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The paradox of marine heterotrophic nitrogen fixation: abundances of heterotrophic diazotrophs do not account for nitrogen fixation rates in the E astern T ropical S outh P acific
Author(s) -
TurkKubo Kendra A.,
Karamchandani Muskan,
Capone Douglas G.,
Zehr Jonathan P.
Publication year - 2014
Publication title -
environmental microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.954
H-Index - 188
eISSN - 1462-2920
pISSN - 1462-2912
DOI - 10.1111/1462-2920.12346
Subject(s) - diazotroph , biology , heterotroph , nitrogen fixation , phylotype , cyanobacteria , trichodesmium , upwelling , symbiosis , ecology , botany , gene , phylogenetics , bacteria , genetics
Summary Results of recent modelling efforts imply denitrification‐influenced waters, such as those in the E astern T ropical S outh P acific ( ETSP ), may support high rates of biological nitrogen fixation ( BNF ), yet little is known about the N 2 ‐fixing microbial community in this region. Our characterization of the ETSP diazotrophic community along a gradient from upwelling‐influenced to oligotrophic waters did not detect cyanobacterial diazotrophs commonly found in other open ocean regions. Most of the nifH genes amplified by polymerase chain reaction ( PCR ) from DNA and RNA samples clustered with γ‐proteobacterial nifH sequences, although a novel T richodesmium phylotype was also recovered. Three quantitative PCR assays were developed to target γ‐proteobacterial phylotypes, but all were found to be present at low abundances. An analysis of the expected BNF rates based on abundances and plausible cell‐specific N 2 fixation rates indicates that these γ‐proteobacteria are unlikely to be responsible for previously reported BNF rates from corresponding samples. Therefore, the organisms responsible for the measured BNF rates remain poorly understood. Furthermore, there is little direct evidence, at this time, to support the hypothesis that heterotrophic N 2 fixation contributes significantly to oceanic BNF rates based on our analysis of heterotrophic cell‐specific N 2 fixation rates required to explain BNF rates reported in previously published studies.