Microbial response to enhanced phosphorus cycling in the North Pacific Subtropical Gyre

Microbial community response to nitrogen (N) amendments and induced phosphorus (P) stress was investigated in the North Pacific Subtropical Gyre (NPSG). Samples amended with reduced sources of N, in the form of nitrate plus ammonium, showed significant increases in microbial cell abundance and biomass and decreases in dissolved inorganic phosphate (Pi) and silicate concentrations during an incubation period of 6 d. Primary productivity, P uptake rates (as both Pi and adenosine-5'-triphosphate [ATP]) and alkaline phosphatase activity (APA) all increased following N amendment. Dissolved organic P (DOP) concentrations did not change, but the large increase in APA and ATP uptake rates suggests that DOP was a dynamic pool and an important source for microbial P nutrition in P-stressed samples. Significant changes were also observed in the structure of the microbial community, with Synecho coccus and picoalgae abundances increasing substantially in the N-amended treatments, while non-pigmented picoplankton abundances were unchanged. Data on P resource partitioning among groups of picoplankton separated by size using membrane filters of different porosities, or by scattering and fluorescence properties using flow cytometry sorting, indicate that Synechococcus could have a greater role in the NPSG P cycling following episodic N inputs. This experimental manipulation of nutrient loading combined with observations at the total population to the microbial group levels constitutes a unique approach to improve our understanding of microbial community structure and function in response to environmental forcing.