Photosynthetic production and light utilization efficiencies in aquatic organisms and microbial communities is determined by the irradiance incident on the system, which on a macroscale is dependent on factors such as, water depth and turbidity. However, on a microscale the light field inside phototrophic tissues and communities is determined by interactions between the incident light and the optical properties of the system, which is influenced by pigmentation, organization of tissue structural components, and the intracellular organisation of phytoelements. Our current understanding of how photosynthesis is influenced by light interactions is largely based on studies of terrestrial plants where canopy interactions have been described across scales; from landscape-level down to the organization of individual chloroplasts. How light interactions and photosynthetic efficiencies are influenced by microstructural heterogeneities in the organization of aquatic tissues and communities is largely unexplored although a few papers have described the importance of community structure on wholecommunity production. In this thesis, it was the aim to investigate if fundamental links exists between the microscale organization of aquatic photosynthetic tissues and biofilm communities, their optical properties, and photosynthetic efficiencies and to investigate whether canopy-like effects are relevant for the microscale regulation of aquatic phototrophs similar to what is found in terrestrial plants.
This was investigated in a range of aquatic phototrophs such as macroalgae, reef-building corals, and photosynthetic biofilms. As a first step, we demonstrate that a microscale stratification of the internal light- and chemical environment exists across the investigated systems, with concomitant internal gradients of photosynthesis and respiration. We further investigate this by compiling a closed radiative energy budget of a coral and find that corals are highly efficient light collectors that can display photosynthetic quantum efficiencies close to the theoretical limit. Using a similar approach, we then investigate i) how community composition affects energy budgets in photosynthetic sediments, ii) the role of incident light field angularity (diffuse/collimated) on radiative energy conservation, and iii) how light-induced migration of cyanobacteria change community-structure and -photosynthetic efficiencies in a natural biofilm. We develop methods for measuring quantum yields inside tissues while considering the actual light availability. Furthermore, physical structures protruding from the surface of a system can change both the light- and chemical microenvironment and the consequence of such changes on plant fitness was studied. In the final chapter, a comparative analysis between terrestrial and aquatic photosynthetic systems is given and it is discussed if aquatic microscale structure/function relationships can be described conceptually similar to terrestrial canopy interactions.