Optical properties of microbial mats: Light measurements with fiber-optic microprobes

Abstract

Photosynthetie microbial mats occur as dense stratified communities in top layers of sediments, where microorganisms and sediment particles are embedded in an extracellular polymer matrix (e.g., Farbstreifensandwatt, Stal et al. 1985) or growing as a thin photosynthetic biofilm on solid substrata, e.g., stone or plant surfaces (Kühl 1993). In extreme environments, like hypersaline salt marshes and hot springs, regular mats several mm’ to cm’ thick and composed of almost pure biomass and exopolymers develop (Cohen and Rosenberg 1989). Photosynthetic microorganisms are the predominant component of these microbial mats, which often exhibit a vertical stratification of different colored layers due to the presence of photosynthetie microalgae and bacteria containing different photopigments with depth (Nicholson et al. 1987; Pierson et al. 1990). High metabolic rates due to the high density of microorganisms in mats combined with molecular diffusion acting as the major transport mechanism result in steep chemical gradients with depth as has been demonstrated by microelectrode measurements at < 50–100 µm spatial resolution (Revsbech and Jørgensen 1986; Revsbech this volume). Production and consumption of the major electron acceptors can be calculated from measured microprofiles and different functional layers can thus be identified in microbial mats from microelectrode measurements. The typical sequence found is an upper oxygenic photosynthetie layer with concurrent oxygen respiration and a lower anoxic layer with denitrification and sulfate reduction as the predominant respiratory processes and with anoxygenic photosynthesis, provided sufficient light is penetrating from above (Jørgensen et al. 1983; Jørgensen and Des Marais 1986b; Revsbech et al. 1989; Kühl 1993).