The microbial and viral loop in aerobic aquatic sediments

Research project P 14220 The microbial and viral loop in aquatic sediments Branko VELIMIROV 06.03.2000 The formulation of the concept of the microbial loop was a major breakthrough in the field of microbial ecology, and together with the recognition of the impact of virus induced lysis on bacterial production and its consequences, it certainly contributed to a better understanding of the functioning of marine and freshwater ecosystems. . However, most processes describing the functioning of the microbial and viral loop were derived from studies on the water column of both oceanic and limnic systems. In contrast to the knowledge emerging from the above mentioned studies, the microbial loop hypothesis was never thoroughly tested for its validity in the adjacent shallow aerobic sediment systems, and there is a total lack of investigations concerning virus-bacteria interactions in those sediments. This leads to the working hypothesis that a) either such relationships do not exist for the community members in aquatic sediments and that a new conceptual framework needs to be defined to understand the connection between processes and obtain predictive models on the direction and magnitude of energy flow in sediment systems or b) there is a general lack of data concerning structure and rate measurements of the microbial community in sediment environments and that no satisfying experimental approach was so far developed to test the validity of the microbial loop hypothesis in aquatic sediments. Our reserach objectives have the aim to test whether the diverse members of the microbial compartment of aerobic aquatic sediments have or have not a comparable role to those from the water column with respect to the microbial loop hypothesis and will concentrate on I) predation of prototzoa on bacteria and protozoan driven remineralization, or else, are they of no quantitative importance for the microbial compartment, II) what is the role of virus induced lysis of sediment bacteria and is the viral impact a controlling force for bacterial production, III) what is the fate of bacterial production If the above mentioned compartments do not remove significant amounts of bacteria, and IV) what is the role of benthic meiofauna in controlling bacterial, protozoan and viral production, or else, do we have to figure out a new carbon pathway in the sediment system. The proposed study represents a first attempt to test the microbial loop system in aerobic aquatic sediments whereby the integration of the viral compartment is seen as a key step which should contribute essentially to the understanding of the system functioning. The experimental connection between the microbial and the viral loop in order to investigate the qualitative and quantitative relationships within the microbial compartment of aquatic sediments is a so far neglected approach.

Because the concept of the microbial loop and the viral loop (which was a major breakthrough in contributing to understand the functioning of marine and freshwater ecosystems) was mainly derived from obserations of water column processes but never never tested in sediment systems, we investigated microbial relationships in the aerobic sediment layer of a freshwater and a marine system. It could be shown that the microbial interrelationships which charaterize both the microbial and the viral loop are also functional in aquatic aerobic sediments but that the magnitude of these processes is negligible as compared to the water column. Within the frame of this investigation a number of findings deserve to be mentioned specifically for the freshwater sediment system: It could be shown that the bacterial compartment was highly dynamic, revealing population doubling times of 0.6-10.0 days over the year, thus being comparable to those of the water column. However, none of the known bacterivorous organisms had a major impact on the bacterial compartment. Heterotrophic nanoflagellates (HNF) and ciliates cropped between 0-14 % of the bacterial production with a yearly mean amounting to only 4%. This unexpected low impact of protozoan grazing on bacteria was due to the low numbers of HNF and ciliates in relation to bacteria. Thus grazing by HNF and ciliates could be ruled out as a parameter regulating bacterial standing stock or production in aquatic sediments. Yet, the control mechanisms acting upon the bacterial population lead to a relative constancy of the bacterial standing stock during a year, which implies that other compartments are functional in the sense of a limiting factor. The investigation of the viral compartment in the sediment with respect to its capability in controlling bacterial production by lysis gave surprising results: It could be demonstrated that viruses did not contribute significantly to the bacterial mortality since on average only 6% of the bacterial production was controlled by viruses. Therefore the fate of some 70% of the bacterial production in aerobic aquatic sediments remains to be investigated.