Simultaneous photocalorimetric and oxygen polarographic measurements on Dunaliella maritima cells reveal a thermal discrepancy that could be due to nonphotochemical quenching

Photocalorimetry has rarely been employed to investigate the processes in photosynthesis. Yet, especially when complemented by simultaneous oxygen polarographic measurements, it can probe: (i) the light-excited decay of chlorophyll molecules to the ground state by the photochemical process in photosynthesis; (ii) light emission as fluorescence; and (iii) thermal dissipation. The possibility of detecting the latter two was examined by these methods. The halotolerant chlorophytic microalga, Dunaliella maritima, was grown routinely at 25 μmol photon m−2 s−1 incident light and in the experiments it was exposed to higher light intensity and salinity in order to observe the energetic consequences of stress in terms of the net heat and oxygen fluxes. The results showed that at 50 and 90 μmol photon m−2 s−1 the net heat flow varied from rapidly negative (endothermic) to sharply positive (exothermic). This was different to the simultaneous data from the oxygen polarographic sensor that always showed oxygen evolution in the light. Energy balances to correct for the slight imbalance in the response to incident radiant light and to convert the oxygen evolution to energy values by the oxycaloric equivalent for glucose (−470 kJ mol−1 O2) revealed an extra source of heat at 15.5 ± 3.3 (S.E.) and at 9.4 ± 3.2 pW per cell for the control and treated cells, respectively, at 90 μmol photon m−2 s−1. This was thought to be due mainly to nonphotochemical quenching (NPQ), characteristically with a small contribution from chlorophyll fluorescence. There was an insignificant (paired t-test: P = 0.131) difference in dark oxygen flux of the control (1% saline) and osmotically stressed (3%) cells, meaning that respiration could provide no more ATP to pump the greater quantity of Na+ ions from the cell at higher salinity. Evidence is provided that cells in the higher salinity had a lower specific growth rate, which may have been due to the ATP being diverted from the synthesis of biomass to the Na+, K+-ATPase pump. The dark heat flux was significantly (P = 0.036) greater in the 3% treatment than in the 1% control, whereas the dark oxygen flux was similar for both of them. This meant that the calorimetric/respirometric (CR) ratio in the control was more negative than the oxycaloric equivalent, and indicated the presence of an anaerobic pathway, presumably the glycolytic synthesis of glycerol that increased with higher salinity