Photosynthetic characteristics of aerial microalga Trentepohlia jolithus during drying and rewetting cycles

Aerial microalga Trentepohlia jolithus is frequently exposed to drying and rewetting cycles. To characterize the adaptive mechanisms allowing survivability of this aerial microalga, the responses of photosynthetic characteristics and photoprotective mechanisms to moderate desiccation (80% and 60% relative air humidity) and severe desiccation (40% relative air humidity) were investigated in T. jolithus grown at 95% relative air humidity. Additionally, the recovery of photosynthetic performance in severe desiccated T. jolithus was monitored after rehydration. In T. jolithus, non-photochemical quenching (NPQ) decreased gradually as the relative air humidity decreased from 95% to 40%, but the actual photochemical efficiency of photosystem II (Phi(PSII)) remained unchanged until the relative air humidity decreased to 40% under high light, indicating that the photoprotective mechanism NPQ was more sensitive to desiccation than photosynthetic linear electron flow. NPQ was maximally induced even under low light during moderate and severe desiccation. Under high light, excess excitation energy would not be well dissipated by NPQ in the desiccated state. However, a fraction of reaction centers were reversibly down-regulated during moderate and severe desiccation under high light. Inactive reaction centers well protected desiccated T. jolithus against photoinhibition by dissipating excess excitation energy as heat. The linear electron flow was inhibited significantly under severe desiccation. However, activated key Calvin cycle enzymes in severe desiccated T. jolithus promoted the surprisingly rapid recovery of linear electron flow after rehydration. The rapid reaction center recovery of severe desiccated T. jolithus might also favor the fast recovery of linear electron flow after rehydration. Therefore, the reversible down-regulation of reaction centers during desiccation, the maintenance of linear electron flow during moderate desiccation, and the rapid recovery of linear electron flow of severe desiccated algal cells after rehydration might be special adaptive mechanisms that allow T. jolithus to survive in an environment with rapidly changing water availability.