The role of microorganisms in the cycling of organic matter in anoxic marine environments A culture-independent approach

Abstract

Organic matter (OM) plays an important part in regulating the marine carbon cycle by linking the atmospheric and oceanic carbon cycles with long-term carbon storage in the sediments. OM originates mainly from inorganic carbon fixed by photosynthetic primary production. However, only a minor part of OM reaches the deep ocean waters or marine sediments. The majority of OM undergoes rapid remineralization in the water column and oxygenated surface sediments, mainly driven by the metabolic activity of microorganisms. The dynamics of OM cycling in the marine environment differ strongly depending on environmental conditions. Oxygen depletion has a major effect on ecosystem functioning, the metabolisms of microbes, making the carbon cycle in anoxic marine environments fundamentally different from less productive, oxygenated marine systems. On the one hand, oxygen is depleted because of respiration linked to the heterotrophic degradation of an excess of OM, while on the other hand, the depletion of oxygen defines strict thermodynamic limits to microbial metabolism. A relatively greater amount of carbon is preserved in anoxic conditions. To understand the processes affecting OM recycling and burial in anoxic marine environments, it is necessary to study the microbial communities involved in its transformation in detail. The majority of the microbial community in anoxic marine environments is uncultured and only known indirectly from environmental studies. Little is known about the effect that the organic carbon pool has on the microbial community and vice versa. It is likely that many uncultured microbial community members in anoxic conditions are dependent on the initial fermentative transformation of complex OM. Further information on how the activities and interactions of uncultured microorganisms are determined in anoxic conditions may help in understanding the limits and future development of the marine organic carbon cycle. The objective of this thesis was to study the relationship of microbial metabolism to OM cycling in anoxic marine environments by using culture-independent methods. The focus was especially on studying the initial degraders of OM and their niche separation based on the type of OM. The closed marine basins of the Black Sea and Baltic Sea were chosen as study sites due to their high OM content and permanent anoxic conditions, as well as the strong redox gradients. Studies into the relationship between microbial community composition and OM composition were made with dissolved OM (DOM) and particulate forms of OM (POM) in the Black Sea water column as well as with POM in Baltic Sea sediments. The results described in this thesis highlight the function of abundant uncultured members of microbial communities in the cycling of the OM pool in anoxic environments. The activity of these microbial communities has been shown to be shaped by the OM form and compound type. In addition, these studies highlight the importance of possible autotrophic sources of carbon to the resident microbial communities in an anoxic water column. These results advance our understanding on which environmental pressures are important in shaping microbial communities and the carbon cycle in anoxic marine systems.