TRANSPARENT EXOPOLYMER PARTICLES AS PLANKTONIC “HOTSPOTS” FOR HETEROTROPHIC N₂ FIXATION

Biological dinitrogen (N₂) fixation is a key biogeochemical process in marine and freshwater environments, performed solely by specialized bacteria and archaeal species termed diazotrophs. N₂ fixation is considered to be (i) energetically expensive, (ii) oxygen-sensitive, and (iii) potentially hindered by the availability of dissolved inorganic nitrogen. Here, we propose a novel paradigm where heterotrophic N₂ fixation is linked with planktonic bio-aggregates in the form of transparent exopolymer particles (TEP). Dedicated bottle-microcosms with water from the coastal eastern Mediterranean Sea (nutrients-poor) and the Qishon estuary (nutrients-rich) were supplemented with mono and polysaccharides as well as inorganic nitrogen and phosphorous. Our results indicate that the diazotrophic community expressing nifH and actively fix nitrogen was ascribed primarily to heterotrophic Proteobacteria. N₂ fixation and heterotrophic bacterial activity increased up-to tenfold once seawater was supplemented with organic carbon substrate in the form of glucose (monosaccharides) or gum-xanthan (polysaccharide surrogate). We suggest that these carbon-rich polysaccharides in the form of TEP enhance heterotrophic N₂ fixation, by forming microenvironments of intense metabolic activity, high carbon: nitrogen ratio, and possibly low O₂ levels. The conclusions of this study indicate that diazotrophic activity is prompt by carbon-rich substrates in the eastern Mediterranean Sea and the Qishon estuary as possibly in many other marine and freshwater environments.