Heat Induction of Cyclic Electron Flow around Photosystem I in the Symbiotic Dinoflagellate Symbiodinium

Abstract

Increases in seawater temperature impair photosynthesis (photoinhibition) in the symbiotic dinoflagellate Symbiodinium within cnidarian hosts, such as corals and sea anemones, and may destroy their symbiotic relationship. Although the degree of photoinhibition in Symbiodinium under heat stress differs among strains, the differences in their responses to increased temperatures, including cyclic electron flow (CEF), which sustains photoprotective thermal energy dissipation, have not been investigated. Here, we examined CEF in cultured Symbiodinium cells or those in an endosymbiotic relationship within a cnidarian host. The light-dependent reduction of the primary electron donor photosystem I, i.e. P700(+), was enhanced in any Symbiodinium cell by increasing temperatures, indicating CEF was induced by heat, which was accompanied by thermal energy dissipation activation. The critical temperatures for inducing CEF were different among Symbiodinium strains. The clade A strains with greater susceptibility to photoinhibition, OTcH-1 and Y106, exhibited higher CEF activities under moderate heat stress than a more phototolerant clade B strain Mf1.05b, suggesting that the observed CEF induction was not a preventive measure but a stress response in Symbiodinium.

Figure 1.

Dark reduction kinetics of photooxidized P700⁺ in Symbiodinium OTcH-1. Five measurements taken at 10-s intervals were averaged and fit with a single-exponential decay. A, Effect of DCMU. The cells were incubated at 25°C for 15 min and subsequently treated with or without DCMU (80 µm) for 5 min in the dark before measurement. Inset, A micrograph of Symbiodinium cells (bar = 10 µm). B, Effects of elevated temperatures. The cells were incubated at 25°C, 33°C, or 36°C for 15 min and subsequently treated with DCMU for 5 min in the dark before measurement. The reduction rate constants shown (k) are means ± sd from three independent experiments.

Figure 2.

Dark reduction kinetics of photooxidized P700⁺ in Aiptasia H2. Aiptasia polyps were incubated at 25°C in the presence of DCMU (200 µm) for 15 min in the dark, and the reduction kinetics of P700⁺ were measured (blue line). The polyps were subsequently incubated at 35°C for 15 min in the dark, and the reduction kinetics of P700⁺ were measured (red line). The reduction rate constants shown (k) are means ± sd from three independent experiments. Inset, Micrograph of an Aiptasia polyp (bar = 2 mm).

Figure 3.

Time course of the P700⁺ reduction rate in Symbiodinium OTcH-1 after transfer from a high (33°C) temperature to the optimal growth (25°C) temperature. The P700⁺ reduction rate was monitored at 25°C (triangles) and 33°C (circles) following preincubation at 33°C for 15 min. The P700⁺ reduction rate was determined in the presence of DCMU. The values are presented as means ± sd from three independent experiments.

Figure 4.

Effects of DCMU and NH₄Cl on NPQ at different temperatures in Symbiodinium OTcH-1. The cells were incubated at 25°C for 30 min in the dark with or without NH₄Cl (100 mm). Next, the cells were incubated at 25°C (A), 33°C (B), or 35°C (C) for 15 min in the dark, and the NPQ development was monitored in the light at 240 µmol photons m⁻² s⁻¹ for 10 min. The relaxation of NPQ was monitored in the dark for another 10 min. DCMU was added to the cells 5 min after the onset of light exposure (arrows). The values are presented as means ± sd of two or three independent experiments.

Figure 5.

Relationships between the extent of CEF and the photoinhibition sensitivity in three different Symbiodinium strains. A, Effects of high temperature on PSII photoinhibition. The cells were preincubated at 25°C (left) and 33°C (right) for 1 h in the dark and were subsequently exposed to light at 500 µmol photons m⁻² s⁻¹. The maximum quantum yield of PSII, i.e. F_v/F_m, was measured at each time point after dark incubation for 10 to 15 min. The asterisks indicate statistically significant differences compared with the corresponding data from Mf1.05b (*P < 0.05 and **P < 0.01, Student’s t tests). B, Effects of elevated temperatures on the P700⁺ reduction rate. The cells were incubated at temperatures ranging from 25°C to 36°C for 15 min in the dark before the measurements. DCMU was subsequently added, and the rate constant of P700⁺ reduction was taken as demonstrated in Figure 1. The values are presented as means ± sd from three independent experiments.