Algal-bacteria Interactions and the Effects on Organic Matter Flux and Carbon Remineralization in the Ocean

Dissolved organic matter (DOM) produced during phytoplankton growth is processed and transformed by heterotrophic bacterial communities that produce extracellular enzymes in order to access and degrade organic matter substrates. Despite their important role for pelagic nutrient fluxes and energy cycling in the ocean, interactions between phytoplankton growth and activities of heterotrophic bacterial communities are rarely measured in concert. We conducted separate laboratory experiments with non-axenic diatom cultures of Thalassiosira pseudonana, Thalassiosira sp. and Chaetoceros sp.; the latter two diatoms were recently isolated from the Northeast Pacific Ocean as well as natural microbial assemblages sampled during a cruise into the Mid-Atlantic Bight, to measure hydrolytic enzyme activities during phytoplankton growth. Diatom monocultures and mixed microbial communities were incubated under turbulence using oscillating grids, and incubation times varied between 4 and 14 days. Subsamples were taken at different times during the incubation to determine bacterial and phytoplankton cell numbers, b-glucosidase and leucine-aminopeptidase activities, and concentrations of transparent exopolymeric particles (TEP) that form from microbial metabolites. The culture experiments revealed species-specific patterns in TEP formation and hydrolytic enzyme activities during the different phytoplankton growth phases. Heterotrophic responses within natural assemblages were more complex, reflecting hydrolytic activities of both bacterial communities associated with phytoplankton and free-living cells in the surrounding seawater.