Oxygenic photosynthesis and light distribution in marine microbial mats

Marine intertidal sediments are often colonized by dense populations of prototrophic microorganisms forming stratified communities with diatoms at the very surface and an underlying population of cyanobacteria (Stal et al. 1985). Underneath the layers of oxygenic phototrophs, purple and green sulfur bacteria frequently form additional colored bands (Nicholson et al. 1987). Microalgae in the top layers shade the underlying sediment of those regions of the light spectrum which they preferentially absorb (Jøgensen et al. 1987; Pierson et al 1987; Lassen et al. 1992b; Ploug et al. 1993). The distinct stratification, which is often observed for different types of phototrophic organisms, may thus be strongly influenced by their complementary utilization of the light spectrum. Below the layers of the oxygenic phototrophs, scalar irradiance in the visible spectrum (400–700 nm, PAR) is depleted 10–100 times more than light in the near infrared spectrum (NIR) (Pierson et al. 1990; Lassen et al. 1992b; Kühland Jørgensen 1992). The NIR absorption bands of the bacteriochlorophyll-protein complexes of the anoxygenic phototrophs are essential for the presence of thedense populations of these organisms underneath the oxygenic phototrophs.