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

Abstract

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 μmol m −2 s −1) and high-light (HL, 400 μmol 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 ( α, 0.591 μmol −1 m 2 s vs. 0.661 μmol −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 μmol m −2 s −1 vs. 44.3 μmol m −2 s −1). Acclimation to HL induced bi-phasic NPQ vs. E curves, with minimum values reached under low E levels (15–25 μmol 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, α RLC, decreased monotonically with E in LL cultures, from 0.68 to 0.25 μmol −1 m 2 s, while varied bi-phasically in HL-acclimated samples. Typically, α RLC of HL cultures increased under low E, reaching a maximum of 0.61 μmol −1 m 2 s under 25–55 μmol m −2 s −1, and decreased gradually under higher E levels to 0.25 μmol −1 m 2 s. RLC maximum rETR, rETR m,RLC, and saturation coefficient E k,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 α 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 α RLC, Δ α 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, Ê 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.