Relationship of rapid light curves of variable fluorescence to photoacclimation and non-photochemical quenching in a benthic diatom

The response of rapid light-response curves (RLCs) of variable fluorescence to changes in short- and long-term photoacclimation status was studied in an estuarine benthic diatom. The diatom Nitzschia palea was grown under low- (LL, 20 mu mol m(-2) s(-1)) and high-light (HL, 400 mu mol m(-2) s(-1)) conditions, with the purpose of characterising the effects of long-term photoacclimation on (i) steady-state light-response curves (LC) of relative electron transport rate, rETR, (ii) the response of RLCs to changes in ambient irradiance (E, the irradiance to which the sample is acclimated to immediately before the RLCs), (iii) the relationship of RLCs to LC parameters and non-photochemical quenching (NPQ). Photoacclimation to LL and HL conditions induced distinct light-response patterns of rETR and NPQ. Higher growth light resulted in rETR vs. E curves with lower initial slopes (alpha, 0.591 mu mol(-1) m(2) s vs. 0.661 mu mol(-1) m(2) s, for HL and LL, respectively) and markedly higher maximum rates (rETR(m), 95.9 vs. 29.3), reached under higher E levels (higher light-saturation coefficient, E-k: 162.4 mu mol m(-2) s(-1) vs. 44.3 mu mol m(-2) s(-1)). Acclimation to HL induced bi-phasic NPQ vs. E curves, with minimum values reached under low E levels (15-25 mu mol m(-2) s(-1)) and not on dark-acclimated samples. The response of RLCs to changes in ambient irradiance varied with the long-term photoacclimation status of the samples. The initial slope, alpha(RLC), decreased monotonically with E in LL cultures, from 0.68 to 0.25 mu mol m(-2) s, while varied bi-phasically in HL-acclimated samples. Typically, alpha(RLC) of HL cultures increased under low E, reaching a maximum of 0.61 mu mol m(-2) s under 25-55 mu mol m(-2) s(-1), and decreased gradually under higher E levels to 0.25 mu mol m(-2) s. RLC maximum rETR, rETR(m,RLC), and saturation coefficient Ek(m,RLC), increased with E following a saturation-like pattern, with the HL cultures presenting markedly higher values for all the E range (maximum rETR(m,RLC) values were 108.6 and 33.4 for HL and LL cultures, respectively). An inverse relationship was consistently found between alpha(RLC) and NPQ, both on LL and HL cultures, causing strong correlations (P < 0.001 in all cases) between NPQ and the high light-induced decrease of alpha(RLC), Delta alpha(RLC). RLCs were confirmed to also provide information on the long-term photoacclimation status, as significant correlations (P < 0.001 both for HL and LL cultures) were verified between E-k and an index based on RLC parameters, (E) over cap (k), both for LL and HL cultures. These results reinforce the usefulness of RLCs as a tool for inferring on the short- and long-term photoacclimation status of samples with different long-term light histories, through the estimation of LC parameters and the monitoring of NPQ levels. (c) 2007 Elsevier B.V. All rights reserved.