NifH expression by five groups of phototrophs compared with nitrogenase activity in coastal microbial mats

Abstract

Diazotrophic (nitrogen-fixing) Cyanobacteria are often structurally dominant in coastal microbial mats but diazotrophs from other bacterial lineages are also present and active. The expression of nifH by four nonheterocystous Cyanobacteria and one member of the Gammaproteobacteria was followed over a 24-h cycle using quantitative reverse transcriptase-PCR. Daily nifH expression patterns were compared with the actual nitrogenase activity (NA) of the entire mat community. Lyngbya sp. was identified as the dominant cyanobacterium but, although recognized as a diazotroph, its cell-specific and abundance-related nifH expression was low. Unexpectedly, the other three cyanobacterial phylotypes dominated community nifH expression at all stations. Also, the gammaproteobacterium showed high levels of cell-specific nifH expression but its nifH copy number was low. Its contribution to the whole community nifH expression was therefore low. These results indicate that there were varying levels of cell-specific expression of nifH in the different mat types and more so, varying contributions to the overall nifH expression by the different diazotrophs. Furthermore, NA did not follow nifH expression patterns.

Factors influencing microbial mat composition, distribution and dinitrogen fixation in three western Indian Ocean coral reefs

“…Diazotrophs in other bacterial lineages, such as anoxygenic phototrophic and heterotrophic bacteria, are also present in these mats and contribute to N 2 fixation, as can be demonstrated by the acetylene reduction technique (Severin & Stal, 2008) and nifH gene expression, but their contribution is second to that of the cyanobacteria (Severin & Stal, 2010b).…”

“…However, there is no evidence of such exclusion for benthic heterocystous cyanobacteria, which have been regularly observed to dominate in tropical regions, in both sun-exposed intertidal habitats (Hoffmann, 1999) and the warm, shallow lagoons of the Pacific (Charpy et al, 2007) and Indian Ocean (Golubic, 1985; this study). Temperature and salinity have been proposed to be of major importance in controlling the degree of mat development and diazotrophy, but do not exclude mat organisms from thermal and hypersaline environments (Severin & Stal, 2010b). Thus, the conspicuous absence of heterocystous cyanobacteria in marine plankton must have other causes than temperature.…”

The cyanobacterium Mastigocladus fulfills the nitrogen demand of a terrestrial hot spring microbial mat

“…N 2 fixation has been assessed by screening for specific nif genes such as nifH (encoding the α-subunit of the nitrogenase enzyme complex), which is the most widely used molecular marker in the search for diazotrophs. Hence, the analysis of the presence of the nifH gene combined with measurements of nitrogenase activity (using the acetylene reduction assay) has been widely used to identify diazotrophs and diazotrophy in microbial mats from diverse environments (Stal et al, 1984;Bergman et al, 1997;Steunou et al, 2006;Díez et al, 2007;Severin and Stal, 2009;Desai et al, 2013).…”

Identification of a novel cyanobacterial group as active diazotrophs in a coastal microbial mat using NanoSIMS analysis

“…Surveys of nifH in microbial mats suggested that heterotrophic bacteria might also have an important role in microbial mat N 2 fixation in addition to cyanobacteria (Zehr et al, 1995;Steppe et al, 1996;Zehr et al, 2003;Omoregie et al, 2004a;Severin et al, 2010;Severin and Stal, 2010b). Analysis and quantification of nifH transcripts have helped to identify the fraction of diazotrophs actively expressing this essential gene for N 2 fixation and has given insights into gene-expression dynamics in the environment (Omoregie et al, 2004b;Moisander et al, 2006;Severin and Stal, 2010a). Previous studies (Steunou et al, 2008;Severin and Stal, 2010a) have revealed discrepancies between the expression of nifH by diazotrophic groups and nitrogenase activity patterns measured by acetylene reduction, illustrating that gene expression does not necessarily correspond to activity.…”

“…Analysis and quantification of nifH transcripts have helped to identify the fraction of diazotrophs actively expressing this essential gene for N 2 fixation and has given insights into gene-expression dynamics in the environment (Omoregie et al, 2004b;Moisander et al, 2006;Severin and Stal, 2010a). Previous studies (Steunou et al, 2008;Severin and Stal, 2010a) have revealed discrepancies between the expression of nifH by diazotrophic groups and nitrogenase activity patterns measured by acetylene reduction, illustrating that gene expression does not necessarily correspond to activity.…”