Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean

Abstract

Fixed nitrogen (N) often limits the growth of organisms in terrestrial and aquatic biomes, and N availability has been important in controlling the CO2 balance of modern and ancient oceans. The fixation of atmospheric dinitrogen gas (N2) to ammonia is catalysed by nitrogenase and provides a fixed N for N-limited environments. The filamentous cyanobacterium Trichodesmium has been assumed to be the predominant oceanic N2-fixing microorganism since the discovery of N2 fixation in Trichodesmium in 1961 (ref. 6). Attention has recently focused on oceanic N2 fixation because nitrogen availability is generally limiting in many oceans, and attempts to constrain the global atmosphere-ocean fluxes of CO2 are based on basin-scale N balances. Biogeochemical studies and models have suggested that total N2-fixation rates may be substantially greater than previously believed but cannot be reconciled with observed Trichodesmium abundances. It is curious that there are so few known N2-fixing microorganisms in oligotrophic oceans when it is clearly ecologically advantageous. Here we show that there are unicellular cyanobacteria in the open ocean that are expressing nitrogenase, and are abundant enough to potentially have a significant role in N dynamics.

Distribution of microbial terpenoid lipid cyclases in the global ocean metagenome

“…In contrast, although the absence of cyanobacterial SHC sequences in the GOS samples does not mean that cyanobacterial producers of hopanoids are absent from the presentday ocean (they surely are present, for example, Crocosphaera watsonii and Trichodesmium erythraeum; Capone et al, 1997;Zehr et al, 2001Zehr et al, , 2007, their proportional density among total hopanoid producers appears to be low.…”

Modeled contributions of three types of diazotrophs to nitrogen fixation at Station ALOHA

“…High abundances and estimated rates of N 2 fixation of unicellular cyanobacterial N 2 fixers in the North Pacific (Zehr et al, 2001;Montoya et al, 2004) suggest that diazotrophs other than Trichodesmium potentially play a significant role in the nitrogen cycle at the North Pacific Station ALOHA, and in tropical and subtropical waters worldwide. This paper describes a diagnostic model to potentially translate abundance measurements into estimates of N 2 fixation for both Trichodesmium and unicellular organisms for comparison of rates among different diazotrophs, and includes sensitivity analyses to parameter choices.…”

“…These relatively large microorganisms can form colonies that are visible to the unaided eye. More recently, the potential of unicellular cyanobacteria to contribute substantially to N 2 fixation in tropical North Atlantic and the subtropical North Pacific has been attributed to their high abundances and estimated rates of N 2 fixation (Zehr et al, 2001Falcon et al, 2002Falcon et al, , 2004Montoya et al, 2004). High abundances of these single-celled microorganisms should make it easier to measure their rates of N 2 fixation, compared to measurements on the less abundant, colony-forming Trichodesmium cells.…”

Bioavailability of iron to Trichodesmium colonies in the western subtropical Atlantic Ocean

“…It has been estimated that these Trichodesmium blooms are responsible for approximately one fourth of the total N, fixation in the ocean (Rueter et al 1990) and that this organism is the largest source of new N in the surface waters of the north Atlantic Ocean (Lipschultz et al 2002). Recent evidence, however, indicates that unicellular diazotrophs make a large contribution in some regions as well, especially in the oligotrophic Pacific (Zehr et al 2001). The regions in the North Atlantic with the greatest seasonal flux of atmospheric Fe dust tend to have an excess of nitrate compared to phosphate, which is presumed to be the result of diazotrophy (Michaels et al 1996).…”