Photosynthetic response to different light exposures and associated environmental conditions of the subaerial, epilithic green alga Trentepohlia umbrina (Chlorophyta, Ulvophyceae)

The potentially biodeteriogenic, subaerial green alga Trentepohlia umbrina is widely distributed on shaded substrates with northern aspects. This preference for low light was confirmed by a recent survey in Lazio, Italy. However, the same survey revealed an isolated occurrence of extensive growths of T. umbrina-dominated biofilms on surfaces with diverse aspects that ranged from highly exposed to shaded. Prior studies of Trentepohlia photosynthesis have been restricted to single sites, so this multi-aspect site was particularly interesting to understand the photoacclimation potential of Trentepohlia. Photosynthetic pigment concentrations and chlorophyll fluorescence (in situ and ex situ) were used to assess the photosynthetic response to changes in light exposure and humidity. Pigment contents and photoinhibition decreased, while the maximum photosynthetic rate and the level of light that saturated photosynthesis increased with an increasing light exposure duration (ranging from c. 5 to 300 min d–1 of exposure to sunlight). Laboratory tests of relative humidity (RH) on biofilm photosynthesis showed no recorded activity up to 32% RH, only 26% of the maximum activity at 77% RH, and full activity at 92% RH. When T. umbrina was in the desiccated state a lowering of basal of chlorophyll a fluorescence emission and a loss of variable fluorescence occurred. This suppression of fluorescence when cells are desiccated is known to result in photoprotection of reaction centres from continuing high irradiances, as the enhanced thermal dissipation replaces slower and potentially damaging energy transfer to the functional reaction centres. It was concluded that T. umbrina has a much wider environmental range than previously thought, but its growth is very slow and may be overlooked during cultural heritage surveillance, leading to inconspicuous chronic degradation of outdoor stone substrata.