Photosynthetic rapid light curves for Padina sanctae-crucis vary with irradiance, aerial exposure, and tides in Hawaii's micro-intertidal zones
Résumé
Environmental variations can have major consequences on photosynthesis and in turn impact the settlement, productivity, or survival of plants in the intertidal zones. Low tides in Hawaii fluctuate < 1 m yet expose intertidal macroalgae to high tropical temperatures and irradiances, which are predicted to have negative physiological consequences. To better understand environmental variations, temperature, exposure duration, and irradiance were measured in two intertidal zones on O'ahu over 8 days with negative tidal heights. Then to test whether these conditions were stressful, the photosynthetic response (via pulse amplitude modulated fluorometry) and relative water content of a common intertidal macroalgae, Padina sanctae-crucis, was measured over varying tidal heights. We found that P. sanctae-crucis was exposed to air from 0 to 6 h on days with tropical irradiance (237-2000 A mu mol photon m(-2) s(-1)) and a range of temperatures (20.4-32.3 A degrees C), yet plants were able to maintain relatively high water content. Photosynthetic parameters for P. sanctae-crucis derived from light response curves were found to vary with irradiance. High irradiances were associated with lowered maximum rates of electron transport (rETR(max)) and effective quantum yield (I broken vertical bar (PSII)). It was also determined that macroalgae exhibited different curves with tidal state. P. sanctae-crucis had relatively increased saturation irradiances (E (k)) and increased rETR(max) but lowered effective quantum yield (I broken vertical bar (PSII)) during negative tidal heights when aerial exposure is more common. P. sanctae-crucis was then exposed to air for up to 40 min in a laboratory experiment to determine the effect on fluorescence parameters. The 40-min aerial exposure, a duration smaller than which occurs at low tide, resulted in reduced rETR(max), alpha, and I broken vertical bar (PSII). This manipulation combined with field observations indicate that aggregations of thalli combined with water motion and minimal tidal fluctuations help to limit water loss and maintain photosynthetic rates. From these results, we can conclude, for this system, high irradiances are a major factor that likely limits production.