Bio-optical characteristics and the vertical distribution of photosynthetic pigments and photosynthesis in an artificial cyanobacterial mat

Zonations of photosynthesis and photopigments in artificial cyanobacterial mats were studied with (i) oxygen and pH microsensors, (ii) fiber-optic mic roprobes for field radiance, scalar irradiance, and PSII fluorescence, and (iii) a light microscope equipped with a spectrometer for spectral absorban ce and fluorescence measurements. Our analysis revealed the presence of sev eral distinct 1-2 mm thick cyanobacterial layers mixed with patches of anox ygenic photosynthetic bacteria. Strong attenuation of visible light confine d the euphotic zone to the uppermost 3 mm of the mat, where oxygen levels o f 3-4 times air saturation and a pH peak of up to pH 8.8 were observed unde r saturating irradiance (413 mu mol photon m(-2) s(-1)). Oxygen penetration was 5 mm in light and decreased to 1 mm in darkness. Volumetric oxygen con sumption in the photic and aphotic zones of illuminated mat was 5.5 and 2.9 times higher, respectively, than oxygen consumption in dark incubated mats . Scalar irradiance reached 100-150% of incident irradiance in the upper 0. 5 mm of the mat due to intense scattering in the matrix of cells, exopolyme rs, and carbonate precipitates. In deeper mat layers scalar irradiance decr eased nearly exponentially, and highest attenuation coefficients of 6-7 mm( -1) were found in cyanobacterial layers, where photosynthesis and photopigm ent fluorescence also peaked. Visible light was attenuated >100 times more strongly than near infrared light. Microscope spectrometry on thin sections of mats allowed detailed spectral absorbance and fluorescence measurements at defined positions relative to the mat surface. Besides strong spectral signals of cyanobacterial photopigments (Chl a and phycobiliproteins), the presence of both green and purple photosynthetic bacteria was evident from spectral signals of Bchl a and Bchl c. Microprofiles of photopigment absorb ance correlated well with microdistributions of phototrophs determined in a n accompanying study.