| Summary: | To understand (1) the role of heterotrophic bacteria in the biogeochemical cycles, (2) the fate of the organic carbon and mineral nutrients, and (3) the flow of energy to higher trophic levels, we need to understand how bacterial growth is controlled in marine environments. The main hypotheses about the control of bacterial growth are based on the studies about bottomup control (limiting resources) and top-down control (predators and viruses), that is, how bacteria interact with their ‘neighbors’ in the microbial food web. Using micro- and mesocosm experiments, comparative analyses, plus an idealized model to test different predictions, this thesis evaluates the role of heterotrophic bacteria in the utilization of organic matter in different marine environments. The effects of mineral nutrients, organic carbon and predator control over bacterial growth are also investigated. Bacterial growth rates can be controlled by carbon or/and mineral nutrients depending on the biological oceanographic conditions (Papers I and II). Both types of bacterial growth limitation (carbon and mineral nutrient) can co-exist simultaneously depending on the structure of the microbial food web, mineral nutrient and labile organic carbon concentration. Dominance of nano-phytoplankton, which can be grazed by rapidly responding micro-zooplankton (e.g. idealized model in Paper IV), will shifts more rapidly to a carbon limited bacterial growth, thus, heterotrophic bacteria may use all labile organic carbon preventing its accumulation in the euphotic zone (Paper I). Contrary, dominance of large diatoms, which can be grazed by slowly responding copepods, can keep labile organic carbon produced by copepod (e.g. sloppy feeding) and bacterial growth can be limited by mineral nutrients (i.e. competition of nutrients with diatoms) (Paper II) shifting the system slowly to carbon limited bacterial growth control. Different rates of organic carbon utilization by bacteria can be estimated in manipulated mesocosm experiments by adding mineral nutrients ...
|
|---|