title: High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation.
abstract: Australia's tropical waters represent predicted 'hotspots' for nitrogen (N2) fixation based on empirical and modelled data. However, the identity, activity and ecology of diazotrophs within this region are virtually unknown. By coupling DNA and cDNA sequencing of nitrogenase genes (nifH) with size-fractionated N2 fixation rate measurements, we elucidated diazotroph dynamics across the shelf region of the Arafura and Timor Seas (ATS) and oceanic Coral Sea during Austral spring and winter. During spring, Trichodesmium dominated ATS assemblages, comprising 60% of nifH DNA sequences, while Candidatus Atelocyanobacterium thalassa (UCYN-A) comprised 42% in the Coral Sea. In contrast, during winter the relative abundance of heterotrophic unicellular diazotrophs (δ-proteobacteria and γ-24774A11) increased in both regions, concomitant with a marked decline in UCYN-A sequences, whereby this clade effectively disappeared in the Coral Sea. Conservative estimates of N2 fixation rates ranged from <1 to 91 nmol l(-1) day(-1), and size fractionation indicated that unicellular organisms dominated N2 fixation during both spring and winter, but average unicellular rates were up to 10-fold higher in winter than in spring. Relative abundances of UCYN-A1 and γ-24774A11 nifH transcripts negatively correlated to silicate and phosphate, suggesting an affinity for oligotrophy. Our results indicate that Australia's tropical waters are indeed hotspots for N2 fixation and that regional physicochemical characteristics drive differential contributions of cyanobacterial and heterotrophic phylotypes to N2 fixation.
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Lauren F
authors.initials: LF
authors.lastname: Messer
authors.affiliation: Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, UK.
authors.forename: Claire
authors.initials: C
authors.lastname: Mahaffey
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Charlotte
authors.initials: C
authors.lastname: M Robinson
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Jeffries
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Kirralee G
authors.initials: KG
authors.lastname: Baker
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Jaime
authors.initials: J
authors.lastname: Bibiloni Isaksson
authors.affiliation: Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney, New South Wales, Australia.
authors.forename: Martin
authors.initials: M
authors.lastname: Ostrowski
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Martina A
authors.initials: MA
authors.lastname: Doblin
authors.affiliation: School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, New South Wales, Australia.
authors.forename: Mark V
authors.initials: MV
authors.lastname: Brown
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
authors.forename: Justin R
authors.initials: JR
authors.lastname: Seymour
mesh_terms: D000818:Animals; D037421:Anthozoa; D001315:Australia; D000458:Cyanobacteria / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D020564:Deltaproteobacteria / Q000254:growth & development / Q000378:metabolism; D004463:Ecology; D052836:Heterotrophic Processes; D009586:Nitrogen Fixation / Q000235:genetics*; D009591:Nitrogenase / Q000235:genetics; D009792:Oceans and Seas; D012621:Seasons; D012623:Seawater / Q000382:microbiology; D017422:Sequence Analysis, DNA; D000072359:Trichodesmium / Q000235:genetics / Q000254:growth & development / Q000378:metabolism
chemical_list: D009591:Nitrogenase