Oxygenic photosynthesis and light distribution in marine microbial mats

Abstract

Introdnction Marine intertidal sediments are often colonized by dense populations of phototrophic 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 (Jmgensen 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 (NlR) (pierson et al. 1990; Lassen et al. 1992b; Kiihl and J mgensen 1992). The NlR absorption bands of the bacteriochlorophyll-protein complexes of the anoxygenic phototrophs are essential for the presence of the dense populations of these organisms underneath the oxygenic phototrophs. It is not known to what extent complementary spectral utilization of the incident light has an ecological significance for the zonation of the oxygenic phototrophs. The extension of the euphotic zone of oxygenic photosynthesis in microbial mats is often only about 1 mm. The aim of the present study was to quantify the efficiency by which available light in sediments is used for photosynthesis, to measure action spectra for photosynthesis and from these data to estimate the extent to which changes in the spectral light composition was of competitive advantage to cyanobacteria over diatoms. The investigated microbial mats were collected in August 1990 and 1991 at sheltered sandy sediments of Limfjorden, Denmark, in areas periodically desiccated. Light gradients in the sediments were measured by 70 ~m wide fiber-optic scalar irradiance (4' 11") sensors (Lassen et al. 1992a). Gross oxygenic photosynthesis was measured by the light-dark-shift technique (Revsbech and J0fgensen 1983) using oxygen microelectrodes with a 90% response time of 0.3-0.4 s (Revsbech 1989). When illuminated at high irradiance, the depth distribution of photosynthesis exhibited a maximum at the very surface and a secondary maximum at 1.0 mm, reflecting the zonation of the dominant oxygenic phototrophs (Fig. 1) . At the depth of the secondary maximum, PAR was only 12 ~E m-2 s-1 i.e. less than 2 % of incident irradiance. The efficiency by which the available light was utilized for oxygenic photosynthesis could be calculated by dividing the photosynthetic rates for each depth interval by the scalar irradiance (PAR) at the top of the interval (Fig. 2). At the secondary maximum, the efficiency was lO-fold higher than the efficiency in the upper 0.0-0.6 mm of the sediment when the sediment was illuminated at high irradiance (60% of in situ irradiance on a clear day).