Change in Size of Chromatium minus Cells in Relation to Growth Rate, Sulfur Content, and Photosynthetic Activity: A Comparison of Pure Cultures and Field Populations.

The size frequency distribution of planktonic cells of purple sulfur phototrophic bacteria was measured at several depths in a bacterial layer of Lake Cisó (Spain). The bacterioplankton was dominated by Chromatium minus (87 to 94% of the total biomass). The largest cells of C. minus were found in the top part of the bacterial layer. In addition, the in situ and potential specific photosynthetic activity (CO(2) fixation and acetate uptake) and specific pigment content were measured in relation to several key environmental parameters that determine the activity of cells. Potential growth rates were estimated from production rates and biomass. A maximal specific growth rate of 0.074 h was found for the top part of the bacterial layer. Photosynthesis versus light and versus sulfide curves among field samples indicated that light was the main limiting factor controlling the activity of C. minus in Lake Cisó. The specific bacteriochlorophyll a content was very high in all samples (0.27 to 0.36 mug mug of C). Results of laboratory experiments performed with pure cultures indicated that the average cell volume changes from 5.9 to 20.0 mum and that differences in growth rate, breakdown, or synthesis of sulfur and glycogen and degradation of the photosynthetic apparatus are the main factors accounting for the observed changes in cell volume across the bacterial layer.