the absence of bioavailable nitrogen. We identified PtNCDs using metagenomics sequencing and detected nitrogenase gene in silico and/or by PCR. We demonstrated nitrogen fixation in PtNCDs and their close genetic affiliation with NCDs from the environment. We showed the wide occurrence of this type of symbiosis with the isolation of NCDs from other microalgae and their identification in the environment and in co-occurrence with photosynthetic microalgae. Overall, this study provides evidence for a previously overlooked symbiosis using a multidisciplinary model-based approach which will consequently help understand the different players driving global marine nitrogen fixation.

Editorial: Algal symbiotic relationships in freshwater and marine environments

“…Another fundamental boost is undoubtedly the existence of established model organisms that open up novel avenues of investigations, which would otherwise be impossible or at least difficult to achieve. A recent study that used both metagenomics sequencing and a model species, the diatom Phaeodactylum tricornutum , successfully identified an overlooked symbiosis between microalgae and non-cyanobacteria diazotrophs (NCDs) which challenges the long-held paradigm of dominance of cyanobacteria interactions with microalgae over NCDs and brings the first hints on how heterotrophic proteobacteria thrive in surface waters and oxygenated areas ( Chandola et al., 2022 ).…”