Effect of nutrient amendments on bacterioplankton production, community structure, and DOC utilization in the northwestern Sargasso Sea

Bacterial abundance, DOC concentrations, and bacterioplankton community structure (using PCR-based techniques) were measured in 5 seawater culture experiments conducted near the Bermuda Atlantic Time-series Study (BATS) site in the northwestern Sargasso Sea. Cultures were amended with inorganic and organic nutrients, alone or in combination, to test the existence of the 'malfunctioning microbial loop' during late spring and summer at BATS. Objectives of the study were to determine whether (1) alleviating grazing pressure and inorganic nutrient limitation stimulated DOC remineralization by bacterioplankton; (2) a combination of organic and inorganic nutrients affect bacterial production and utilization of seasonally accumulated DOC; and (3) shifts in bacterioplankton community structure are associated with nutrient amendment and DOC utilization. In unamended cultures natural assemblages of surface bacterioplankton did not utilize detectable amounts of naturally occurring 'semi-labile' DOC over time-scales of days to weeks. Neither bacterial production nor utilization of DOC was enhanced with the addition of inorganic N or P (alone or in combination). Labile DOC amendments stimulated bacterial production and DOC utilization, even in the absence of measurable inorganic nutrients, indicating that the bacterioplankton assemblage was initially energy limited, but did not stimulate utilization of seasonally accumulated DOC. The combination of inorganic N and P with labile DOC enhanced both bacterial production and utilization of 'semi-labile' DOC. Changes in bacterioplankton community rDNA gene profiles were minor in the control and inorganic treatments; however, utilization of 'semi-labile' DOC in the organic plus inorganic nutrient treatments coincided with significant changes in bacterioplankton community structure. These data suggest that bacterioplankton community structure, as well as nutrient regime, may be important factors governing the utilization of recalcitrant DOC substrates in the northwestern Sargasso Sea.