Photosynthetic electron transport of beachrock microbial mats growing in the intertidal zone of Heron Island (Great Barrier Reef, Australia) was investigated with a pulse amplitude modulation chl fluorometer providing four different excitation wavelengths for preferential excitation of the major algal groups (cyanobacteria, green algae, diatoms/dinoflagellates). A new type of fiberoptic emitter-detector unit (PHYTO-EDF) was used to measure chl fluorescence at the sample surface. Fluorescence signals mainly originated from cyanobacteria, which could be almost selectively assessed by 640-nm excitation. Even after desiccation for long time periods under full sunlight, beachrock showed rapid recovery of photosynthesis after rehydration in the light (t 1/2 ف 15 min). However, when rehydrated in the dark, the quantum yield of energy conversion of PSII remained zero over extended periods of time. Parallel measurements of O 2 concentration with an oxygen microoptode revealed zero oxygen concentration in the surface layer of rehydrated beachrock in the dark. Upon illumination, O 2 concentration increased in parallel with PSII quantum yield and decreased again to zero in the dark. It is proposed that oxygen is required for preventing complete dark reduction of the PSII acceptor pools via the NADPH-dehydrogenase/chlororespiration pathway. This hypothesis is supported by the observation that PSII quantum yield could be partially induced in the dark by flushing with molecular oxygen. Key index words: beachrock; cyanobacteria; photosynthesis; oxygen; chl fluorescence; microsensor; state 1/state 2 Abbreviations: EDF, emitter-detector unit; F o , fluorescence yield of dark-adapted sample; F m , maximal fluorescence yield measured during saturation pulse; F v , variable fluorescence yield; LED, light-emitting diode; PAM, pulse amplitude modulation; PQ, plastoquinoneBeachrock consists of carbonate-cemented rock of varying composition occuring in the upper tidal zone of many subtropical and tropical marine environments. Beachrock is colonized by a variety of microorganisms with a predominance of epilithic, chasmolithic, and endolithic cyanobacteria, which can form a dense microbial mat at the beachrock surface. Although geological and geochemical studies of beachrock are relatively abundant in the literature, very little is known about the biology and biogeochemistry of beachrock. Biological studies have mostly focused on faunistic and floristic accounts (e.g. Cribb 1966, Brattström 1992 and of the possible role of microorganisms in the formation of beachrock (e.g. Neumeier 1999, Webb et al. 1999. To our knowledge, the comprehensive study of Krumbein (1979) represents the most detailed account of beachrock biogeochemistry, and very little is known about the ecophysiology of microbial communities in beachrock.The beachrock on Heron Island, Great Barrier Reef consists of microbialites and micritic aragonite cement, the formation of which is induced by biological activity (Davies and Kinsey 1973, Webb et al. 1999). Three characteri...
