Uptake of dissolved organic matter and phosphate by phototrophic and heterotrophic bacteria

Bacterioplankton communities play a crucial role in marine biogeochemical cycles because they mediate the flux of dissolved organic matter (DOM), which is equal to about half of primary production in the ocean. These bacterial communities are also known to be very diverse and are comprised of bacteria from several different phylogenetic groups. Most biogeochemical studies of marine environments use the dichotomy of grouping microorganisms into two boxes, photoautotrophs and heterotrophs. My dissertation is about organisms and processes not described by these two boxes.;An important component of my research work was the identification and quantification of organic matter uptake by marine cyanobacteria Prochlorococcus and Synechococcus. The goal of the first part of my dissertation was to identify the microbial groups responsible for light-dependent leucine incorporation and to examine the effect of light on the uptake of amino acids added at tracer levels. My hypothesis was that stimulation of bacterial production by light was due to photoheterotrophy by Prochlorococcus. My results indicated that this was in fact the case, but other groups of photoheterotrophic bacteria contributed to the light effect as well. The uptake of essential elements in reduced organic forms could provide an additional source of macronutrients to be used in protein synthesis, and help explain the success of Prochlorococcus and Synechococcus in oligorophic environments. These marine cyanobacteria are capable of accessing a wide variety of organic compounds. These results allowed me to hypothesize that these marine cyanobacteria take up dissolved organic matter for limiting elements like nitrogen and phosphorus.;To determine if these cyanobacteria were also capable of assimilating non-limiting organic compounds without nitrogen or phosphorus, uptake rates of amino acids and glucose by Prochlorococcus, Synechococcus and heterotrophic bacteria were determined in the Sargasso Sea in May and September of 2008 using flow cytometry. This study revealed that glucose uptake by Synechococcus was significantly higher than uptake by both Prochlorococcus and heterotrophic bacteria in all samples, while uptake by Prochlorococcus and heterotrophic bacteria was similar. These results suggested that coccoid cyanobacteria are competitive at assimilating dissolved organic compounds without nitrogen or phosphorus, though these microbes accounted for a small fraction of total uptake.;Phosphorus potentially limits the growth and productivity of microbial communities in many oligotrophic oceans. Both dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) could be an important resource in these regimes. Assimilation rates of inorganic and organic phosphorus by marine cyanobacteria and heterotrophic bacteria in the oligotrophic ocean are not well quantified. For the last chapter of my dissertation, I used radioisotope tracers of orthophosphate and ATP combined with flow cytometry sorting to quantify assimilation by heterotrophic bacteria, Prochlorococcus and Synechococcus during the fall of 2007 and 2008 and the spring of 2009 in the western Sargasso Sea. Phosphate and ATP uptake rates were 50-fold and 80-fold higher for Synechococcus compared to the other two groups. However, there was no significant difference between ATP and phosphate uptake by Prochlorococcus and heterotrophic bacteria. Total uptake of phosphate and ATP was dominated by heterotrophic bacteria, while uptake by Prochlorococcus and Synechococcus was a smaller fraction of the total. This coincided with the lower abundance of these cyanobacteria (25% and 2.5% of total prokaryotes respectively) compared to heterotrophic bacteria during my study. Phosphate and ATP turnover was surprisingly similar for the three groups analyzed, suggesting P uptake by marine cyanobacteria is not that different from uptake by heterotrophic bacteria. Collectively, these data suggest that inorganic and organic P play an important role in the ecological success of marine cyanobacteria in the Sargasso Sea.;The current study added to the growing information regarding the role of specific bacterial groups, such as cyanobacteria, in DOM assimilation. In particular, this study was the first to unfold the contribution of photoheterotrophic microbes to leucine assimilation in the North Atlantic Ocean. My results not only provided new evidence of carbon uptake by phototrophic picoplankton, they lay the foundation for a number of new interesting questions regarding these extremely successful microorganisms.