Effects of temperature and irradiance on a benthic microalgal community: A combined two-dimensional oxygen and fluorescence imaging approach

The effects of temperature and light on both oxygen (O₂) production and gross photosynthesis were resolved in a benthic microalgal community by combining two-dimensional (2D) imaging of O₂ and variable chlorophyll a (Chl a) fluorescence. Images revealed a photosynthetically active community with spatial heterogeneity at the millimeter scale. Irradiance strongly increased pore-water O₂ concentration, sediment net O₂ production, and gross photosynthesis. The latter was derived from measurements of the electron transfer rate (rETR) in Photosystem II. The onset of light saturation for gross photosynthesis was approximately twofold higher than for net O₂ production, reflecting significant light-stimulated O₂ consumption at higher light (> 75 μmol photons m−2 s−1). Temperature stimulated O₂ consumption more than photosynthesis, turning the community more heterotrophic at elevated temperatures. Thus, the compensation irradiance (i.e., the irradiance at which community O₂ production and consumption balance) increased fivefold (from 6 to 30 μmol photons m−2 s−1) with a temperature increase from 10°C to 25°C, corresponding to a temperature coefficient (Q10) of 2.9. Whereas net O₂ production had a temperature optimum at ∼ 20°C, no optimum was observed for gross photosynthesis within the investigated range (10°C to 25°C). The resolved 2D net O₂ production and rETR exhibited a significant exponential relationship, demonstrating predictable correlations between the net community production and gross photosynthesis for a complex microbial community, at different temperatures. The present imaging approach demonstrates a great potential to study consequences of environmental effects on photosynthetic activity and O₂ turnover in complex phototrophic benthic communities.