Heterotrophic bacteria fulfill two major functions in the oceanic carbon cycle. They utilize dissolved organic carbon (DOC) for biomass production but at the same time, convert a large part of this DOC into carbon dioxide. The aim of this thesis is to advance our knowledge on the dynamics of bacterial production (BP) and bacterial respiration (BR) in the open ocean. BP and BR together determine the bacterial growth efficiency (BGE) which is an important, albeit rarely studied index for ocean carbon cycling models.

Although the currency in carbon cycling measurements is logically carbon, respiration in water is most often calculated from the decline in oxygen concentrations in enclosed samples over time. To measure oxygen concentrations a system was developed where a continuous flow-through analyzer was coupled to a custom made auto-sampler for oxygen bottles. Compared to conventional titration techniques, the automated setup allows for rapid oxygen analysis and measurements in the subtropical North Atlantic proved that even low respiration rates are detectable.

The sea-surface microlayer (SML) is the boundary layer between the ocean and the atmosphere and its chemical composition and biological activity might substantially influence the gas exchange between the ocean and the atmosphere. In the underlying water (30 cm depth) of the North Atlantic, BGE exhibited the typical range for open ocean surface waters, however, BGE in the SML was extremely low. Despite the high concentrations of free amino acids in the SML, usually taken up efficiently by bacteria, BP was consistently low whereas BR was highly elevated in the underlying water. This suggests that the dissolved organic matter accumulating in the SML is not readily available to bacteria.

Shelf seas such as the North Sea are sites of high primary production and heterotrophic microbial activity. Seasonal variability in BGE was mainly determined by the dynamics in BP and estimated bioavailability of DOC to support BP was linked to the dynamics in particulate primary production. Thus, despite the high input of terrigenous organic matter via rivers into the North Sea, autochthonously produced organic matter determines BP and BGE. We also investigated the richness of the bacterial community by T-RFLP fingerprinting of 16S rRNA gene fragments in the North Sea. The monthly variability of cell-specific BP as a function of seasonal bacterioplankton richness suggested, that increasing richness is related to enhanced variability of cell-specific bacterial growth. However, cell-specific BR remained remarkably constant over the range of observed bacterioplankton richness. This suggests that despite the shifts in the community composition, the main function of heterotrophic bacterioplankton, i.e., the remineralization of DOC to CO2 is rather stable over the seasonal cycle.

Reported biological activity in the deep sea is low, however, evidence accumulates suggesting that the local carbon export from the surface water does not match the respiration rates estimated for the dark ocean. Measured BGE was low in the dark ocean of the North Atlantic and the resulting high bacterial carbon demand measured cannot be explained by conventional models of carbon fluxes. This discrepancy highlights the need to perform rate measurements under realistic pressure conditions but also indicates that deep water prokaryotes have the potential to exhibit higher metabolic activity than hitherto assumed.
More information is available at: http://dissertations.ub.rug.nl/faculties/science/2006/t.reinthaler/