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Duhamel S.,University of Hawaii at Manoa | Duhamel S.,Center for Microbial Oceanography Research and Education | Dyhrman S.T.,Woods Hole Oceanographic Institution | Dyhrman S.T.,Center for Microbial Oceanography Research and Education | And 2 more authors.
Limnology and Oceanography | Year: 2010

Alkaline phosphatase (AP) activity (APA) was measured at several stations in the North Pacific Subtropical Gyre in July 2008, and in a series of nutrient addition experiments: nitrate plus ammonium (+N) or phosphate (+P), to study APA regulation and to evaluate the capacity of picoplankton organisms (i.e., in the 0.2-2-μm size range) to access the AP-hydrolyzable fraction of dissolved organic phosphorus (DOP). The data indicated a primary limitation of the biomass by nitrogen. Both total (measured with a soluble DOP analog) and cell-specific (measured with the enzyme-labeled fluorescence [ELF] phosphate cell labeling method) APA were enhanced in the +N samples and reduced in the +P samples, suggesting that DOP is an important resource for picoplankton nutrition. Cell-free APA represented > 65% of the APA in all samples, but its contribution to total APA significantly decreased in the +N treatment as microbial biomass increased. In the +N treatment, < 5% and up to 96% of the cells in the heterotrophic bacteria-enriched and picophytoplankton-enriched fractions, respectively, were ELF-alcohol-labeled after 5 d. Following N enrichment, the microbial assemblage shifted from cell-free phosphatase dominated under N limitation and P stress (i.e., physiological response) to picophytoplankton-based phosphatase dominated under P limitation (i.e., production or growth rate limitation). If, as predicted, the ocean evolves towards P limitation, DOP availability would become of major importance to sustain productivity. © 2010, by the American Society of Limnology and Oceanography, Inc.


Wilson S.T.,Center for Microbial Oceanography Research and Education | Wilson S.T.,University of Hawaii at Manoa | Foster R.A.,Center for Microbial Oceanography Research and Education | Foster R.A.,University of California at Santa Cruz | And 4 more authors.
Aquatic Microbial Ecology | Year: 2010

Diazotrophic cyanobacteria are important components of marine ecosystems, where they contribute to primary production and provide a source of fixed nitrogen (N). During biological fixation of atmospheric nitrogen (N 2), hydrogen is produced as an obligate by-product. The present study investigated the potential contribution of 4 marine diazotrophs to the pool of dissolved H2 in the oceans. N2 fixation, as measured by acetylene reduction, and H2 production rates were monitored throughout the diel period in cultures of the filamentous Trichodesmium erythraeum strain IMS101, and the unicellular organisms Cyanothece sp. strain ATCC 51142 and Crocosphaera watsonii strains WH8501 and WH0002. H2 production coincided with diel variations in N2 fixation for each strain regardless of whether N2 fixation peaked during the day or night. Chlorophyll-normalized rates of H2 production ranged 100-fold from a maximum of 3 nmol μg chl a-1 h-1 in T. erythraeum IMS101 cultures to 0.03 nmol μg chl a-1 h-1 in Crocosphaera watsonii WH0002. Overall, the ratio of net H2 produced to N2 fixed varied from 0.05 to 0.003 in the unicellular cyanobacteria, compared to 0.3 in the filamentous T. erythraeum IMS101, indicating that unicellular cyanobacteria produce less, or alternatively, re-assimilate more of the H2 produced during N2 fixation. Crocosphaera watsonii has recently been identified as a significant source of fixed N in the marine environment, and an efficient recycling of H2 would provide a valuable source of energy to their respiratory electron transport chain. Furthermore, the magnitude of H2 produced by T. erythraeum IMS101 strongly implicates this organism in the production of H2 in the upper ocean. © Inter-Research 2010.

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