University of Liverpool

North Atlantic

Zinc, iron and phosphorus co-limitation in the ocean

At present, on-going warming is predicted to reduce the physical supply of essential nutrients, nitrate and phosphate to the surface ocean, and hence control future trends in productivity in the subtropical gyres.

However, this view ignores the significant additions of nitrogen to the subtropical ocean from both natural and anthropogenic atmospheric input that decouples nitrate and phosphate, causing phosphate stress for phytoplankton. This will drive an ‘arms race’ towards accessing alternative forms of phosphorus, such as dissolved organic phosphorus.

Recent work by our team and others suggests that availability of trace metals, specifically zinc and iron, regulates the uptake of alternative phosphorus pools. If such phenomena are widespread, this suggests that predictions of future trends in biological activity are inaccurate and instead we need to consider expansion and intensification of a phosphate-deplete ocean. By combining novel observational and modelling experiments, we will gain a more complete quantitative understanding of how trace metals regulate phosphorus acquisition and thus biological activity in the contemporary and future ocean.

During JC150, our aim is to quantitatively assess the role of zinc and iron in regulating phosphorus acquisition by key phytoplankton groups, specifically the nitrogen fixer, Trichodesmium and the cyanobacteria, Prochlorococcus and Synechococcus. We will achieve this aim by performing a series of trace metal clean bioassay experiments at 6 stations along a westward transect from Guadeloupe to Tenerife at a latitude of ~ 20°N.

In these low phosphate subtropical waters, phytoplankton are known to deploy alternative strategies to acquire organic phosphorus. We will collect seawater using the trace metal clean titanium CTD rosette and sampling bottles. We will add a range of concentrations of zinc and iron (alongside other nutrients) and determine the impact of these trace metals on the rate of organic phosphorus acquisition, growth and carbon and nitrogen fixation and incubate under in situ light and temperature conditions for 48 hours. In addition, we will collaborate with international experts in the field of single cell elemental quotas and proteomics to determine the impact of trace metal additions on cell quotas and protein production.

In addition to trace metal clean bioassay experiments, we will exploit the natural gradients in inorganic nutrients and trace metals along the transect to determine the natural variability in rates of phosphorus acquisition, carbon and nitrogen fixation, cell quotas and proteomics.

Twitter account for JC150    @MahaffeyLab