Growth and bacteriochlorophyll a formation in taxonomically diverse aerobic anoxygenic phototrophic bacteria in chemostat culture: Influence of light regimen and starvation

Abstract

The influence of light and starvation on bacteriochlorophyll a (bchl a) and biomass formation of some aerobic anoxygenic photosynthetic bacteria was investigated in chemostat culture. Three species recently isolated from dinoflagellate cultures were compared, Dinoroseobacter shibae, which contained relatively high amounts of bchl a, and Stappia sp. DFL-11 and Hoeflea phototrophica, which both have very low amounts of photosynthetic pigments. Simulating day/night alternation D. shibae was subjected to 7 light:dark cycles = 8 h:16 h and 3 light:dark cycles = 16 h:8 h. Using a dilution rate of 0.1 h −1 and succinate as a substrate (8.5 mmol/l) the concentration of bchl a decreased during the light period due to inhibition by light and recovered always to about the previous level during the dark period. Biomass increased during the light period showing that additional energy was generated in the light. Nutrient deficiency in the absence or presence of light was studied by exposing continuous cultures growing in the dark under the conditions described above to 8 h of illumination, starvation, i.e. interruption of medium supply, or both. The cultures of D. shibae exhibited a bchl a base level of 2.5 nmol/mg protein, which decreased reversibly in the light and increased significantly during starvation in the dark to reach a level of 4 nmol/mg protein 16 h after medium supply was resumed, indicating a slow regulatory response towards periodic starvation in D. shibae. Under simultaneous illumination and starvation conditions, these two effects apparently cancelled one another out, resulting in unchanged levels of bchl a. By contrast, the cultures of Stappia sp. and H. phototropica were characterized by bchl a contents of almost zero in the dark, little change under illumination or starvation, but a very marked increase after simultaneous illumination and starvation, reaching 0.16 and 0.24 nmol bchl a/mg protein, respectively, 16 h after medium supply was resumed. These data suggest the presence of a regulatory mechanism integrating light stimuli and starvation related metabolic signals. The different physiological behaviour of the two groups is discussed in terms of the significance of aerobic anoxygenic photosynthesis in their natural environment.