An endogenous periodicity exhibited in the activity of a natural bacterioplankton community isolated in mesocosms

In July 1996, bacterial abundance and incorporation of [H-3]thymidine (H-3-TdR) were determined every 4 h during a mesocosm experiment initially designed to study the effects of different intensities of ultraviolet-B (UVB) radiation on the summer planktonic community of the lower St. Lawrence Estuary. Water was obtained from the quay of the Maurice Lamontagne Institute (Mont-joli, Que.) and incubated in experimental mesocosms (1500 L total volume, n = 8) with continuous mixing provided by a pumping system,During 72 h, different UVB intensities showed no significant effects on the bacterial incorporation of H-3-TdR. This indicates that in the presence of other trophic levels and with continuous mixing, bacterioplankton responses to UVB are substantially different from those reported in axenic bacterial cultures or even whole-water incubations exposed to UVB at fixed depths. In conjunction with this observation, H-3-TdR incorporation exhibited a significant periodic variation within all experimental treatments. The periodicity consisted of a 16-h cycle occurring independently of the time of the day. When the H-3-TdR incorporation was normalized to cell abundance, the resulting cell-specific thymidine incorporation exhibited the same periodic oscillatory pattern. On the other hand, other factors suspected of inducing such a variability showed no consistent oscillation. In addition to suggesting an endogenously controlled activity of the studied bacterial community, the results of the present study indicate that failure of taking temporal variations of bacterial activity into account may introduce an error of almost 50% in the estimation of the daily thymidine incorporation rates. This represents a considerable error, because several studies rely on this measurement to estimate bacterial carbon production and to establish carbon budgets within different oceanic provinces.