MICROENVIRONMENTAL CONTROL OF PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN AN EPILITHIC CYANOBACTERIAL BIOFILM

The photosynthetic performance of an epilithic cyanobacterial biofilm was studied in relation to the in situ light field by the use of combi ned microsensor measurements of O-2, photosynthesis, and spectral scal ar irradiance. The high density of the dominant filamentous cyanobacte ria (Oscillatoria sp.) embedded in a matrix of exopolymers and bacteri a resulted in a photic zone of <0.7 mm. At the biofilm surface, the pr evailing irradiance and spectral composition were significantly differ ent from the incident light. Multiple scattering bed to an intensity m aximum for photic light (400-700 nm) of ca. 120% of incident quantum i rradiance at the biofilm surface. At the bottom of the euphotic zone i n the biofilm, light was attenuated strongly to <5-10% of the incident surface irradiance. Strong spectral signals from chlorophyll a (440 a nd 675 nm) and phycobilins (phycoerythrin 540-570 nm, phycocyanin 615- 625 nm) were observed as distinct maxima in the scalar irradiance atte nuation spectra in the upper 0.0-0.5 mm of the biofilm. The action spe ctrum for photosynthesis in rite cyanobacterial layer revealed peak ph otosynthetic activity at absorption wavelengths of phycobilins, wherea s only low photosynthesis rates were induced by light absorption of ca rotenoids (450-550 nm). Respiration rates in light- and dark-incubated biofilms were determined using simple flux calculations on measured O -2 concentration profiles and photosynthetic rates. A significantly hi gher areal O-2 consumption was found in illuminated biofilms than in d ark-incubated biofilms. Although photorespiration accounted for part o f the increase, the enhanced areal O-2 consumption of illuminated biof ilms could also be ascribed to a deeper oxygen penetration in light as well as an enhanced volumetric O-2 respiration In and below the Photi c zone. Gross photosynthesis was largely unaffected by increasing flow velocities, whereas the O-2 flux out of the photic zone, that is, net photosynthesis, increased with flow velocity. Consequently, the amoun t of produced O-2 consumed within the biofilm decreased with increasin g flow velocity. Our data indicated a close coupling of photosynthesis and respiration in biofilms, where the dissolved inorganic carbon req uirement of the photosynthetic population may largely be covered by th e respiration of closely associated populations of heterotrophic bacte ria consuming a significant part of the photosynthetically produced ox ygen and organic carbon.