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

Abstract

A diagnostic model based on biomass and growth was used to assess the relative contributions of filamentous nonheterocystous Trichodesmium and unicellular cyanobacteria, termed Groups A and B, to nitrogen fixation at the North Pacific Station ALOHA over a 2-year period. Average (and 95% confidence interval, CI) annual rates of modeled monthly values for Trichodesmium, Group B and Group A were 92 (52), 14 (4) and 12 (8) mmol N per m 2 per year, respectively. The fractional contribution to modeled instantaneous nitrogen fixation by each diazotroph fluctuated on interannual, seasonal and shorter time scales. Trichodesmium fixed substantially more nitrogen in year 1 (162) than year 2 (12). Group B fixed almost two times more nitrogen in year 1 (17) than year 2 (9). In contrast, Group A fixed two times more nitrogen in year 2 (16) than year 1 (8). When including uncertainties in our estimates using the bootstrap approach, the range of unicellular nitrogen fixation extended from 10% to 68% of the total annual rate of nitrogen fixation for all three diazotrophs. Furthermore, on a seasonal basis, the model demonstrated that unicellular diazotrophs fixed the majority (51%-97%) of nitrogen during winter and spring, whereas Trichodesmium dominated nitrogen fixation during summer and autumn (60%-96%). Sensitivity of the modeled rates to some parameters suggests that this unique attempt to quantify relative rates of nitrogen fixation by different diazotrophs may need to be reevaluated as additional information on cell size, variability in biomass and C:N, and growth characteristics of the different cyanobacterial diazotrophs become available.

Colimitation of the unicellular photosynthetic diazotroph Crocosphaera watsonii by phosphorus, light, and carbon dioxide

“…For instance, nonoxygenic diazotrophs (e.g., Candidatus Atelocyanobacterium thalassa; Thompson et al 2012) do not fix CO 2 , and the primary benefit of high P CO 2 seems to be associated CO 2 fixation, whereas N 2 fixation likely receives secondary benefits (Law et al 2012). Thus, assuming that oceanic P CO 2 will increase within the next 100 yr, N 2 fixation by unicellular oxygenic diazotrophs will likely be affected by future concentrations of P CO 2 , mostly in areas where they reach high densities, which are known to include tropical regions of the Atlantic Ocean (Langlois et al 2008) and parts of the Pacific Ocean near northern Australia and Hawaii (Montoya et al 2004;Church et al 2005;Goebel et al 2007).…”

Active nitrogen-fixing heterotrophic bacteria at and below the chemocline of the central Baltic Sea

“…The Baltic Sea is among the world's largest brackish seas and regularly experiences extensive blooms of diazotrophic heterocystous cyanobacteria (Stal et al, 2003) providing N-input almost as large as the riverine load (Larsson et al, 2001). Cyanobacterial N 2 fixation is also significant in open oceans (Goebel et al, 2007), but molecular techniques targeting the nifH gene of the nitrogenase enzyme have revealed that heterotrophic diazotrophs are also present and nifH transcribed in diverse marine and estuarine environments (reviewed in Riemann et al, 2010). However, their significance to N 2 fixation and the factors controlling their activity and distribution patterns are not yet understood.…”

Phylogenetic diversity and spatio-temporal ­distribution of nitrogenase genes (nifH) in the northern South China Sea

“…More recently, unicellular diazotrophic cyanobacteria, named group A (UCYN-A), group B (UCYN-B) and group C (UCYN-C), have been discovered on the basis of molecular techniques (Zehr et al 2001, Langlois et al 2005, Foster et al 2007, and their contribution to total nitrogen fixation was estimated to be equal to, or even greater than, that of Trichodesmium spp. (Falcón et al 2004, Goebel et al 2007.…”

“…More recently, unicellular diazotrophic cyanobacteria, named group A (UCYN-A), group B (UCYN-B) and group C (UCYN-C), have been discovered on the basis of molecular techniques (Zehr et al 2001, Langlois et al 2005, Foster et al 2007, and their contribution to total nitrogen fixation was estimated to be equal to, or even greater than, that of Trichodesmium spp. (Falcón et al 2004, Goebel et al 2007.Although most cyanobacterial diazotrophic groups can be detected simultaneously in the marine environment (Church et al 2005a, Foster et al 2007, Fong et al 2008, different diazotrophic groups have shown different patterns of distribution in the ocean (Riemann et al 2010). Trichodesmium spp.…”