Heterotrophic microbial activity in lake sediments: effects of organic electron donors

Allochthonous and autochthonous organic matter deposited in benthic sediments are mineralized by microbial communities, resulting in release of nutrients to the water column. Lakes with different trophic states may have sediments with different carbon and nutrient concentration with consequently different microbial communities. Microbial diversity of surface sediments of three subtropical lakes of different trophic state was investigated by measuring catabolic response to a wide variety of carbon-substrates. Basal carbon dioxide and methane production rates were highest in Lake Apopka (hypereutrophic), followed by Lake Annie (oligo-mesotrophic) and Lake Okeechobee (eutrophic) sediments. The oligo-mesotrophic Lake Annie showed the highest metabolic quotient (qCO2; proportion of basal respiration per unit of microbial biomass, 0.008 ± 0.001) indicating inefficient use of energy. The low qCO2 found in Lake Apopka sediment indicated higher efficiency in using energy. Lake Okeechobee sediments had intermediary values of qCO2 (M9 0.005 ± 0.001; M17 0.006 ± 0.0003; KR 0.004 ± 0.001) as compared with other lakes (lake Apopka 0.004 ± 0.14). Lake Apopka’s sediment catabolic diversity was higher than that observed in other sediments. Addition of organic electron donors to sediment samples from all lakes stimulated heterotrophic activity; however, the extent of the response varied greatly and was related to microbial biomass. The hypereutrophic Lake Apopka sediments had the highest respiration per unit of microbial biomass with the addition of electron donors indicating that these sediments respired most of the C added. These results showed that sediments with different biogeochemical properties had microbial communities with distinct catabolic responses to addition of the C sources.