| Light is the ultimate source of energy for the growth and development of all living things and is a limiting factor in phototrophic metabolism. In water, the light intensity and quality vary with depth. Wavelengths of light lower than 300 nm and higher than 600 nm are readily absorbed or scattered within the top few meters of water. The metabolic rate of anaerobic photosynthetic bacteria is coupled to the selective absorption of wavelengths by bacteriochlorophylls and carotenoids. In this study, we examined the effects of different light regimes on representative cultures of anaerobic phototrophs that synthesize bacteriochlorophylls a, b, c, d, and e. Cultures were placed in full spectrum light, in infrared radiation (IR) (wavelengths above 700 nm), in photosynthetically active radiation (PAR) (wavelengths between 350-700 nm), and in darkness. Purple bacterial cultures that live in shallow waters had higher maximum population concentrations when grown under IR than any of the other light conditions, suggesting that these organisms are more dependent on their bacteriochlorophylls (a or b) for light harvesting than they are on their carotenoids. Green sulfur bacteria grow in the deeper regions of lakes than purple bacteria and are thus more dependent upon carotenoids for light harvesting. As expected, no growth under PAR only conditions was observed in cultures of the green sulfur bacteria; however, they generally grew best in a mixture of 40% IR and 60% PAR, apparently due to their utilization of carotenoids for additional energy. Our findings are generally consistent with the known ecology of anaerobic phototrophic bacteria. |
