Resting cell formation in the marine diatom Thalassiosira pseudonana

Non-cyanobacterial diazotrophs support the survival of marine microalgae in nitrogen-depleted environment

BACKGROUND

Non-cyanobacteria diazotrophs (NCDs) are shown to dominate in surface waters shifting the long-held paradigm of cyanobacteria dominance. This raises 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.

RESULTS

Here, via a multidisciplinary approach, we identify the presence of NCDs within the phycosphere of the model diatom Phaeodactylum tricornutum (Pt), which sustain the survival of Pt in nitrogen-depleted conditions. Through bacterial metacommunity sequencing and genome assembly, we identify multiple NCDs belonging to the Rhizobiales order, including Bradyrhizobium, Mesorhizobium, Georhizobium, and Methylobacterium. We demonstrate the nitrogen-fixing ability of PtNCDs through in silico identification of nitrogen fixation genes and by other experimental assays. We show 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.

CONCLUSIONS

Our study underscores the importance of microalgae interactions with NCDs to 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.

Broad active metabolic pathways, autophagy, and antagonistic hormones regulate dinoflagellate cyst dormancy in marine sediments

This work aimed to reveal the molecular machinery regulating the dormancy of dinoflagellate resting cysts buried in marine sediments. Dinoflagellates play pivotal roles in marine ecosystems, particularly as major contributors of harmful algal blooms. Despite vital roles of cysts in blooming cycles and dinoflagellate ecology, the molecular processes controlling cyst dormancy have largely remained unexplored due to technological difficulties. Using DinoSL as a dinoflagellates-specific mRNA "hook" and SMRT sequencing, we analyzed metatranscriptomes of sediment-buried dinoflagellate cyst assemblages. The data show that most major metabolic and regulatory pathways, except photosynthesis, were transcriptionally active. This suggests the crucial importance of broad metabolic pathways in sustaining cyst viability and germination potential. Further expression analyses of 11 genes (relevant to autophagy and phytohormone gibberellin), lysosome/autolysosome staining, and germination experiments revealed vital roles of autophagy in energy generation, nutrient recycling, and of phytohormones abscisic acid/gibberellin in modulating dormancy/germination of resting cysts. Our findings lay a cornerstone for elucidating the molecular machinery regulating dinoflagellate cyst dormancy.

Some fall to sleep slowly: cell biophysics and metabolism of quiescence in diatom resting cells

This article is a Commentary on Wang et al. (2024), 243: 1347–1360.

Resting cells of Skeletonema marinoi assimilate organic compounds and respire by dissimilatory nitrate reduction to ammonium in dark, anoxic conditions

Diatoms can survive long periods in dark, anoxic sediments by forming resting spores or resting cells. These have been considered dormant until recently when resting cells of Skeletonema marinoi were shown to assimilate nitrate and ammonium from the ambient environment in dark, anoxic conditions. Here, we show that resting cells of S. marinoi can also perform dissimilatory nitrate reduction to ammonium (DNRA), in dark, anoxic conditions. Transmission electron microscope analyses showed that chloroplasts were compacted, and few large mitochondria had visible cristae within resting cells. Using secondary ion mass spectrometry and isotope ratio mass spectrometry combined with stable isotopic tracers, we measured assimilatory and dissimilatory processes carried out by resting cells of S. marinoi under dark, anoxic conditions. Nitrate was both respired by DNRA and assimilated into biomass by resting cells. Cells assimilated nitrogen from urea and carbon from acetate, both of which are sources of dissolved organic matter produced in sediments. Carbon and nitrogen assimilation rates corresponded to turnover rates of cellular carbon and nitrogen content ranging between 469 and 10,000 years. Hence, diatom resting cells can sustain their cells in dark, anoxic sediments by slowly assimilating and respiring substrates from the ambient environment.