The anoxic switch: Strain variation in H2 exchange during repeated illumination after dark anoxia
Hydrogen (H2) exchange during transitions from dark incubation to sustained illumination provides a readout of physiological history in green microalgae. In Chlamydomonas reinhardtii, the first illumination after prolonged dark anoxia gives a higher H2 production rate than later illuminations, an anoxic switch linked to photosystem II (PSII) photosynthetic control. Membrane-inlet mass spectrometry was used to compare 54 independently grown cultures from 17 strains across sequential irradiances and 48 cultures from 15 strains during seven repeated cycles of 2 min of light and 3 min of darkness at strain-specific irradiances. For the primary early-response 30s endpoint, first sustained-light rates exceeded the mean of cycles 5-7 in 43 of 48 cultures. Ratios of strain mean first to later rates exceeded 1 in 14 of 15 strains and ranged from 0.63 to 17.48, and strain accounted for most of the variance in culture-level ratios. G1122 was the only strain with higher later rates. The predominance of higher first rates persisted across alternative rate windows and after excluding cultures with elevated pre-light O2, although exact magnitudes depended on the window. O2 and nominal CO2 signal changes varied among strains, without a detected cross-strain correlation with the H2 ratio. An enhanced first-light H2 response was therefore the predominant pattern in strains of the Chlorellaceae, Scenedesmaceae, Selenastraceae and Chlamydomonadales, but its magnitude differed by more than an order of magnitude, identifying candidate strains for testing whether PSII photosynthetic control explains this variation. Keywords: anoxia; green microalgae; hydrogen; illumination history; membrane-inlet mass spectrometry; photosynthetic induction