Continuous monitoring of growth detects photoperiod-dependent oscillations in growth rates of Chlorella vulgaris

Abstract

The green alga Chlorella vulgaris Beij. exhibits minimal capacity to adjust exponential growth rates in response to photon flux density (PFD) when monitored on a discontinuous basis. We hypothesized that modulation of maximum growth rates in C. vulgaris by PFD is a photoperiod-dependent phenomenon. The use of the photobioreactors to monitor continuous growth allowed us to detect repetitive daily oscillations in growth which were photoperiod-dependent. The rate of change in optical density (OD735) during the daily light period was two-fold greater in cells grown at 28 °C with a PFD of either 2000 or 150 μmol photons·m−2·s−1 when C. vulgaris was grown under a daily light–dark cycle. Concomitantly, oscillations of the chlorophyll fluorescence parameters paralleled the oscillations observed in growth rate. When cultures were shifted from a 12 h photoperiod with low light to continuous light (CL), the growth oscillations disappeared. In contrast, oscillations in the fluorescence parameters persisted even after the shift from a 12 h photoperiod to CL. We suggest that the nocturnal catabolism of starch reserves in conjunction with changes in cellular volume coupled with the diurnal changes in DNA content, as quantified by changes in Vybrant Green fluorescence yield, indicate that these growth oscillations reflect synchronized cellular division in C. vulgaris that is not evident when growth is assayed discontinuously.