Zonations of photosynthesis and photopigments in artificial cyanobacterial mats were studied with
(i) oxygen and pH microsensors, (ii) fiber-optic microprobes for field radiance, scalar irradiance,
and PSII fluorescence, and (iii) a light microscope equipped with a spectrometer for spectral
absorbance and fluorescence measurements. Our analysis revealed the presence of several distinct
1–2 mm thick cyanobacterial layers mixed with patches of anoxygenic photosynthetic bacteria.
Strong attenuation of visible light confined the euphotic zone to the uppermost 3 mm of the mat,
where oxygen levels of 3–4 times air saturation and a pH peak of up to pH 8.8 were observed under
saturating irradiance (413 µmol photon m−2 s
−1). Oxygen penetration was 5 mm in light and
decreased to 1 mm in darkness. Volumetric oxygen consumption 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, exopolymers, and carbonate precipitates. In
deeper mat layers scalar irradiance decreased nearly exponentially, and highest attenuation coefficients
of 6–7 mm−1 were found in cyanobacterial layers, where photosynthesis and photopigment
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 absorbance correlated well with microdistributions of phototrophs determined
in an accompanying study.