The effects of copper on the photosynthetic response of Phaeocystis cordata

Abstract

We investigated the effects of limiting (1.96 × 10(-9) mol l(-1) total Cu, corresponding to pCu 14.8; where pCu = -log [Cu(2+)]) and toxic Cu concentrations up to 8.0 × 10(-5) mol l(-1) total Cu (equivalent to pCu 9.5) on growth rates and photosynthetic activity of exponentially grown Phaeocystis cordata, using batch and semi-continuous cultures. With pulse amplitude modulated (PAM) fluorometry, we determined the photochemical response of P. cordata to the various Cu levels, and showed contrasting results for the batch and semi-continuous cultures. Although maximum photosystem II (PSII) quantum yield (Φ(M)) was optimal and constant in the semi-continuous P. cordata, the batch cultures showed a significant decrease in Φ(M) with culture age (0-72 h). The EC50 for the batch cultures was higher (2.0 × 10(-10) mol l(-1), pCu9.7), than that for the semi-continuous cultures (6.3 × 10(-11) mol l(-1), pCu10.2). The semi-continuous cultures exhibited a systematic and linear decrease in Φ(M) as Cu levels increased (for [Cu(2+)] < 1.0 × 10(-12) mol l(-1), pCu12.0), however, no effect of high Cu was observed on their operational PSII quantum yield (Φ'(M)). Similarly, semi-continuous cultures exhibited a significant decrease in Φ(M), but not in Φ'(M), because of low-Cu levels. Thus, Cu toxicity and Cu limitation damage the PSII reaction centers, but not the processes downstream of PSII. Quenching mechanisms (NPQ and Q (n)) were lower under high Cu relative to the controls, suggesting that toxic Cu impairs photo-protective mechanisms. PAM fluorometry is a sensitive tool for detecting minor physiological variations. However, culturing techniques (batch vs. semi-continuous) and sampling time might account for literature discrepancies on the effects of Cu on PSII. Semi-continuous culturing might be the most adequate technique to investigate Cu effects on PSII photochemistry.