Phosphorus Cycling

While the broad outlines of phosphorus cycling in the ocean are known, a mechanistic understanding of P cycling, especially at or near the sediment-water interface, is lacking. This has far-reaching implications, in part because fundamental conclusions about nutrient cycling and nitrogen fixation are based on the N:P ratios and Fe:P ratios. Slight changes in the C:P ratios of particulate material delivered to the sediment, or the C:P ration of particles exported to the euphotic zone may strongly propagate through the microbial food webs (Deutsch et al., 2012).

A tool for examining P biogeochemistry is these systems is the use of ³³P radiotracer on both surface sediments (see Goldhammer et al., Nature Geoscience, 2010) and in the subeuphotic water column. In Geophysical Research Letters, (Sokoll et al., GRL 2017; doi:10.1002/2016GL072183) we describe experiments using radiolabeled phosphate that show the rapid uptake of phosphate into particles in sub-euphotic waters off the coasts of Mauritania and Namibia. The experiments and analysis demonstrate that this uptake is biologically mediated, and thus P cycling in the dark ocean is more intense than originally thought.

In Meador et al., 2020 (Science Advances) we collaborated with Travis Meador and Martin Koenneke (MARUM) to evaluate P and C uptake by the marine nitrifying archaea Nitrosopumilus maritimus with the help of radiotracers ³³P and ¹⁴C.

We have also developed a method to image ³³P uptake using nanoSIMS in Schoffelen et al. 2018;Using this method, we could show that key cyanobacteria rely on different forms of phosphorus throughout the summer bloom in the Baltic Sea.