In situ specific loss and growth rates of purple sulfur bacteria in Lake Cisó

Abstract Growth rates and population dynamics of phototrophic bacteria in Lak Cisó were analysed by measuring bacterial abundances and determining specific rates of growth and loss. Net growth rates were calculated from actual changes in biomass assuming exponential growth. Values ranged between −0.072 and 0.037 per day (d⁻¹) for Chromatium , and between −0.043 and 0.022 d⁻¹ for Amoebobacter . Exponential loss rates through sedimentation, decomposition and washout were determined independently. Values ranged between 0 and 0.025 d⁻¹ in the case of Chromatium and between 0 and 0.015 d⁻¹ in the case of Amoebobacter . Finally, gross growth rates were calculated by adding net growth to losses. Maximal values were 0.063 d⁻¹ for Chromatium and 0.037 d⁻¹ for Amoebobacter . In the case of Chromatium , population growth rates were found to be correlated with the amount of light available per unit of growing biomass. It was concluded that growth of phototrophic bacteria in Lake Cisó was limited by light availability. Altogether, purple sulfur bacteria seemed to maintain a very large biomass with very slow growth, thanks to very slow losses during stratification. During holomixis the situation was more dynamic. Washout of cells and disappearance of algal cells allowed more light to reach the bacteria. Therefore, high growth rates were found towards the end of the winter. A similar pattern repeated itself from year to year. These are the first estimates of in situ growth rates for populations of phototrophic bacteria.