Intensified coastal eutrophication can result in an overgrowth of seagrass leaves by epiphytes, which is a major threat to seagrass habitats worldwide, but little is known about how epiphytic biofilms affect the seagrass phyllosphere. The physico-chemical microenvironment of Zostera marina L. leaves with and without epiphytes was mapped with electrochemical, thermocouple and scalar irradiance microsensors as a function of four irradiance conditions (dark, low, saturating and high light) and two water flow velocities (approx. 0.5 and 5 cm s −1 ), which resemble field conditions. The presence of epiphytes led to the build up of a diffusive boundary layer and a thermal boundary layer which impeded O 2 and heat transfer between the leaf surface and the surrounding water, resulting in a maximum increase of 0.8°C relative to leaves with no epiphytes. Epiphytes also reduced the quantity and quality of light reaching the leaf, decreasing plant photosynthesis. In darkness, epiphyte respiration exacerbated hypoxic conditions, which can lead to anoxia and the production of potential phytotoxic nitric oxide in the seagrass phyllosphere. Epiphytic biofilm affects the local phyllosphere physico-chemistry both because of its metabolic activity (i.e. photosynthesis/respiration) and its physical properties (i.e. thickness, roughness, density and back-scattering properties). Leaf tissue warming can lead to thermal stress in seagrasses living close to their thermal stress threshold, and thus potentially aggravate negative effects of global warming.

Biofilms are surface-attached and matrix-enclosed microbial communities that dominate microbial life in numerous ecosystems. Using flumes and automated optical coherence tomography, we studied the morphogenesis of phototrophic biofilms along a gradient of hydraulic conditions. Compact and coalescent biofilms formed under elevated bed shear stress, whereas protruding clusters separated by troughs formed under reduced shear stress. This morphological differentiation did not linearly follow the hydraulic gradient, but a break point emerged around a shear stress of~0.08 Pa. While community composition did not differ between high and low shear environments, our results suggest that the morphological differentiation was linked to biomass displacement and reciprocal interactions between the biofilm structure and hydraulics. Mapping oxygen concentrations within and around biofilm structures, we provide empirical evidence for biofilm-induced alterations of oxygen mass transfer. Our findings suggest that architectural plasticity, efficient mass transfer, and resistance to shear stress contribute to the success of phototrophic biofilms.

Phototrophic biofilms form complex spatial patterns in streams and rivers, yet, how community patchiness, structure and function are coupled and contribute to larger-scale metabolism remains unkown. Here, we combined optical coherence tomography with automated O2 microprofiling and amplicon sequencing in a flume experiment to show how distinct community patches interact with the hydraulic environment and how this affects the internal distribution of oxygen. We used numerical simulations to derive rates of community photosynthetic activity and respiration at the patch scale and use the obtained parameter to upscale from individual patches to the larger biofilm landscape. Our biofilm landscape approach revealed evidence of parallels in the structure-function coupling between phototrophic biofilms and their streambed habitat.

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