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 desig ned to study the effects of different intensities of ultraviolet-B (UVB) ra diation on the summer planktonic community of the lower St. Lawrence Estuar y. Water was obtained from the quay of the Maurice Lamontagne Institute (Mo nt-joli, Que.) and incubated in experimental mesocosms (1500 L total volume , n = 8) with continuous mixing provided by a pumping system,During 72 h, d ifferent UVB intensities showed no significant effects on the bacterial inc orporation of H-3-TdR. This indicates that in the presence of other trophic levels and with continuous mixing, bacterioplankton responses to UVB are s ubstantially 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 perio dic 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-s pecific thymidine incorporation exhibited the same periodic oscillatory pat tern. On the other hand, other factors suspected of inducing such a variabi lity showed no consistent oscillation. In addition to suggesting an endogen ously controlled activity of the studied bacterial community, the results o f 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 c onsiderable error, because several studies rely on this measurement to esti mate bacterial carbon production and to establish carbon budgets within dif ferent oceanic provinces.