Analogous nutrient limitations in unicellular diazotrophs and Prochlorococcus in the South Pacific Ocean

Abstract

Growth limitation of phytoplankton and unicellular nitrogen (N2) fixers (diazotrophs) were investigated in the oligotrophic Western South Pacific Ocean. Based on change in abundances of nifH or 23S rRNA gene copies during nutrient-enrichment experiments, the factors limiting net growth of the unicellular diazotrophs UCYN-A (Group A), Crocosphaera watsonii, γ-Proteobacterium 24774A11, and the non-diazotrophic picocyanobacterium Prochlorococcus, varied within the region. At the westernmost stations, numbers were enhanced by organic carbon added as simple sugars, a combination of iron and an organic chelator, or iron added with phosphate. At stations nearest the equator, the nutrient-limiting growth was not apparent. Maximum net growth rates for UCYN-A, C. watsonii and γ-24774A11 were 0.19, 0.61 and 0.52 d−1, respectively, which are the first known empirical growth rates reported for the uncultivated UCYN-A and the γ-24774A11. The addition of N enhanced total phytoplankton biomass up to 5-fold, and the non-N2-fixing Synechococcus was among the groups that responded favorably to N addition. Nitrogen was the major nutrient-limiting phytoplankton biomass in the Western South Pacific Ocean, while availability of organic carbon or iron and organic chelator appear to limit abundances of unicellular diazotrophs. Lack of phytoplankton response to nutrient additions in the Pacific warm pool waters suggests diazotroph growth in this area is controlled by different factors than in the higher latitudes, which may partially explain previously observed variability in community composition in the region.

A microarray for assessing transcription from pelagic marine microbial taxa

“…This hypothesis is consistent with the similarity in responses of diazotrophs and Prochlorococcus HL spp. to Fe addition, both in this study and in a study in the Southwest Pacific Ocean (Moisander et al, 2012).…”

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

“…The significant negative correlation observed between silicate and UCYN-A1 and γ-24774A11 transcripts, and UCYN-A1 and fucoxanthin, could be indicative of shifting phytoplankton communities between the ATS and Coral Sea, given that UCYN-A is known to live in association with a prymnesiophyte host (Thompson et al, 2012;Hagino et al, 2013;Krupke et al, 2013). Currently, the lifestyle (for example, free-living, particle attached or symbiont) of γ-24774A11 remains unknown (Langlois et al, 2015); however, it has been speculated that it may depend upon phytoplankton-produced dissolved organic carbon (Moisander et al, 2012(Moisander et al, , 2014. Although strongly co-linear variables were removed from our analyses, silicate was inversely correlated to salinity and positively correlated to temperature, therefore it could also be indicative of a water mass tracer rather than a biological causation.…”

“…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.…”

“…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. Although we did not measure dFe throughout the ATS and Coral Sea during the present study, previous evidence suggests that concentrations of particulate Fe are relatively high in the Timor region of the ATS (Waite et al, 1995), but dFe concentrations may be relatively low (Sohm et al, 2008), and Trichodesmium colonies have been shown to experience Fe limitation in this region (Kustka et al, 2003).…”

Nutrient uplift in a cyclonic eddy increases diversity, primary productivity and iron demand of microbial communities relative to a western boundary current

“…The physiology or genome content 531 of these taxa remains completely unknown, but they seem to be abundant in warm, oligotrophic 532 surface waters globally (Langlois et al, 2005). In the southwestern Pacific, the abundance of this 533 group has been shown to be correlated to DOC concentration, and to increase with Fe and P 534 addition (Moisander et al, 2012). In contrast, the dominant diazotroph in the CCE was an OTU 535 sharing 95 % NifH amino acid identity with Trichodemsium erythraeum (OTU181) and OTUs 536 matching Candidatus Atelocyanobacterium thalassa (UCYN-A; OTU1321 and OTU50).…”