Filamentous forms of cyanobacteria, the earliest known oxygenic photosynthesising organisms, are the primary structuring agents in modern-day benthic microbial mat consortia. Under conditions of low sedimentation rates and slow water flow, gliding and colliding of the filaments create peaked and ridged structures with regular centimeter-scale spacing. Petroff et al. (2010) suggest that this spacing is set to minimise competition between individual structures. In support of this hypothesis they show that the spacing between structures corresponds to diffusive distances generated by a rhythmically fluctuating metabolism with a period of approximately 20 hours — the periodicity of photosynthesis. Fossil stromatolites show similar structures and spacing, which supports the still-contentious view that they are evidence of early photosynthetic communities and provides constraints on the origin of phototrophic communities in the early evolution of life on earth. Ridge-peak microbial mats of the type and spacing considered by Petroff are common in ice-covered Antarctic lakes. Such lakes provide a useful test of the diffusive mechanism of pinnacle spacing, since the weak diurnal variability in irradiance reaching the mats could be hypothesised as insufficient to generate diurnal metabolic periodicities. Centimetre-scale structures cannot relate to diffusive distances if metabolic cycles do not generate a marked diel pattern in resource availability in the water between structures, and the universal applicability of the Petroff model would be undermined. To test this model, we simultaneously measured, for the first time, a 30-hour time-series of in situ vertical oxygen concentration gradients in a pinnacle mat and incident irradiance. We found that the oxygen concentration gradient and irradiance were significantly linearly correlated despite a diurnal irradiance change of only 1.8 μmol quanta m-2 s-1. This result supports the model by Petroff et al. and confirms our own previous conclusions regarding the mats photosynthetic competence.