title: Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications.
abstract: 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 HCO3-to CO2can occur, i.e. can "use" external HCO3-. They all had detectable extracellular carbonic anhydrase activity, suggesting that HCO3-use could involve catalysis of external CO2production, a view supported to some extent by experiments with an inhibitor of carbonic anhydrase. All of the algae tested had CO2compensation concentrations at pH 8 which were lower than would be expected from diffusive entry of CO2supplying RUBISCO as the initial carboxylase, consistent with the operation of energized entry of HCO3-and / or CO2acting as a "CO2concentrating 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 CO2compensation concentrations, and lower K1/2values 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 "CO2concentrating mechanism" in CO2acquisition by these plants. Other species tested have characteristcs which suggest a less effective HCO3-use and "CO2concentrating 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.
authors.affiliation: Department of Biological Sciences, University of Dundee, DD1 4HN, Dundee, Scotland, UK.
authors.forename: Misni B
authors.initials: MB
authors.lastname: Surif
authors.affiliation: Department of Biological Sciences, University of Dundee, DD1 4HN, Dundee, Scotland, UK.
authors.forename: John A
authors.initials: JA
authors.lastname: Raven
keywords: Carbon dioxide compensation concentration; Carbonic anhydrase; Ecology; Inorganic carbon assimilation; Phacophyta; Taxonomy