• microbial mat;
  • microcoleus;
  • microsensor;
  • optodes;
  • oxygen;
  • photosynthesis;
  • respiration

By applying planar optodes and imaging techniques to a benthic photosynthetic mat, we demonstrated an extensive vertical and horizontal variation in O2 concentrations, O2 consumption, and O2 production. In light, the oxic zone could be divided into three horizons: 1) an upper zone dominated by diatoms that had a moderate net O2 production, 2) another zone dominated by Microcoleus-like cyanobacteria with a high net O2 production, and 3) a lower zone with disintegrating microalgae and cyanobacteria with a high O2 consumption rate. From the O2 images, the net O2 production/consumption was calculated at a spatial resolution of 130 μM. This allowed us to identify microsites with high rates of O2 turnover within the photic zone. Sites with high net O2 consumption (>1.5 nmol·cm−3·s−1) were typically situated next to sites with a relatively high net production (>2 nmol·cm−3·s−1), revealing a mosaic in which the highest O2 consumption sites were surrounded by the highest O2 production sites. This suggested a tight spatial coupling between production and consumption of O2 within the photic zone. Light stimulated the O2 consumption within the photic zone. At irradiances above 400 μmol photons·m−2·s−1, the stimulated O2 production was almost completely balanced by enhanced O2 consumption at microsites exhibiting net consumption of O2 even at maximum irradiance (578 μmol photons·m−2·s−1). Our observations strongly supported the idea that light-stimulated respiration was caused by stimulated heterotrophic activity fueled by organic carbon leakage from the phototrophs. Despite microsites with high net O2 consumption, anoxic microniches were not encountered in the investigated mat. Images of gross photosynthetic rates also revealed an extensive horizontal variation in gross rates, with microsites of low or no photosynthesis within the otherwise photic zone. Calculations based on the obtained images revealed that at maximum light (578 μmol photons·m−2·s−1), 90% of the O2 produced was consumed within the photic zone. The presented data demonstrate the great potential offered by planar optode for studies of benthic photosynthetic communities.