Light fluctuations and the photosynthesis of marine algae. II. Photosynthetic response to frequency, phase ratio and amplitude

Photosynthesis of three marine macroalgae (Hormosira banksii [Turn.] Dene., Carpophyllum maschalocarpum [Turn.] Grev., Ecklonia radiata [C.Ag.] J.Ag.) was measured by O-2 electrode and infra-red gas analysis (IRGA) methods in both steady and fluctuating light regimes Using immersed material of Carpophyllum or Ecklonia in the O-2 electrode the response of mean photosynthetic rate ((P) over bar) to changes in the frequency (i.e. phase period, T) at different relative amplitudes of fluctuating light was examined. (P) over bar was also compared to rates predicted from steady state responses on the basis of either integration of photosynthate (P-S) or integration of irradiance (P-f). In alternations of saturating and limiting light (defined with reference to steady P-I response of the same plant material) (P) over bar was > P-S at T < 300 s, increased markedly as T decreased, and approached P-f at T = 0.25 s. The enhancement of <(P)over bar> at high frequencies was less marked in alternations of sub-saturating and limiting light. (P) over bar showed little or no change with T for alternations of irradiance within the limiting range. For emersed material in the IRGA the response of (P) over bar to T showed a dependence on relative amplitude of light fluctuation similar to that obtained in the O-2 electrode. Frequency response patterns ((P) over bar versus log T) in saturating/limiting amplitudes were comparable for all three species. Irrespective of the amplitude of the fluctuations (P) over bar -> P-S at T > 300s and (P) over bar -> P-f at T < 0.25s. Enhancement of <(P)over bar> at high and intermediate frequencies is attributed to the integrated influence of two transients of CO2 exchange viz. induction loss and post-illumination fixation. The complex interaction of these and other transients such as post-illumination bursts of CO2 means that previous models of the (P) over bar versus T response based only on two time constants are probably inadequate. However, for these macroalgae, a modified form of the Thornley (1974) model can be used to predict (P) over bar at the extremes of the frequency range.