LAUSR

Short-term heat exposure in tropical regions can generate severe stress in the photosynthetic activity of soil crust cyanobacteria. We investigated the responses of two filamentous cyanobacteria, Scytonema tolypothrichoides and Tolypothrix bouteillei , to 1hr exposure at 35, 45, and 55 °C using variable chlorophyll fluorescence. Protocols for maximum quantum yield ( F V / F M ) and dark recovery of chlorophyll a fluorescence (OJIP) transient were applied. Heat exposure caused damage to the donor side of PSII, indicated by a decrease in F V / F M and a rapid increase in F 0 . After heat stress, photochemical energy utilization ( φ Po , φ ETo , and φ RE1o ) declined and energy dissipation ( φ DIo ) increased. At 45 °C, the photosynthetic apparatus was reversibly damaged, since full recovery was observed after 7 days of relaxation. S. tolypothrichoides was more resistant to heat stress than T. bouteillei , confirming better adaptation to higher temperatures as observed in growth experiments.