Responses of desiccation-tolerant cyanobacteria to environmental extremes

Intertidal cyanobacterial communities regularly experience periods of desiccation between tidal flows. For a Lyngbya cf. aestuarii dominated mat from Baja California Sur, Mexico, periods of hydration occur about every two weeks and last for only a few days. Upon rehydration of the Lyngbya mat, respiratory activity recovered in about 10-30 minutes followed by the full recovery of photosynthetic activity in about 2-3 hours. The rapid recovery of photosynthesis in these mats suggests that they are able to take full advantage of each period of hydration. However, salinities outside the natural range for the local environment (i.e. 35--50 ppt) were detrimental to recovery by not only slowing recovery but limiting maximum photosynthetic activity, as well. Photosynthetic activity recovered more rapidly under higher light intensities, but excessive light became inhibitory later when photosynthesis had recovered almost fully.; Based on chlorophyll fluorescence analysis, sequential changes in photochemistry were measured during rehydration of the Lyngbya mat. Photosynthetic recovery was primarily limited by the recovery of functional PSII complexes. The slower recovery of photosynthesis in mat stressed by high salt was probably due to direct inhibition of PSII centers as well as the disruption of energy transfer from phycobilisomes to the PSII centers. Functional PSII centers were closed initially, probably due to the reduction of the electron transport chain by respiratory activity. The time that PSII centers remained closed early in recovery increased with light above ∼50 W·m-2. This closure can lead to the generation of destructive singlet oxygen.; Many desiccation-tolerant cyanobacteria produce an ultraviolet radiation (UVR) screening compound, scytonemin, that is located in the extracellular polysaccharide sheath. Scytonemin is an extremely stable molecule that can provide protection from UVR even when cells are desiccated and metabolically inactive. In two out of three strains of cyanobacteria exposed to UVA, scytonemin content increased in response to periodic desiccation stress. In addition, the desiccation-tolerant cyanobacterium Nostoc punctiforme ATCC 29133 produced scytonemin only when allocating large amounts of ATP to fixing N2. These experiments support the theory that cyanobacteria increase scytonemin content when metabolically stressed.