Mechanistic understanding of diazotroph aggregation and sinking: "A rolling tank approach"

Diazotrophs are ubiquitous in the surface (sub)tropical ocean, where they sustain most new primary production. Several recent studies also report the presence of diverse cyanobacterial diazotrophs in the mesopelagic and bathypelagic ocean, suggesting that they gravitationally sink, potentially supporting organic matter export and the biological carbon pump. Yet, the mechanisms leading to their export are not elucidated. Here, we simulated the sinking of diazotrophs in the water column by using rolling tanks, and measured the aggregation capacity and sinking velocities of four globally distributed strains having different sizes, shapes, and abilities to produce transparent exopolymer particles (TEP): two filamentous diazotrophs (Trichodesmium erythraeum, Calothrix sp.) and two unicellular cyanobacterial diazotrophs (UCYN-B, Crocosphaera watsonii) and (UCYN-C, Cyanothece sp.). All diazotrophs tested, regardless their size and shape, were capable of forming aggregates and sunk, albeit at different velocities depending on the aggregation capacity. Overall, UCYN formed aggregates as large as those formed by the filamentous diazotrophs (7000-32,014 mu m ESD, equivalent spherical diameter), and sunk at 100-400 m d(-1), i.e., at the same velocity as filamentous diazotrophs (92-400 m d(-1)). Although TEP are generally considered as enhancers of aggregation, TEP did not clearly influence aggregation rates nor sinking velocities during our study. We conclude that diazotrophs may be important contributors to carbon export in the ocean and need to be considered in future studies to improve the accuracy of current regional and global estimates of export.