FRANKLIN, LA, SEATON, GGR, LOVELOCK, CE & LARKUM, AWD 1996, 'Photoinhibition of photosynthesis on a coral reef', Plant, Cell & Environment, vol. 19, no. 7, pp. 825-836.
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ABSTRACTPhotoinhibition of macroalgae in the epilithic algal community (KAC) of coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. Fv/Fm (variable to maximum fluorescence, darkened samples) of shallow macroalgae declined by 50% on fine summer and winter days, recovering in late afternoon. Within a species, thalli from low‐light habitats were more photoinhibited (2h at 1400μimol m−2 s−1) than those from high‐light habitats. The sensitivity of Lobophora variegata (Phacophyta) and Chlorodesmis fastigiata (Chlorophyta) increased with depth (1 versus 20 m). However, shallow Halimeda tuna (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas.Photoinhibition and recovery were depth‐ and species‐specific. Shallow Lobophora and Chlorodesmis maintained a greater degree of QA oxidation during photoinhibition. In deep thalli, reduced effective quantum yield of open photosystem II centres reflected lower proportions and excitation capture efficiencies of open centres. In Lobophora, zeaxanthin formation accompanied non‐photochemical fluorescence quenching (NPQ), but in Chlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may compensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity.
James, PL & Larkum, AWD 1996, 'Photosynthetic inorganic carbon acquisition of Posidonia australis', Aquatic Botany, vol. 55, no. 3, pp. 149-157.
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Lockhart, PJ, Larkum, AW, Steel, M, Waddell, PJ & Penny, D 1996, 'Evolution of chlorophyll and bacteriochlorophyll: the problem of invariant sites in sequence analysis.', Proceedings of the National Academy of Sciences, vol. 93, no. 5, pp. 1930-1934.
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Competing hypotheses seek to explain the evolution of oxygenic and anoxygenic processes of photosynthesis. Since chlorophyll is less reduced and precedes bacteriochlorophyll on the modern biosynthetic pathway, it has been proposed that chlorophyll preceded bacteriochlorophyll in its evolution. However, recent analyses of nucleotide sequences that encode chlorophyll and bacteriochlorophyll biosynthetic enzymes appear to provide support for an alternative hypothesis. This is that the evolution of bacteriochlorophyll occurred earlier than the evolution of chlorophyll. Here we demonstrate that the presence of invariant sites in sequence datasets leads to inconsistency in tree building (including maximum-likelihood methods). Homologous sequences with different biological functions often share invariant sites at the same nucleotide positions. However, different constraints can also result in additional invariant sites unique to the genes, which have specific and different biological functions. Consequently, the distribution of these sites can be uneven between the different types of homologous genes. The presence of invariant sites, shared by related biosynthetic genes as well as those unique to only some of these genes, has misled the recent evolutionary analysis of oxygenic and anoxygenic photosynthetic pigments. We evaluate an alternative scheme for the evolution of chlorophyll and bacteriochlorophyll.
Lockhart, PJ, Steel, MA & Larkum, AWD 1996, 'Gene duplication and the evolution of photosynthetic reaction center proteins', FEBS Letters, vol. 385, no. 3, pp. 193-196.
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We investigate the evolutionary relationships between photosynthetic reaction center proteins (D1, D2, L and M) and demonstrate that the pattern of nucleotide substitution in these is more complicated than has been assumed in previous phylogenetic analyses. We show that there are serious violations of methodological assumptions in previous published studies. We conclude that there is equal support for hypotheses indicating (i) a single gene duplication of an ancestral reaction center protein followed by diversification and (ii) two independent gene duplications giving rise to proteins in oxygenic and anoxygenic systems.
Lockhart, PJ, Steel, MA & W.D. Larkum, A 1996, 'Gene duplication and the evaluation of photosynthetic reaction center proteins', FEBS Letters, vol. 390, no. 2, pp. 242-242.
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Lukins, PB, Post, A, Walker, PJ & Larkum, AWD 1996, 'P680+ reduction in oxygen-evolving Photosystem II core complexes', Photosynthesis Research, vol. 49, no. 3, pp. 209-221.
