Radiative Energy Budgets of Phototrophic Surface-Associated Microbial Communities and their Photosynthetic Efficiency Under Diffuse and Collimated Light

Abstract

We investigated the radiative energy budgets of a heterogeneous photosynthetic coral reef sediment and a compact uniform cyanobacterial biofilm on top of coastal sediment. By combining electrochemical, thermocouple and fiber-optic microsensor measurements of O2, temperature and light, we could calculate the proportion of the absorbed light energy that was either dissipated as heat or conserved by photosynthesis. We show, across a range of different incident light regimes, that such radiative energy budgets are highly dominated by heat dissipation constituting up to 99.5% of the absorbed light energy. Highest photosynthetic energy conservation efficiency was found in the coral sediment under low light conditions and amounted to 18.1% of the absorbed light energy. Additionally, the effect of light directionality, i.e., diffuse or collimated light, on energy conversion efficiency was tested on the two surface-associated systems. The effects of light directionality on the radiative energy budgets of these phototrophic communities were not unanimous but, resulted in local spatial differences in heat-transfer, gross photosynthesis, and light distribution. The light acclimation index, Ek, i.e., the irradiance at the onset of saturation of photosynthesis, was >2 times higher in the coral sediment compared to the biofilm and changed the pattern of photosynthetic energy conservation under light-limiting conditions. At moderate to high incident irradiances, the photosynthetic conservation of absorbed energy was highest in collimated light; a tendency that changed in the biofilm under sub-saturating incident irradiances, where higher photosynthetic efficiencies were observed under diffuse light. The aim was to investigate how the physical structure and light propagation affected energy budgets and light utilization efficiencies in loosely organized vs. compact phototrophic sediment under diffuse and collimated light. Our results suggest that the optical properties and the structural organization of phytoelements are important traits affecting the photosynthetic efficiency of biofilms and sediments.

Effect of temperature and feeding on carbon budgets and O2 dynamics in Pocillopora damicornis

“…Corals are counted among the most efficient aquat ic photosynthetic organisms (Al-Najjar et al 2010, Brodersen et al 2014, Lichtenberg et al 2017, and the high photosynthetic activity of P. damicornis observed here and in Lyndby et al (2019) supports that notion. However, carbon energy budgets established here show evidence that a large fraction of assimilated carbon is lost quickly, either as respiration or released as dissolved inorganic carbon, in agreement with a previous study on energy and carbon dynamics in P. eydouxi (Davies 1984).…”

Bio-optical properties and radiative energy budgets in fed and unfed scleractinian corals (Pocillopora sp.) during thermal bleaching

“…In contrast, studies on energy use efficiency in other highly pigmented photosynthetic systems (e.g. benthic biofilms and coral sediments) reveal low energy use efficiencies due to their more uniform topography and high optical density (Al-Najjar et al 2010, Lichtenberg et al 2017.…”

“…Studies of photosynthetic efficiency and radiative energy budgets accounting for the fate of incident and absorbed light energy in benthic marine systems have so far focused on sediments, biofilms, and microbial mats (Al-Najjar et al 2010, Lichtenberg et al 2017, with the exception of a single study on a symbiont-bearing coral showing that light is used very efficiently for photosynthesis in corals (Brodersen et al 2014). It was found that while the majority (> 96%) of incident light energy was absorbed and dissipated as heat, the local photosynthetic energy efficiency of zooxanthellae in the tissue of corals measured under low to moderate irradiance approached the theoretical maximum of 0.125 mol O 2 per mol photons.…”

Understanding photosynthetic biofilm productivity and structure through 2D simulation

“…It is interesting to note that the thickness of the active biofilm (defined as the area where the photosynthesis rate is larger than 1% of its maximal) reaches a constant value of 75 ± 7.5 μ m after 5 days. This means that the superficial layers of the biofilm are the most photosynthetically active, as supported by several experimental studies on phototrophic biofilms [ 43 – 45 ]. Typically, the factor driving the vertical distribution of the active layers in photosynthetic biofilms is light, which is imposing the extent of the euphotic zone [ 43 ].…”