Abstract
Sinking particles transport carbon and nutrients from the surface ocean into the deep sea and are considered hot spots for bacterial diversity and activity. In the oligotrophic oceans, nitrogen (N2)-fixing organisms (diazotrophs) are an important source of new N but the extent to which these organisms are present and exported on sinking particles is not well known. Sinking particles were collected every 6 h over a 2-day period using net traps deployed at 150 m in the North Pacific Subtropical Gyre. The bacterial community and composition of diazotrophs associated with individual and bulk sinking particles was assessed using 16S rRNA and nifH gene amplicon sequencing. The bacterial community composition in bulk particles remained remarkably consistent throughout time and space while large variations of individually picked particles were observed. This difference suggests that unique biogeochemical conditions within individual particles may offer distinct ecological niches for specialized bacterial taxa. Compared to surrounding seawater, particle samples were enriched in different size classes of globally significant N2-fixing cyanobacteria including Trichodesmium, symbionts of diatoms, and the unicellular cyanobacteria Crocosphaera and UCYN-A. The particles also contained nifH gene sequences of diverse non-cyanobacterial diazotrophs suggesting that particles could be loci for N2 fixation by heterotrophic bacteria. The results demonstrate that diverse diazotrophs were present on particles and that new N may thereby be directly exported from surface waters on sinking particles.
Highlights
Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Marine environments exhibit microscale gradients of organic carbon and nutrients, representing diverse microniches for microorganisms to exploit
Particulate organic matter (POM) is ubiquitous in seawater and composed of a mixture of living microorganisms and dead detrital matter, providing ‘hotspots’ for microbial activity and organic matter remineralization that significantly contribute to biogeochemical element cycling rates [1]
We investigate the bacterial composition and the associated populations of diazotrophs on both individually picked and bulk samples of sinking particles in the North Pacific Subtropical Gyre (NPSG), and compare the community composition with that of the surrounding seawater
Summary
Marine environments exhibit microscale gradients of organic carbon and nutrients, representing diverse microniches for microorganisms to exploit. Sinking particles transport organic and inorganic material into the deep ocean, a process known as the ocean’s biological pump [2]. Because of their importance, sinking particles and the composition and function of their associated community is receiving increasing interest [3,4,5]. The representation of picoplankton in sinking particles has generally been thought to be negligible due to their small size, slow sinking velocity and tight regulation through grazing [6]. In recent years the possibility of export of small picoplankton through incorporation into aggregates or fecal pellets has been increasingly recognized [7, 8] and has been proposed to be in proportion to their total net primary production [9, 10]
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