Photosynthetic use of light and inorganic carbon in air and water by the intertidal alga Petalonia fascia (Scytosiphonales, Phaeophyta)

The photosynthetic performance of the intertidal alga Petalonia fascia (0. F. Muller) Kuntze (Scytosiphona-ceae, Phaeophyta) has been investigated, both in air and water, by analyzing the relationship between apparent photosynthesis rate and photon irradiance and inorganic carbon. In relation to the use of photon irradiance, it was found that the net photosynthetic capacity in water was 5.7 times that in air (fully hydrated thallus). The light compensation point was achieved at 5.9 and 3.0 μmol photons m^?2 s^?1 in air and water, respectively. The light onset-saturation parameter and the photosynthetic efficiency were 77% and 25% greater in water than in air, respectively. The dark respiration rate was one-third greater when emersed in comparison to submersion conditions. These data suggest that the photosynthetic response to irradiance in P. fascia is similar to that in infralittoral species rather than the intertidal species. This assessment can be explained by the winter seasonality of the bladed stage of growth, when storms and waves permit a permanent hydrated status of P. fascia that in the intertidal zone. Moreover, the minimum tissue water content that permitted active photosynthesis in the alga was around 20%. The net photosynthetic capacity as a function of inorganic carbon (C) concentration in water was 1.5 times that in air. Photosynthesis was saturated in both media with respect to the availability of inorganic C in natural conditions. The affinity to inorganic C, and the carbon conductance, were two orders of magnitude higher in air than in water. However, the higher photosynthetic capacity when submerged in comparison to emersion conditions suggests that P. fascia can assimilate the external HCO₃,– or the occurrence of a CO₂ concentrating mechanism in this species.