Effects of short-term irradiation on photoinhibition and accumulation of mycosporine-like amino acids in sun and shade species of the red algal genus Porphyra.

The effect of irradiance (40 and 840 micromol photons m(-2) s(-1)) of short-term (48 h) irradiation on photosynthetic activity (estimated as oxygen evolution and as chlorophyll fluorescence), specific absorption and fluorescence excitation spectra, photosynthetic pigment accumulation (chlorophyll a and biliproteins) and UV-absorbing compounds (mycosporine-like amino acids, MAAs) was investigated in sun and shade species of the red algal genus Porphyra collected in Trondheimsfjord (Norway). In the sun type, high irradiance exposure (840 micromol photons m(-2) s(-1)) did not alter the Chl a concentration, however, exposure to a lower irradiance (40 micromol photons m(-2) s(-1)) for 48 h significantly increased the chlorophyll concentration. The content of MAAs was significantly higher in the suntype than in the shade type algae. Porphyra-334 is the main MAA in this species followed by shinorine. The total content of MAAs significantly (P<0.05) increased in the sun type after 48 h exposure to both high and low irradiances. However, in the shade type, porphyra-334 significantly decreased (P<0.05) after both high and low irradiance exposure. Photosynthetic activity (as oxygen evolution) and the optimal quantum yield (F(v)/F(m)), as an indicator of photoinhibition, decreased under low and high irradiance in the shade type algae and no full recovery was observed when the algae were transferred to very low irradiation. The sun type algae presented a higher capacity of acclimation to increased irradiance than the shade type algae. This high acclimation of sun type algae to short term high irradiance exposure (48 h) is explained by the higher thermal dissipation. This was estimated as the ratio of nonphotochemical quenching related to the light dose (q(N):dose) and by the accumulation of MAAs.

Potassium drives daily reversible thallus enlargement in the marine red alga Porphyra leucosticta (Rhodophyta).

Growth rate in terms of area expansion per 30 min was measured in the marine red algae Porphyra leucosticta under light/dark cycles of 6:6 h. Thalli grown in artificial seawater (ASW), under controlled ionic concentrations, showed a rapid thallus expansion just after light-on (morning-peak). Dark phase began with a significant thallus contraction (dark-peak) but no growth was observed until the next light phase. The removal of K+ from the medium inhibited the reversible growth peak that this species shows after light-on. On the other hand, the removal of Na+ did not have an apparent effect on growth pattern. Addition of Rb+ to K+-free ASW restored the morning-peak to 60% of its value in ASW, but addition of Li+ failed to restore the morning-peak. Intracellular ion analyses revealed that after light-on, the internal K+ content of the cells of this species increased 4-fold in 90 min, reaching an intracellular concentration of up to 300 micromol K+ per gram fresh weight, and that this value remained fairly constant over the light phase. Addition of 100 mM of tetraethylammonium, a specific K+-channel blocker, inhibited the morning-peak by 40%. The Na+ and Cl- contents in the cells increased rapidly in the first 45 min of the light phase, but then the internal concentrations of both ions decreased to their minimum values in the light phase. The carbonic anhydrase inhibitors acetazolamide and 6-ethoxyzolamide did not affect thallus expansion during the light phase. In contrast, thallus expansion in the light phase was completely inhibited by diethylstilbestrol, an inhibitor of the putative primary pump, and by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosynthesis, but not by sodium vanadate, a specific inhibitor of the plasma-membrane H+-ATPase. We suggest that the periodic oscillation of the short-term growth rate of P. leucosticta occurs in response to potassium fluxes, which control the osmotic pressure and eventually the relative cell volume. The possible effects of the loosening of the cell wall and the internal K+ concentration on the growth of this species are also discussed.