Influence of light on bacterioplankton production and respiration in a subtropical coral reef

The influence of sunlight on bacterioplankton production [ super(14)C-leucine (Leu) and super(3)H-thymidine (TdR) incorporation; changes in cell abundances] and O sub(2) consumption was investigated in a shallow subtropical coral reef located near Key Largo, Florida, USA. Quartz (light) and opaque (dark) glass biological oxygen demand (BOD) bottles containing 0.8 mu m filtered reef water amended with C, N and P were incubated in situ and exposed to natural variations in solar radiation over a 48 h period. Photoinhibition of Leu and TdR incorporation was observed at all depths during both daylight periods. Photoinhibition of bacterial production decreased with depth and was significantly higher during the first day of exposure. Bacterial abundances also decreased during daylight periods particularly during the second day of exposure. Leu and TdR incorporation rates and bacterial abundances exhibited recovery during periods of darkness. Light treatment bacterial O sub(2) consumption was inhibited at all depths during Day 1 but enhanced relative to dark treatments at all depths during Day 2. Estimates of light treatment bacterial gross growth efficiencies (GGE) determined during the evening of Day 1 were similar to dark treatment estimates. Light treatment GGE determined during Day 2, however, were lower than dark treatments but increased with depth. Recovery of bacterial production and respiration during the second day of exposure suggested photoinduced selection for light tolerant cells and/or physiological adaptation to ambient light regimes occurred over the duration of exposure. The results of this experiment suggested that solar radiation may have a significant effect on bacterial metabolism in this shallow euphotic marine ecosystem.