Trichodesmium: a nitrogen fixing cyanobacteria

Trichodesmium is a genus of globally distributed nitrogen fixing marine cyanobacteria. It provides up to 50% of new nitrogen in some regions. They can form large macroscopic colonies that host an association of diverse microbes distinct from that in the surrounding sea water.

Interactions between the cyanobacteria and its community are recognised as being important by the way they influence the physiology and nitrogen fixation of the Trichodesmium host. When located in equatorial regions there is a lack of heterocysts in the host. It is suggested that with the host provisioning carbon and nitrogen aiding the growth of the heterotrophs, they intern respire resulting in microenvironments of decreased oxygen concentration. These sub oxic environments become a haven for the host and in turn benefit the community through more nitrogen fixation.

Due to their provision of a substantial proportion of new nitrogen there have been studies on its response to changes in the ocean. Under increased CO₂ conditions nitrogen fixation also increases. The community increase in respiration, seen through dedicating more transcription of relevant proteins, due to the rise in nitrogen fixation.

Another study¹ showed that increasing CO₂ without any other changes increased growth of Trichodesmium. Low growth rates are attributed to high energy demands to establish the carbon concentrating mechanism which in turn limits energy for nitrogen fixation. At high CO₂ the mechanism is fully saturated allowing for upregulation of nitrogen fixation, indirectly increasing growth.

Conversely CO₂, and associated ocean acidification has been shown by other studies² to be detrimental to the cyanobacteria. Low pH causes fixation rates to drop as the pH in the cytosol where nitrogen is located decreases with that of the surrounding sea water, thus lowering the efficiency nitrogenase. Effects of low pH differ depending on what is the limiting factor. If carbon is not limiting it can dominate over the effects of acidification. It is possible that when in a microbial community even if carbon is not too high, the heterotrophs could provision enough or through lowering oxygen still enable reasonable levels of nitrogen fixation at low pH.

The community is made up of organisms which are key in the cycling of DOM including Bacteriodetes, gammaproteobacteria and alphaproteobacteria. Bacteriodetes aid the maintenance of low oxygen environments through contributing to the extracellular matrix as it restricts oxygen diffusion. Alphaproteobacteria dominated by rhodobacterales and rhizobiales demonstrated significantly higher gene expression of amino acid transport, corresponding with their nature of consuming the amino acid component of marine DOM.  It is likely not a coincident that these major taxanomic groups that can together degrade DOM are dominantly found in association like Trichodesmium. This association means that how the climate will affect this cyanobacteria may also affect these DOM processing bacteria, and so it is important to look into these community structures and how they respond to elemental fluxes to better understand individual taxa responses.

Reviewed Paper

Lee, M., Webb, E., Walworth, N., Fu, F., Held, N., Saito, M. and Hutchins, D. (2017). Transcriptional Activities of the Microbial Consortium Living with the Marine Nitrogen-Fixing Cyanobacterium Trichodesmium Reveal Potential Roles in Community-Level Nitrogen Cycling. Applied and Environmental Microbiology, 84(1).

References

1.Boatman, T., Lawson, T. and Geider, R. (2017). A Key Marine Diazotroph in a Changing Ocean: The Interacting Effects of Temperature, CO2 and Light on the Growth of Trichodesmium erythraeum IMS101. PLOS ONE, 12(1), p.e0168796.

2.H. Hong et al., Science 10.1126/science.aal2981 (2017).