Nitrogenase (nifH) gene expression in diazotrophic cyanobacteria in the Tropical North Atlantic in response to nutrient amendments

Abstract

The Tropical North Atlantic (TNAtl) plays a critical role in the marine nitrogen cycle, as it supports high rates of biological nitrogen (N 2 ) fixation, yet it is unclear whether this process is limited by the availability of iron (Fe), phosphate (P) or is co-limited by both. In order to investigate the impact of nutrient limitation on the N 2 -fixing microorganisms (diazotrophs) in the TNAtl, trace metal clean nutrient amendment experiments were conducted, and the expression of nitrogenase (nifH) in cyanobacterial diazotrophs in response to the addition of Fe, P, or Fe+P was measured using quantitative PCR. To provide context, N 2 fixation rates associated with the <10 µm community and diel nifH expression in natural cyanobacterial populations were measured. In the western TNAtl, nifH expression in Crocosphaera, Trichodesmium, and Richelia was stimulated by Fe and Fe+P additions, but not by P, implying that diazotrophs may be Fe-limited in this region. In the eastern TNAtl, nifH expression in unicellular cyanobacteria UCYN-A and Crocosphaera was stimulated by P, implying P-limitation. In equatorial waters, nifH expression in Trichodesmium was highest in Fe+P treatments, implying co-limitation in this region. Nutrient additions did not measurably stimulate N 2 fixation rates in the <10 µm fraction in most of the experiments, even when upregulation of nifH expression was evident. These results demonstrate the utility of using gene expression to investigate the physiological state of natural populations of microorganisms, while underscoring the complexity of nutrient limitation on diazotrophy, and providing evidence that diazotroph populations are slow to respond to the addition of limiting nutrients and may be limited by different nutrients on basin-wide spatial scales. This has important implications for our current understanding of controls on N 2 fixation in the TNAtl and may partially explain why it appears to be intermittently limited by Fe, P, or both.

Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume

“…The riverine fixed N concentration is high enough in low-salinity waters to negate the advantage of N 2 fixation (Subramaniam et al, 2008), and thus fewer diazotrophs are found in these waters. Furthest from the Amazon River influence, Trichodesmium is the dominant diazotroph in the more oceanic environment, as has been observed previously (Foster et al, 2007;Turk-Kubo et al, 2012). In transitional waters between the river input and open ocean, enough fixed N has been assimilated by the community, but riverine P, Fe and Si are still in sufficiently high concentrations to create ideal conditions for diazotrophs, especially those in association with diatoms (Yeung et al, 2012;Goes et al, 2014).…”

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

“…Conversely, phosphate availability is known to directly influence N 2 fixation and nifH expression in natural populations of diazotrophs (Sañudo-Wilhelmy et al, 2001;Rees et al, 2006;Turk-Kubo et al, 2012), as well as the oceanic distribution of diazotrophs in general (Sohm et al, 2011). In the present study, phosphate concentrations were relatively high (for example, 0.27 μmol l − 1 and 0.26 μmol l − 1 ) in the ATS where Trichodesmiumand δ-proteobacterial-dominated communities were observed.…”

“…Therefore, other factors may have had a significant influence on diazotroph distribution and activity, for instance, dissolved iron (dFe) availability is known to limit marine N 2 fixation (see Sohm et al, 2011) and dFe additions can stimulate diazotrophic activity (Moisander et al, 2012;Turk-Kubo et al, 2012). In the western South Pacific ocean, Moisander et al, (2012) demonstrated that the abundances of unicellular diazotrophs, including UCYN-A and γ-24774A11, not only increased substantially in response to dFe amendments but also exhibited signs of iron and phosphorous co-limitation.…”

The effect of nutrients on carbon and nitrogen fixation by the UCYN-A–haptophyte symbiosis

“…Currently, little is known about how UCYN-A responds to nutrient limitation. A recent study investigating the response of nifH expression of different diazotrophs to nutrient additions indicated that UCYN-A might be P limited in the tropical North Atlantic (Turk-Kubo et al, 2012). To date, there are no reports on the effect of nutrient limitation on the photosynthetic partner of UCYN-A and/or this symbiosis.…”