Aspects of photoadaptation in the intertidal red alga Gracilaria chilensis
The intertidal red alga, Gracilaria chilensis Bird, McLachlan et Oliveira (Rhodophyta, Gracilariales), lives in an environment in which light is highly variable in terms of both amplitude and duration. A laboratory investigation of the photophysiology of G. chilensis was conducted to assess the response of the photosynthetic apparatus to light variability characteristic of the natural environment. Freshly collected Gracilaria chilensis was found to exhibit an endogenous rhythm of photosynthesis in conditions of constant limiting light and temperature. However, such a phenomenon was not observed in saturating light. Rhythms of phycobiliprotein concentration and dark respiration were also observed but were not as well-defined and could nor account for the photosynthetic rhythm. The photosynthetic response of G. chilensis to light fluctuations of various durations (0.25 to 900 seconds) and light levels was compared to that in static light. G. chilensis was able to utilise rapidly fluctuating light (< 1 second) more efficiently than fluctuations of longer duration (60-900 second). Mean photosynthetic rates were enhanced by up to l50% in fluctuating light of less than 60 sec duration over that predicted from steady-state. The photosynthetic apparatus of freshly collected G. chilensis was found to have many low-light "shade" acclimation characteristics. These included a low compensation point (5 µmolm-2 s-l) and onset of saturation (80 µmolm-2 s-l) suggesting sensitivity to photoinhibition. However, (laboratory) low-light acclimated G. chilensis was able to tolerate periods of constant high light (2000 µmolm-2 s-l)for periods of six hours without detectable detrimental effect on photosynthetic capacity, although photosynthetic efficiency was significantly inhibited after two hours of this treatment. The time course and characteristics of photoacclimation were determined by culturing G. chilensis in low- (15 µmol m-2 s-l) and high- (180 µmol m-2 s-l) light and high- and low-nitrogen regimes. The observed change in photosynthetic characteristics and pigment concentration indicated that acclimation began after a time lag of l-2 days, was complete after approximately a week and was reversible. Acclimation to growth light included changes in growth rate, P-I response curves, pigment concentration and composition and other biochemical components (e.g. carbon/nitrogen ratio). The nitrogen regime significantly affected pigment concentration in the high-light grown plants and the response suggests pigments play a role in nitrogen storage as well as light harvesting. These various physiological characteristics described above were interpreted as important mechanisms that enable G. chilensis to optimise photosynthetic response in the highly dynamic and stressful zone of the intertidal environment.