Algae have adopted two primary strategies to maximize the performance of Rubisco in photosynthetic CO2 fixation. This has included either the development of a CO2-concentrating mechanism (CCM), based at the level of the chloroplast, or the evolution of the kinetic properties of Rubisco. This review examines the potential diversity of both Rubisco and chloroplast-based CCMs across algal divisions, including both green and nongreen algae, and seeks to highlight recent advances in our understanding of the area and future areas for research. Overall, the available data show that Rubisco enzymes from algae have evolved a higher affinity for CO2 when the algae have adopted a strategy for CO2 fixation that does not utilise a CCM. This appears to be true of both Green and Red Form I Rubisco enzymes found in green and nongreen algae, respectively. However, the Red Form I Rubisco enzymes present in nongreen algae appear to have reduced oxygenase potential at air level of O2. This has resulted in a photosynthetic physiology with a reduced potential to be inhibited by O2 and a reduced need to deal with photorespiration. In the limited number of microalgae that have been examined, there is a strong correlation between the existence of a high-affinity CCM physiology and the presence of pyrenoids in all algae, highlighting the potential importance of these chloroplast Rubisco-containing bodies. However, in macroalgae, there is greater diversity in the apparent relationships between pyrenoids and chloroplast features and the CCM physiology that the species shows. There are many examples of microalgae and macroalgae with variations in the presence and absence of pyrenoids as well as single and multiple chloroplasts per cell. This occurs in both green and nongreen algae and should provide ample material for extending studies in this area. Future research into the function of the pyrenoid and other chloroplast features, such as thylakoids, in the operation of a chloroplast-based CCM needs to be addressed in a diverse range of algal species. This should be approached together with assessment of the coevolution of Rubisco, particularly the evolution of Red Form I Rubisco enzymes, which appear to achieve superior kinetic characteristics when compared with the Rubisco of C3 higher plants, which are derived from green algal ancestors.Key words: Rubisco, CO2-concentrating mechanism, carbonic anhydrase, aquatic photosynthesis, algae, pyrenoids, inorganic carbon.

The oxygen isotope compositions of three wood constituents (the solvent-extractable portion, lignin and α-cellulose) were measured for samples collected from Quercus and Pinus trees around the world. Among Pinus samples all wood constituents were positively related to modelled δ18O of source water at the site, while among Quercus samples whole wood, lignin and α-cellulose showed positive relationships. The data support the hypothesis that many oxygen atoms in lignin exchanged with unenriched stem water during synthesis, rather than retaining the full isotopic signal from the molecular oxygen added during hydroxylation of the aromatic ring. The data also suggest that extraction of α-cellulose from wood samples is unnecessary for isotope studies looking at correlations with site parameters.

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A simple, but very reproducible, method for measuring the relative specificity of ribulosebisphosphate carboxylase-oxygenase for CO2, as opposed to O2, is described. The method uses [1-14C]ribulose bisphosphate as substrate and combines the advantages of supplying both gaseous substrates from the gas phase with HPLC separation of the labelled products. Volumetric or gravimetric accuracy is not required at any stage of the procedure and variations in ionic strength and pH have little effect on the measurements. This leads to excellent reproducibility without the need for normalisation. The average standard deviation was 1.3% of the measured CO2/O2 specificity. Use of very low ribulose bisphosphate concentrations ensures that the gaseous substrates cannot be depleted appreciably during the reaction and enhances the attractiveness of the procedure for measurements with crippled mutant enzymes. The procedure's ability to resolve small differences in relative specificity is demonstrated by its easy detection of the 5% increase in specificity that accompanies substitution of four residues at positions 338-341 of the cyanobacterial large subunit with the analogous higher-plant residues. This resolving power is essential for detecting small differences in the specificities of higher-plant ribulosebisphosphate carboxylases which may be the signature of continuing evolutionary refinement.

Efficiencies of photosynthetic energy conversion were determined in sun and shade leaves of several mangrove species, growing in an open intertidal habitat in North Queensland, by measuring the maximum photon yield of O2 evolution and 77K chlorophyll fluorescence characteristics. Preliminary meas- urements confirmed that mangrove leaves have low water potentials, low stomatal conductances and low light-saturated CO2 exchange rates. Mangrove sun leaves therefore received a very large excess of excitation energy. Mangrove shade leaves had as high a photon yield of O2 evolution as non-mangrove leaves and their fluorescence characteristics were normal, showing that the energy conversion efficiency was unaffected by the high salinity. Mangrove sun leaves had markedly depressed photon yields and fluorescence was severely quenched showing that the efficiency of the photochemistry of photosystem II was reduced. The efficiency of energy conversion decreased with an increased radiation receipt. No such depression was detected in sun leaves of non-mangrove species growing in adjacent non-saline sites. Shading of man- grove sun leaves resulted in an increase in the efficiency of energy conversion but, in most species, more than 1 week was required for these leaves to reach the efficiency of shade leaves. Leaves exposed to direct sunlight had somewhat higher efficiencies in mangrove plants cultivated in 10% seawater as compared with full-strength seawater but the salinity of the culture solution had little effect on the increase in the efficiency upon shading. Field and laboratory fluorescence measurements indicated that the reduced efficiency of energy conversion in mangrove sun leaves resulted from a large increase in the rate constant for radiationless energy dissipation in the antenna chlorophyll rather than from damage to the photosystem II reaction centres. We propose that this increase in radiationless energy dissipation serves to protect the reaction centres against damage by excessive excitation.