RESUMO
Ten species of brown macroalgae (five eulittoral and one submersed species of the Fucales; four submersed species of the Laminariales) from a rocky shore at Arbroath, Scotland, were examined for characteristics of emersed photosynthesis in relation to the partial pressure of CO2 and O2. The five eulittoral species of the Fucaceae were approaching CO2 saturation for light-saturated photosynthesis at normal air levels of CO2 (35 Pa) in 21 kPa O2. The normally submersed algae are further from CO2 saturation under these conditions, especially in the case of the four members of the Laminariales. The rate of net photosynthesis in the Fucaceae is O2-independent in the range 2-21 kPa O2 over the entire range of CO2 partial pressure tested (compensation up to 95 Pa). For the other five algae tested, net photosynthesis is slightly inhibited by O2 at 21 kPa relative to 2 kPa over the entire range of CO2 partial pressures tested (compensation up to 95 Pa). CO2 compensation partial pressures are low (<0.5 Pa) for the Fucaceae and independent of O2 in the range 2-42 kPa. For the other five algae, the CO2 compensation partial pressure are higher, and increased with O2 partial pressure in the range 2-42 kPa. These gas exchange data show that the Fucaceae exhibit more C4-like characteristics of their photosynthetic physiology than do the other five species tested, although even the Laminariales and Halidrys siliquosa are not classic C3 plants in their photosynthetic physiology. These data suggest that, in emersed conditions as well as in the previously reported work on submersed photosynthesis, a "CO2 concentrating mechanism" is operating which, by energized transmembrane transport of inorganic C, accumulates CO2 at the site of RUBISCO and, at least in part, suppresses the oxygenase activity. Work with added extracellular carbonic anhydrase (CA), and with a relatively membrane-impermeant inhibitor of the native extracellular CA activity (acetazolamide), suggests that, in emersed conditions as well as in the previously reported work on algae submersed in seawater at pH 8, HCO inf3sup- is the major inorganic C species entering the cell. At optimal hydration, the rate of emersed photosynthesis in air is not less than the rate of photosynthesis when submersed in seawater, at least for the Fucaceae. δ13C ratios of organic C for the Fucaceae are slightly more negative than is the case for the other five algae; these data are consitent with substantial (half or more of the entering inorganic C) leakage of CO2 from the accumulated pool, and with some contribution of atmospheric CO2 to the organic C gain by the eulittoral algae. The predicted increase in N use efficiency of photosynthesis in the Fucaceae, with their more strongly developed CO2 concentrating mechanism, is consistent with data on emersed, but not submersed, photosynthesis for the algae collected from the wild and thus at a poorly defined N status. The more C4-like gas exchange charateristics of photosynthesis in the eulittoral Fucaceae may be important in increasing the water use efficiency of emersed photosynthesis from the limited capital of water available for transpiration by a haptophyte.
RESUMO
Characteristics of inorganic carbon assimilation by photosynthesis in seawater were investigated in six species of the Fucales (five Fucaceae, one Cystoseiraceae) and four species of the Laminariales (three Laminariaceae, one Alariaceae) from Arbroath, Scotland. All of the algae tested could photosynthesise faster at high external pH values than the uncatalysed conversion of HCO 3- to CO2 can occur, i.e. can "use" external HCO 3- . They all had detectable extracellular carbonic anhydrase activity, suggesting that HCO 3- use could involve catalysis of external CO2 production, a view supported to some extent by experiments with an inhibitor of carbonic anhydrase. All of the algae tested had CO2 compensation concentrations at pH 8 which were lower than would be expected from diffusive entry of CO2 supplying RUBISCO as the initial carboxylase, consistent with the operation of energized entry of HCO 3- and / or CO2 acting as a "CO2 concentrating mechanism". Quantitative differences among the algae examined were noted with respect to characteristics of inorganic C assimilation. The most obvious distinction was between the eulittoral Fucaceae, which are emersed for part of, or most of, the tidal cycle, and the other three families (Cystoseiraceae, Laminariaceae, Alariaceae) whose representatives are essentially continually submersed. The Fucaceae examined are able to photosynthesise at high pH values, and have lower CO2 compensation concentrations, and lower K1/2 values for inorganic C use in photosynthesis, at pH 8, than the other algae tested. Furthermore, the Fucaceae are essentially saturated with inorganic C for photosynthesis at the normal seawater concentration at pH 8 and 10°C. These characteristics are consistent with the dominant role of a "CO2 concentrating mechanism" in CO2 acquisition by these plants. Other species tested have characteristcs which suggest a less effective HCO 3- use and "CO2 concentrating mechanism", with the Laminariaceae being the least effective; unlike the Fucaceae, photosynthesis by these algae is not saturated with inorganic C in normal seawater. Taxonomic and ecological implications of these results are considered in relation to related data in the literature.