Growth and photosynthetic rates of Phaeodactylum tricornutum Bohlin in a cyclical light field

Growth rates, cell chlorophyll a concentrations, and carbon uptake rates of Phaeodactylum tricornutum Bohlin grown in cyclical and constant light regimes at three mean irradiances (PFD) were compared. Light cycles with periods 5–38 min were used. Under high PFD the cyclical light regime resulted in both increased and decreased growth rates dependent on the signal frequency. Growth rates were universally depressed under mid and low PFD. Minimal influence of the light field was apparent on cell chlorophyll concentrations. The most marked effect of the fluctuating light was recorded on the carbon uptake rates with maximal influence demonstrated in those samples both grown and incubated in fluctuating light at high as opposed to mid and low PFDs. From the combination of light fields employed, the existence of both short and long term physiological controls of the carbon uptake rates were deduced, these being related to the time scales of carbon uptake and cell growth. A partial explanation to account for the observed increases in carbon uptake in terms of short period non-linear uptake kinetics was derived from two further experiments.