Franklin, LA & Larkum, AWD 1997, 'Multiple strategies for a high light existence in a tropical marine macroalga', Photosynthesis Research, vol. 53, no. 2/3, pp. 149-159.
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Acclimation to high light conditions on the top of coral reefs was examined in the coenocytic, filamentous green macroalga Chlorodesmis fastigiata (C. Ag.) Ducker. Despite having a pool of violaxanthin, high light does not induce formation of zeaxanthin in this macroalga. Exposure to 11 and 33% of surface irradiance resulted in parallel, reversible declines in F(v)/F(m) and in the number of functional PSII centers. The quantum requirement for PSII inactivation was calculated to be approx. 2 x 107 photons. Recovery of PSII activity after low photon exposures did not depend on protein synthesis, unlike at higher photon exposures, where recovery was inhibited by 50% in the presence of lincomycin. Accumulation of inactive, quenching PSII centers is proposed as a mechanism of energy dissipation; only some of these centers require protein synthesis for reactivation. In natural-sized populations, midday photoinhibition was greater in filament tips than in bases, but the number of inactive PSII centers within entire filaments did not significantly change over the course of the day. It is proposed that the higher chlorophyll concentration in the tips provides protective shading to chloroplasts in lower regions, and that cytoplasmic streaming of chloroplasts within this siphonous alga limits the cumulative exposure to high light, thereby providing another level of protection from high light stress.
La Roche, J, Van Der Staay, GWM, Partensky, F, Ducret, A, Aebersold, R, Li, R, Golden, SS, Hiller, RG, Wrench, PM, Larkum, AWD & Green, BR 1997, 'Evolution of chlorophyll-binding proteins', Trends in Plant Science, vol. 2, no. 4, pp. 123-123.
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Ritchie, RJ, Trautman, DA & Larkum, AWD 1997, 'Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942', Plant and Cell Physiology, vol. 38, no. 11, pp. 1232-1241.
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Phosphate uptake rates in Synechococcus R-2 in BG-11 media (a nitrate-based medium, not phosphate limited) were measured using cells grown semi-continuously and in continuous culture. Net uptake of phosphate is proportional to external concentration. Growing cells at pHo 10 have a net uptake rate of about 600 pmol m-2s-1 phosphate, but the isotopic flux for 32P phosphate was about 4 nmol m-2 s-1. There appears to be a constitutive over-capacity for phosphate uptake. The Km and Vmax of the saturable component were not significantly different at pHo 7.5 and 10, hence the transport system probably recognizes both H2PO4- and HPO42-. The intracellular inorganic phosphate concentration is about 3 to 10 mol m-3, but there is an intracellular polyphosphate store of about 400 mol m-3. Intracellular inorganic phosphate is 25 to 50 kJ mol-1 from electrochemical equilibrium in both the light and dark and at pHo 7.5 and 10. Phosphate uptake is very slow in the dark (≈100 pmol m-2s-1) and is light-activated (pHo 7.5 ≈1.3 nmol m-2 s-1, PHo 10≈600 pmol m-2S-1). Uptake has an irreversible requirement for Mg2+ in the medium. Uptake in the light is strongly Na+-dependent. Phosphate uptake was negatively electrogenic (net negative charge taken up when transporting phosphate) at pHo 7.5, but positively electrogenic at pHo 10. This seems to exclude a sodium motive force driven mechanism. An ATP-driven phosphate uptake mechanism needs to have a stoichiometry of one phosphate taken up per ATP (1 PO4 in /ATP) to be thermodynamically possible under all the conditions tested in the present study.
