Biogeography of nitrogen fixers in the Australian coastal ocean — ASN Events
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  2. Marine microbiology
  3. Marine Microbiology
  4. Biogeography of nitrogen fixers in the Australian coastal ocean

Biogeography of nitrogen fixers in the Australian coastal ocean (#56)

Lauren F Messer 1 , Claire Mahaffey 2 , Mark Doubell 3 , Martina Doblin 1 , Mark V Brown 4 , Justin R Seymour 1
  1. Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, NSW, Australia
  2. Department of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, University of Liverpool, Liverpool, United Kingdom
  3. Aquatic Sciences, South Australian Research and Development Institute, Adelaide, SA, Australia
  4. School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, NSW, Australia

Nitrogen fixing bacteria (diazotrophs) play a fundamental role in alleviating nitrogen limitation in oligotrophic oceanic waters. Recent evidence suggests that the marine waters of northern Australia are a potential hotspot of nitrogen (N2) fixation activity, yet the microbial community underpinning this activity has not yet been explored. In addition, little information exists regarding spatiotemporal patterns in the identity and activity of diazotrophs within the largely oligotrophic marine environment surrounding Australia. To address this gap, we surveyed a number of important but distinct oceanographic provinces spanning tropical to temperate latitudes, and coupled amplicon sequencing of nifH DNA and cDNA to 15N-N2 incubation experiments. Discrete diazotrophic assemblages were observed within the tropical and temperate provinces, driven by differences in the relative abundance of globally significant photoautotrophic and photoheterotrophic cyanobacteria and heterotrophic bacteria. Specifically, shifts from Trichodesmium dominated assemblages (up to 90 % of sequences) in the shallow seas of tropical northern Australia to Candidatus Atelocyanobacterium thalassa (UCYN-A; up to 80 % of sequences) and gammaproteobacterial (up to 60 %) dominated communities in temperate systems were observed. Rates of N2 fixation were highly variable across spatiotemporal scales, ranging from < 1 to 91 nmol N L-1 d-1 in the tropics, from 1 to 88 nmol N L-1 d-1 in the subtropical Tasman Sea, and from 6 to 47 nmol N L-1 d-1 in temperate south Australian waters. Network analysis revealed sea surface temperature and salinity to be overarching structuring factors for diazotroph assemblages. In addition, network analysis highlighted the significant correlation between phosphate concentration and the relative abundance of taxa specific nifH transcripts, such as UCYN-A. Our findings show that Australian marine waters harbour globally significant and highly active diazotroph assemblages, and that regional physicochemical characteristics may drive the differential contribution of cyanobacterial and heterotrophic phylotypes to N2 fixation.

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