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Heterotrophic processes are considered to prevail in lakes with high terrestrial loading and a high content of dissolved organic carbon. However, previous studies have shown abundant methanotrophic bacteria (MOB) and photoautotrophic green sulphur bacteria (GSB) in some boreal lakes. We used genetic and lipid biomarkers to follow the seasonal succession of the abundance and carbon biomass of the major bacterial groups in Lake Mekkojarvi, a small Finnish polyhumic lake with a large anoxic hypolimnion. Biomarkers were based on length heterogeneity analysis of 16S rRNA gene amplification by polymerase chain reaction (PCR) (LH-PCR) and on phospholipid fatty acid (PLFA) analysis. Representative strains closely affiliated to the dominant bacterial groups in the lake were cultured and their characteristic PLFA biomarkers were selected. According to these bio- markers, the relative abundance and biomass of the anaerobic photo autotrophic bacteria (e.g. GSB) were extremely high in the pelagial during summer stratification (on average 47% of the bacterial rRNA genes and 77% of the total particulate organic carbon). Type I MOB were especially abundant during autumn mixing, forming on average 32% — but occasionally up to 73% — of the carbon bio- mass. Heterotrophic bacteria affiliated to Actinobacteria and Poly nucleobacter spp. together con- tributed 1 to 8% of the seasonal carbon biomass in the lake, although their genetic biomarkers were much more common. The results indicate that a significant proportion of the bacterial biomass was formed through methanotrophic and photoautotrophic production, both being comparable with the summer primary production rates of phytoplankton. These findings challenge the current model of the heterotrophic microbial pathway as the principal basis for microbial mobilisation of carbon and energy in freshwater food webs in oxygen-stratified lakes.

Methane-oxidizing and photoautotrophic bacteria are major producers in a humic lake with a large anoxic hypolimnion