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The kinetics of P680+ reduction in oxygen-evolving spinach Photosystem II (PS II) core particles were studied using both repetitive and single- flash 830 nm transient absorption. From measurements on samples in which PS II turnover is blocked, we estimate radical-pair lifetimes of 2 ns and 19 ns. Nanosecond single-flash measurements indicate decay times of 7 ns, 40 ns and 95 ns. Both the longer 40 ns and 95 ns components relate to the normal S- state controlled Y(z) → P680+ electron transfer dynamics. Our analysis indicates the existence of a 7 ns component which provides evidence for an additional process associated with modified interactions involving the water- splitting catalytic site. Corresponding microsecond measurements show decay times of 4 μs and 90 μs with the possibility of a small component with a decay time of 20-40 μs. The precise origin of the 4 μs component remains uncertain but appears to be associated with the water-splitting center or its binding site while the 90 μs component is assigned to P680+-Q(A)- recombination. An amplitude and kinetic analysis of the flash dependence data gives results that are consistent with the current model of the oxygen- evolving complex.
Post, A, Lukins, PB, Walker, PJ & Larkum, AWD 1996, 'The effects of ultraviolet irradiation on P680+ reduction in PS II core complexes measured for individual S-states and during repetitive cycling of the oxygen-evolving complex', Photosynthesis Research, vol. 49, no. 1, pp. 21-27.
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Flash-induced absorbance measurements at 830 nm on both nanosecond and microsecond timescales have been used to characterise the effect of ultraviolet light on Photosystem II core particles. A combination of UV-A and UV-B, closely simulating the spectrum of sunlight below 350 nm, was found to have a primary effect on the donor side of P680. Repetitive measurements indicated reductions in the nanosecond components of the absorbance decay with a concomitant appearance and increase in the amplitude of a component with a 10 μs time constant attributed to slow reduction of P680+ by Tyr(z) when the function of the oxygen evolving complex is inhibited. Single-flash measurements show that the nanosecond components have amplitudes which vary with S-state. Increasing UV irradiation inhibited the amplitude of these components without changing their S-state dependence. In addition, UV irradiation resulted in a reduction in the total amplitude, with no change in the proportion of the 10 μs contribution.
Ritchie, RJ, Nadolny, C & Larkum, AWD 1996, 'Driving Forces for Bicarbonate Transport in the Cyanobacterium Synechococcus R-2 (PCC 7942)', Plant Physiology, vol. 112, no. 4, pp. 1573-1584.
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Abstract Air-grown Synechococcus R-2 (PCC 7942) cultures grown in BG-11 medium are very alkaline (outside pH is 10.0) and use HCO3- as their inorganic carbon source. The cells showed a dependence on Na+ for photosynthesis, but low Na+ conditions (1 mol m-3) were sufficient to support saturating photosynthesis. The intracellular dissolved inorganic carbon in the light was greater than 20 mol m-3 in both low-Na+ conditions and in BG-11 medium containing the usual [Na+] (24 mol m-3, designated high-Na+ conditions). The electrochemical potential for HCO3- in the light was in excess of 25 kJ mol-1, even in high-Na+ conditions. The Na+-motive force was greater than -12 kJ mol-1 under both Na+ conditions. On thermodynamic grounds, an Na+-driven co-port process would need to have a stoichiometry of 2 or greater ([greater than or equal to]2Na+ in/HCO3–1 in), but we show that Na+ or K+ fluxes cannot be linked to HCO3- transport. Na+ and K+ fluxes were unaffected by the presence or absence of dissolved inorganic carbon. In low-Na+ conditions, Na+ fluxes are too low to support the observed net 14C-carbon fixation rate. Active transport of HCO3- hyperpolarizes (not depolarizes) the membrane potential.
Roche, JL, van der Staay, GWM, Partensky, F, Ducret, A, Aebersold, R, Li, R, Golden, SS, Hiller, RG, Wrench, PM, Larkum, AWD & Green, BR 1996, 'Independent evolution of the prochlorophyte and green plant chlorophyll a/b light-harvesting proteins', Proceedings of the National Academy of Sciences, vol. 93, no. 26, pp. 15244-15248.
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The prochlorophytes are oxygenic prokaryotes differing from other cyanobacteria by the presence of a light-harvesting system containing both chlorophylls (Chls) a and b and by the absence of phycobilins. We demonstrate here that the Chl a/b binding proteins from all three known prochlorophyte genera are closely related to IsiA, a cyanobacterial Chl a -binding protein induced by iron starvation, and to CP43, a constitutively expressed Chl a antenna protein of photosystem II. The prochlorophyte Chl a/b protein ( pcb ) genes do not belong to the extended gene family encoding eukaryotic Chl a/b and Chl a/c light-harvesting proteins. Although higher plants and prochlorophytes share common pigment complements, their light-harvesting systems have evolved independently.
