Photosynthesis and inorganic carbon accumulating mechanisms in marine intertidal macroalgae
The characteristics of photosynthesis and inorganic carbon accumulation were investigated in two species of marine intertidal macroalgae: Porphyxa umbilicalis and Ulva lactuca. Both species have a similar mid-intertidal location, although P. umbilicalis grows on bare rock faces while U. 1actuca-is found submersed-in rock pools. The similar morphology of the species allows a direct comparison of both the gas exchange and the light response characteristics. For P. umbilicalis-the low light compensation and saturation points are comparable to the light requirements of shade adapted terrestrial plants. Photosynthetic capacity is greater-in seawater although the efficiency of light utilization is no higher than that in air. In the summer population, however, the photosynthetic response is reduced. In the absenceof-any corresponding effect on the the light requirements, this-can be related in part to seasonal variation in the-total chlorophyll concentration. Even after acclimation the summer-population can be photoinhibited by periods of excess PAR. P. umbilicalis is capable of using both HC03 and C02 as a substrate for photosynthesis. The maximum photosythetic capacity is greater in air, and corresponds to a higher affinity-for'C02. The gas exchange characteristics are 02-insensitive indicating low photorespiratory activity in this species. The-suppression of RuBPc-oxygenase may be attributable, to-an effective method of inorganic carbon uptake. The mechanism in P. umbilicalis involves the diffusive uptake of C02, supplied directly or by the Carbonic Anhydrase (CA) dependent catalysis of HC03-, external to the plasma membrane. An intracellular CA maintains the rate of transfer of inorganic carbon to the site C02 fixation in the chloroplast. U. 1actuca shows very similar light response characteristics to those of P. umbilicalis. The low light-requirements are the same as for the summer population, while the decrease in the Vmax of the response is again independent of an effect on the photosynthetic efficiency. The change in the capacity corresponds to a seasonal variation in the total chlorophyll content and corresponding ratio of chlorophyll a: b. Photoinhibition of the photosynthetic response significantly alters the characteristics of the light response, even though this species has the ability to maintain efficient rates of carbon fixation. In U. 1actuca the mechanism of inorganic carbon accumulation involves the direct uptake of both HC03- and C02, independent of any external CA activity. C02 will diffuse across the plasma membrane, while HC03- must be actively transported. Internal CA activity modifies the photosynthetic capacity of this species although more than one possible location is proposed for this enzyme. The difference in the characteristics of photosynthesis and inorganic carbon accumulation may be determined by the specific habitat of the two species. P. umbilicalis, more often exposed, shows a greater photosynthetic efficiency in air and a higher affinity for C02. In contrast, the photosynthetic system in U. 1actuca appears to be adapted to longer periods of submersion. In relation to the inorganic carbon accumulating mechanism, there is evidence that those marine intertidal macroalgae more often exposed have a greater affinity for C02, dependent to a degree on the activity of an external CA.