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

Abstract

The effects of temperature and light on both oxygen (O2) production and gross photosynthesis were resolved in a benthic microalgae 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 mm scale. Irradiance strongly increased porewater 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 ~2 fold 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 five-fold (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. While 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