Prokaryotic respiration and production in the open ocean

The aim of this thesis was to advance our knowledge on the dynamics of bacterial production and bacterial respiration in the open ocean and linking microbial activity to the physico-chemical environment. 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. The method of choice is the Winkler titration technique because it provides sufficient precision to allow measurements in productive as well as in oligotrophic oceanic systems where respiration rates are usually extremely low. Thus far, the Winkler titration was a tedious task and time consuming. The use of a spectrophotometric determination of the concentration of total iodine and later refinements of the method made it possible to analyze samples more rapidly, however, for respiration measurements in oligotrophic regions this approach was found not to be sufficiently sensitive.We describe a method using the spectrophotometric Winkler approach in conjunction with an automated continuous-flow analyzing system. On board measurements along a gradient from high to low productivity proved, that the method allowed for precise and accurate measurements of oxygen concentrations even in oligotrophic environments. The sea-surface microlayer (SML) represents the boundary layer between the ocean and the atmosphere. It has been shown that dissolved organic matter in the SML is often enriched compared to the underlying water for reasons that are not entirely clear. Heterotrophic activity of the prokaryotic community in the SML could give important insight into exchange processes between the ocean and the atmosphere.We measured bacterial production and respiration and linked these parameters to patterns of potential substrate sources for bacteria. Seasonal studies in the open ocean are generally scarce because of constraints due to the weather conditions and availability of shiptime. In the southern North Sea, we conducted a seasonal survey studying the dynamics in bacterial respiration and production in relation to DOC and primary production. In total, we occupied 150 stations and compiled 102 BGE estimates. The dark ocean is one of the most under-sampled environments in our biosphere. Reported biological activity in the deep sea is low, however, until now methods were generally not sensitive enough to allow rate measurements at depths below 500 m. Model estimates on carbon fluxes suggest that respiration in the dark ocean represents up to half of the total respiration in the upper layers. However, even the highest current estimates on carbon input into the deep ocean do not match mineralization rates measured in the deep. In this chapter, bacterial production and respiration was measured in the meso-and bathypelagic of the North Atlantic supporting the current notion, that the carbon flux in the dark ocean mediated by the prokaryotic community might be either higher than previously assumed or that decompression of the prokaryotes leads to a stimulation of their activity. Currently, there is considerable scientific debate on the relation between diversity and ecosystem functioning. We investigated the relation between changes in the phylogenetic composition of the bacterioplankton community and the main function of bacteria in the carbon cycling, i.e., the remineralization of organic carbon, over seasonal cycles in the southern North Sea. The remineralization activity was found to be largely independent of the phylogenetic composition of the bacterioplankton community.