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 photosynthet ic 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 di atoms 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 low er zone with disintegrating microalgae and cyanobacteria with a high O-2 co nsumption 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 identif y 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 situa ted 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 sur rounded 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 irradiance s above 400 mu mol photons.m(-2).s(-1), the stimulated O-2 production was a lmost completely balanced by enhanced O-2 consumption at microsites exhibit ing 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-stimul ated respiration was caused by stimulated heterotrophic activity fueled by organic carbon leakage from the phototrophs. Despite microsites with high n et O-2 consumption, anoxic microniches were not encountered in the investig ated mat, Images of gross photosynthetic rates also revealed an extensive h orizontal variation in press rates, with microsites of low or no photosynth esis within the otherwise photic zone, Calculations based on the obtained i mages 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 be nthic photosynthetic communities.