Environmental forcing versus endogenous control of photosynthesis in intertidal epilithic microalgae

Abstract

Photoadaptation and photosynthetic capacity of an intertidal epilithic microflora were measured hourly over a period of 8 d. The algae were photoadapted to the dim light intensities experienced when submerged and showed little resistance to the extreme irradiances received when exposed. During neap tide when the community was permanently submerged, the photosynthetic response of algae collected hourly in the field followed a circad~an cycle. The endogenous control of this cycle was demonstrated by isolation experiments. During spring tide, photosynthet~c processes were photoinactivated at low tide when extreme irradiances reached the algae. These results lead to the general hypothesis that short-term variability in the photosynthetic capacity of intertidal epilithic microalgae is controlled primarily by endogenous phasing at neap tide and tidal forclng a t spring tide. Time senes of in situ production were reconstructed from solar irradiance measurements and hourly estimates of the parameters of the Photosynthesis-Irradiance curve. Despite the additional stress of low tide exposure, average daily production was comparable between spring and neap tides.