Heterogeneity of oxygen production and consumption in a photosynthetic microbial mat as studied by planar optodes

By applying planar optodes and imaging techniques to a benthic photosynthetic mat, we demonstrated an extensive vertical and horizontal variation in O-2 concentrations, O-2 consumption, and O-2 production, In light, the oxic zone could be divided into three horizons: 1) an upper zone dominated by diatoms that had a moderate net O-2 production, 2) another zone dominated by Microcoleus-like cyanobacteria with a high net O-2 production, and 3) a lower zone with disintegrating microalgae and cyanobacteria with a high O-2 consumption rate, From the O-2 images, the net O-2 production/consumption was calculated at a spatial resolution of 130 mu M, This allowed us to identify microsites with high rates of O-2 turnover within the photic zone, Sites with high net O-2 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 O-2 consumption sites were surrounded by the highest O-2 production sites, This suggested a tight spatial coupling between production and consumption of O-2 within the photic zone, Light stimulated the O-2 consumption within the photic zone, At irradiances above 400 mu mol photons.m(-2).s(-1), the stimulated O-2 production was almost completely balanced by enhanced O-2 consumption at microsites exhibiting net consumption of O-2 even at maximum irradiance (578 mu mol 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 O-2 consumption, anoxic microniches were not encountered in the investigated mat, Images of gross photosynthetic rates also revealed an extensive horizontal variation in press 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 mu mol photons.m(-2).s(-1)), 90% of the O-2 produced was consumed within the photic zone, The presented data demonstrate the great potential offered by planar optode for studies of benthic photosynthetic communities.