Nitrogen fixation is a fundamental process within the ocean, helping to alleviate nitrogen limitation thereby supporting primary production and the locking away of carbon to the deep sea. Based on experimental and modelled data Australia’s tropical waters represent predicted “hotspots” for this process and nitrogen fixing bacteria, diazotrophs, such as the Cyanobacteria Trichodesmium have long been recognised as important members of the phytoplankton communities in Australia’s northern waters. C3 led research now gives scientists the first detailed description of active diazotroph populations in the region.
The research, published in The ISME Journal, provided some unexpected results says lead author, C3 PhD candidate and Ocean Microbiology team member, Lauren Messer.
“Because we were able to sample over a large ocean scale, over two seasons we got a much bigger picture about what is really happening in this part of the ocean,” she explains.
“We surveyed two distinct oceanographic regions in northern Australia, the Arafura and Timor Shelf, or ATS, and the Coral Sea. Both have important roles in global climate and global circulation. The ATS is considered highly productive, especially during winter, whilst annual primary production in the Coral Sea is relatively low and believed to be nitrogen limited,” Ms Messer says.
The team not only found distinct differences between populations of diazotrophs in the two regions but also that N2 fixation was higher in winter and that cyanobacteria weren’t the only bacteria responsible.
“This provides evidence that other bacteria may fix nitrogen in the ocean at high rates,” Messer says.
“Whilst our observations support the view that the tropical waters of Northern Australia are hotspots of diazotrophs in the Southern Hemisphere our data shows there is significant temporal and spatial variability in N2 fixation in this region.”
“The highly dynamic and diverse nature of diazotrophs results in substantial changes in rates of N2 fixation in the region and the subsequent addition of newly fixed nitrogen to the ocean.”
“Spatial and seasonal shifts in diazotroph diversity and activity must be considered in future regional and global marine nitrogen cycle budgets and modelling efforts,” Ms Messer added.
The research was supported by the ARC Discovery Grant Scheme. Sampling took place aboard the R/V Southern Surveyor during ss2012_t07 and ss2013_t03.
Publication details:
High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation
Lauren F Messer, Claire Mahaffey, Charlotte M Robinson, Thomas C Jeffries, Kirralee G Baker, Jaime Bibiloni Isaksson, Martin Ostrowski, Martina A Doblin, Mark V Brown and Justin R Seymour
The ISME Journal advance online publication, 27 November 2015; doi:10.1038/ismej.2015.205