| dc.description.abstract | The dissolved organic carbon (DOC) pool is a highly diverse and important part of the marine carbon cycle and can be altered by microorganisms. This study was conducted to gain further knowledge about changes in DOC composition in altered redox stages. Depending on the availability of electron acceptors different microbial redox reactions, which yield different reaction products, occur. Using sludge from a recirculating aquaculture system (RAS) makes it possible to further address issues related with the reaction product hydrogen sulphide (H2S) in connection to high amounts of labile DOC. RAS sludge and seawater were incubated in closed bottles and kept in the dark at 12°C throughout the experiment (25 days). Half of these sample bottles were treated with a nitrate addition and sampling was done daily from treated as well as untreated bottles to characterise the redox processes occurring. Based on daily iron and H2S measurements samples for the DOC characterisation were taken on four days for each nitrate added and control samples. Solid phase extraction with PPL cartridges was performed and extracted DOC was subsequently analysed using UPLC-QTOF-MS in ESI positive (ESI+) and ESI negative (ESI-) mode. This approach represents an inherently qualitative non-target approach, leading to data, which should be compared only using relative abundances. The results of the characterisation elucidate changes in DOC composition, which can most likely be associated with microbial activity. A high variability between results derived from ESI+ and ESI- mode was found. Whereas in ESI- mode changes of elemental ratios in the two treatments were observed time shifted, no obvious difference between nitrate added and control samples was noted. In ESI+ though linear negative trends for the elemental compositions N:C, P:C, S:C and N:P were found. A high percentage of formulas classified as aliphatic was observed in all samples. Changes in average elemental ratios and in composition regarding different classifications often were in disagreement with results from previous studies. This experiment reveals that there are many, not fully understood, factors affecting the highly diverse DOC pool and microbial community, which need additional research in order to make meaningful interpretations of experimental data. Furthermore, different approaches being used for DOC characterisation create a strong bias, consequently making a comparison between studies highly uncertain. To gain a better understanding of the DOC pool and its connection to different microbial redox reactions enabling these comparisons would be of great benefit. The experiment discussed in this thesis presents an approach of answering questions related to both the natural environment under the influence of hypoxia and eutrophication, as well as the aquaculture industry. | |