Response of the methanogenic microbial community of a profundal lake sediment (Lake Kinneret, Israel) to algal deposition

An algal bloom of Peridinium gatunenese generally precedes the annual maximum of methane release from the profundal sediment of Lake Kinneret. Therefore, we investigated the response of the sediment methanogenic microbial community to simulated algal deposition. Addition of algal biomass on top of sediment cores resulted in increased CH4 production rates and concentrations of the fermentation products acetate and propionate in the upper 4-cm layers with maximum values at 1-cm depth. Addition of algae to sediment slurries also resulted in increased CH4 production rates and a transient increase of H2, propionate, and acetate concentrations within the first 10 d after addition. The composition of the active microbial community was determined by analysis of terminal restriction fragment polymorphism (T-RFLP) targeting ribosomal RNA and cloning and sequencing of reverse-transcribed 16S rRNA. Analysis of the sediment in the presence and absence of algae indicated that among the Bacteria, members of Deltaproteobacteria and Clostridiales responded by synthesis of ribosomes after 1 d of incubation and those of Bacteroidetes after 6 d. Among the Archaea, ribosomal RNA of the Methanosaetaceae (i.e., acetate-utilizing methanogens) slightly increased after 6 d. Algal deposition apparently stimulated ribosomal synthesis in these sediment microorganisms, thus resulting in increased activity. We conclude that these microorganisms were involved in degradation of the algal biomass resulting in transient release of acetate and other fermentation products and increased production of CH4. Availability of organic material is one of the key factors controlling structure and activity of microbial communities in anoxic freshwater lake sediment (Graf et al. 1982; Sander and Kalff 1993). Therefore, input of organic material into the sediment associated with annual algae blooms results in a response of the microbial community within the sediment (Goedkoop et al. 1997; Tornblom and Rydin 1998), for example, in the microbial production of CH4 (Schulz and Conrad 1995). In freshwater sediments, CH4 is usually the most important end product of the degradation of organic matter (Rudd and Taylor 1980). Previous studies of the decomposition of algal biomass in freshwater sediments have been limited to activity measurements and determination of total bacterial biomass (Schulz and Conrad 1995; Zohary et al. 2000a). However, the phylogenetic diversity, composition, and dynamics of the microbial community involved in the decomposition of sedimenting algal material have not been yet been investigated. In subtropical, meso-eutrophic, and monomictic Lake Kinneret, CH4 production is one of the dominating biogeochemical processes involved in carbon cycling in the lake sediment (Eckert and Conrad 2007). The seasonal plankton succession in Lake Kinneret is characterized by an algal bloom formed almost exclusively by Peridinium gatunense Nygaard during winter and spring (Hickel and Pollingher 1988). The highest input of organic matter into the profundal sediment occurs during the decline of this bloom between May and July (Pollingher 1986). Subsequently (July–November), high CH4 emission from the profundal sediment is observed (Ostrovsky 2003). The increase in CH4 bubbling intensity is probably triggered by the settling of organic matter onto the sediment (Eckert and Conrad 2007), similarly as observed in Lake Constance (Schulz and Conrad 1995). Therefore, we studied the response of the microbial community in Lake Kinneret sediment to the input of algal biomass. We investigated both activity and composition of the microbial communities. We targeted ribosomal RNA as marker for microbial populations since 16S rRNA not only allows phylogenetic characterization but also is a good predictor for the size and activity of individual populations (Kemp et al. 1993). By combining biogeochemical process measurements with molecular analysis of 16S rRNA phylogenetic markers, we aimed to enhance our understanding of the processes operating during anaerobic degradation of algal deposits in freshwater sediment, which is an important event in the annual carbon cycle of a lake. 1 Corresponding author ([email protected]).