Direct real-time determination of photosynthetic efficiency by photo-calorespirometry using Synochocystis sp. PCC 6803 as model cyanobacterium.

Photosynthetic efficiency (PE) is key to evaluating phototrophic organisms in biotechnological applications. However, current methods offer limited, indirect insights with poor time resolution. To address this, photo-calorespirometry (Photo-CR) was developed, a novel, non-invasive technique for real-time, direct quantification of photosynthetic energy conversion. Photo-CR simultaneously measures metabolic heat flow and oxygen evolution, capturing both light and dark reactions, including all heat-dissipating side-reactions. Using Synechocystis sp. PCC 6803, it revealed a maximum conserved energy flux of 5.2 ± 0.8 Wg and a light energy yield of 26.4 ± 5.8 kJg. A strong correlation between heat and oxygen flux yielded an oxycaloric equivalent of 488.5 ± 0.3 kJmol. PE dynamics, including light saturation and adaptation to fluctuating light, were characterized, showing a maximum PE of 2.3 ± 0.4 % and half-saturation at 360 μmolms. Photo-CR integrates thermodynamic and respiratory data, offering novel insights into side-reactions like the Mehler cycle and enabling advanced screening and optimization in photo-biotechnology.