Roles of diatom nutrient stress and species identity in determining the short- and long-term bioavailability of diatom exudates to bacterioplankton

Abstract

Phytoplankton exude carbon-rich dissolved organic matter (DOM) upon nutrient stress, yet the ecological role of this exudate in stimulating bacterial growth is not well understood. We harvested DOM produced by four coastal diatoms (Skeletonema marinoi, Chaetoceros socialis, Thalassiosira weissflogii, and Odontella aurita) subjected to depletion of nitrogen (N), silicon (Si), or N+Si simultaneously in batch culture and assessed its bioavailability to natural bacterioplankton communities using dilution batch-culture remineralization bioassays. Short-term, ecologically-relevant responses were affected both by diatom source species and by the nutrient stress under which the DOM was produced. Si-stress DOM was generally more bioavailable over the first week than that produced under N or N+Si stress, and led to higher bacterial growth efficiencies. In contrast, the amount of diatom-derived total organic carbon (TOC) that persisted over months in the same experiments differed among source diatom species, with no evidence of a nutrient stress effect. These results nuance the carbon overflow hypothesis of DOM release, which has been suggested to be maladaptive due to the possibility of DOC release allowing bacterioplankton to better compete for the same nutrient that is limiting the phytoplankton. Our results suggest that bacterial activity is most promoted when diatoms are limited by Si, a nutrient that is not subject to competition with bacteria. They imply that the identity of the nutrient that terminates a diatom bloom impacts heterotrophic activity in the short term; however, source diatom species has the greater influence on DOC persistence in surface waters and its potential export from the system.