Turk-Kubo, K. A. | Search bioRxiv

bioRxiv2023

A novel, N2-fixing cyanobacterium present and active in the global oceans

Marine N2-fixing cyanobacteria, including the unicellular genus Crocosphaera, are considered keystone species in marine food webs. Crocosphaera are globally distributed and provide new sources of nitrogen (N) and carbon (C), which fuel oligotrophic microbial communities and upper trophic levels. Despite their ecosystem importance, only one species, Crocosphaera watsonii, has ever been identified and characterized as widespread in the oligotrophic oceans. Herein, we present a novel species, candidatus Crocosphaera waterburyi (C. waterburyi hereafter), enriched from the North Pacific Ocean, active in situ, and globally distributed in environmental datasets. C. waterburyi is morphologically, phylogenetically, and physiologically distinct from C. watsonii; therefore, description of this novel species provides a new window into previously uncharacterized diversity and ecology of unicellular N2-fixing cyanobacterial taxa and further highlights their importance in the global N cycle.

genomics↗

bioRxiv2021

Nitrogen fixation in mesoscale eddies of the North Pacific Subtropical Gyre: patterns and mechanisms

1.Mesoscale eddies have been shown to support elevated dinitrogen (N2) fixation rates (NFRs) and abundances of N2-fixing microorganisms (diazotrophs), but the mechanisms underlying these observations are not well understood. We explored relationships among NFRs and cyanobacterial diazotroph abundances in eddy pairs of opposite polarity sampled in the North Pacific Subtropical Gyre and compared our observations to seasonal trends from the Hawaii Ocean Time-series (HOT) program. Consistent with previous reports, we found that NFRs were anomalously high for this region (up to 3.7-fold above previous monthly HOT observations) in the centers of the sampled anticyclones, coinciding with elevated abundances of Crocosphaera in the summertime. We then coupled our field-based observations, together with transcriptomic analyses of nutrient stress marker genes and ecological models, to evaluate potential mechanisms controlling diazotroph abundance and activity at the mesoscale. Specifically, we evaluated the role of biological (via estimates of growth and grazing rates) and physical controls on populations of Crocosphaera, Trichodesmium, and diatom symbionts. Our results suggest that increased Crocosphaera abundances associated with summertime anticyclones resulted from the alleviation of phosphate limitation, allowing cells to grow at rates exceeding grazing losses. In contrast, distributions of larger, buoyant taxa (Trichodesmium and diatom symbionts) appeared less affected by eddy-driven biological controls. Instead, they appeared driven by physical dynamics along frontal boundaries that separate cyclonic and anticyclonic eddies. Together, the interplay of eddy-specific changes in bottom-up control, top-down control, and the physical accumulation of cells likely explains the elevated diazotroph abundances and NFRs associated with anticyclones and eddy fronts.

ecology↗