The fate of nitrogen fixed by diazotrophs in the ocean

Abstract

Abstract. While we now know that N 2 fixation is a significant source of new nitrogen (N) in the marine environment, little is known about the fate of this N (and associated C), despite the importance of diazotrophs to global carbon and nutrient cycles. Specifically, does N fixed during N 2 fixation fuel autotrophic or heterotrophic growth and thus facilitate carbon (C) export from the euphotic zone, or does it contribute primarily to bacterial productivity and respiration in the euphotic zone? For Trichodesmium, the diazotroph we know the most about, the transfer of recently fixed N 2 (and C) appears to be primarily through dissolved pools. The release of N varies among and within populations and as a result of the changing physiological state of cells and populations. The net result of trophic transfers appears to depend on the co-occurring organisms and the complexity of the colonizing community. In order to understand the impact of diazotrophy on carbon flow and export in marine systems, we need a better understanding of the trophic flow of elements in Trichodesmium-dominated communities and other diazotrophic communities under various defined physiological states. Nitrogen and carbon fixation rates themselves vary by orders of magnitude within and among studies of Trichodesmium, highlighting the difficulty in extrapolating global rates of N 2 fixation from direct measurements. Because the stoichiometry of N 2 and C fixation does not appear to be in balance with that of particles, and the relationship between C and N 2 fixation rates is also variable, it is equally difficult to derive global rates of one from the other. This paper seeks to synthesize what is known about the fate of diazotrophic production in the environment. A better understanding of the physiology and physiological ecology of Trichodesmium and other marine diazotrophs is necessary to quantify and predict the effects of increased or decreased diazotrophy in the context of the carbon cycle and global change.

High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation

“…N 2 fixed by diverse diazotrophic taxa may have different fates within the marine environment (Glibert and Bronk 1994;Mulholland, 2007;Foster et al, 2011;Karl et al, 2012;, and therefore characterisation of the composition of active N 2 -fixing assemblages, combined with size-fractionated N 2 fixation rates, is necessary to determine the differential contribution of newly fixed N to pelagic ecosystems. Here we report changes in the biogeographical distribution and activity of diazotrophs across a broad tropical region, which has been identified as a potential global 'hotspot' for marine N 2 fixation (Montoya et al, 2004;Monteiro et al, 2010;Luo et al, 2014).…”

Programmed cell death in the marine cyanobacterium Trichodesmium mediates carbon and nitrogen export

“…Bloom crashes may result from viral lysis (Hewson et al, 2004) or autocatalytic programmed cell death (PCD), which is induced by nutrient (iron (Fe) starvation) or high light (oxidative) stress in both laboratory and natural populations (Berman-Frank et al, 2004, 2007. PCD has indeed been well documented in a variety of diverse phytoplankton lineages including bloom formers like Trichodesmium (Berman-Frank et al, 2004;Bidle and Falkowski, 2004), implicating it as an important feature of algal ecophysiology.…”

“…Our previous findings indicate that PCD in Trichodesmium may trigger rapid sinking due to concomitant internal cellular degradation, vacuole loss and increased production of extracellular polysaccharide aggregates, operationally defined as transparent exopolymeric particles (TEPs) (Berman-Frank et al, 2004, 2007. In the present study, we used a customized, experimental water column to specifically test and quantify for the first time whether PCD-induced-bloom demise of Trichodesmium mechanistically regulates increased vertical fluxes of C and N, thereby facilitating its rapid export from surface waters to depth.…”

“…Indeed, the fate and quantitative contribution of Trichodesmium blooms to the vertical export or recycling of newly fixed nitrogen (N) and carbon (C) in the ocean following bloom collapse is poorly studied (Mulholland, 2007;Bergman et al, 2012), despite its global-scale importance Tg N per year and 2.1-18 Tg C per year) (Luo et al, 2012). Trichodesmium may release up to 50-80% of recently fixed N 2 in natural communities and cultures as dissolved organic N and ammonium (NH 4 þ ) (Mulholland, 2007).…”

“…Trichodesmium may release up to 50-80% of recently fixed N 2 in natural communities and cultures as dissolved organic N and ammonium (NH 4 þ ) (Mulholland, 2007). Low d 15 N of exported material collected from the HOTS and BATS time series stations indicates that recently fixed N is transferred out of the euphotic zone (Karl et al, 2002), although Trichodesmium is rarely recovered in sediment traps and is only grazed by three representatives of the Miraciidae family of pelagic harpacticoid copepods, mostly by Macrosetella gracilis (O'Neil, 1998).…”