Maintenance of a photosynthetic capacity by Trentepohlia umbrina in stressful subaerial environments

Subaerial green algae are exposed to extreme hydration stress, yet cells need to remain fully hydrated and ultrastructurally intact in order to function physiologically. To compound the problems of dehydration, there is also an increased possibility of photochemical damage from high levels of irradiance. Algae can retain their chlorophyll viability during desiccation, but under high light these pigments can continue to absorb damaging radiation even though the chemical reactions of photosynthesis are stalled. Cells need effective protective mechanisms to allow harmless dissipation of this excess excitation energy. Species of the genus Trentepohlia are known to produce significant quantities of the photo-protective compounds and suppress photosynthesis during periods of desiccation, with rates of photosynthesis associated with diurnal changes in relative humidity (T. odorata) and full, rapid recovery of photosynthesis upon rewetting cells following extended periods of desiccation of 40 days (T. umbrina). Studies of the desiccation tolerance of Trentepohlia have been conducted ex situ and restricted to samples taken from single environments, yet, ecological ranges of some species, e.g. T. umbrina, can be quite wide and so in situ investigations of such species would be highly informative regarding stress tolerance. The study site, the Grassi Hospital, Ostia, is very particular as it has multiple sites with extensive growths of T. umbrina on substrates with multiple aspects and a wide range of light intensity and associated environmental conditions. Four sites were chosen that ranged from highly exposed to highly shaded. For each site, the environmental conditions were characterised and biofilm samples for chlorophyll a and b and total carotenoids were taken. Photosynthetic parameters were measured in situ with a mini-PAM. Diurnal environmental conditions that would normally inhibit photosynthesis occurred at each site. Pigment analysis of the biofilms showed a high variability (chl a and b, CV 70%; carotenoids, CV 40%) with higher concentrations in biofilms associated with the higher light exposure. The ratio of total carotenoids and total chlorophyll was lowest at the shaded site (3:1) compared the sites receiving direct light (mean 12.5:1). Estimations of biomass showed that the lowest biomass occurred at the exposed site and the highest in the shaded site. A 3 to 6-fold increase in the yields of photosynthesis following re-hydration of biofilms was identified at all sites. The results showed that T. umbrina could physiologically adapt to diverse solar irradiation exposure and dehydration. All biofilms suffered diurnal desiccation yet maintained chlorophyll viability allowing T. umbrina to be photosynthetically active during the briefest window when conditions are met for photosynthesis. Adjusting pigment content to adapt environmental stress allows T. umbrina to maximise its rate of photosynthesis before inhibitory levels of light and relative humidity are reached throughout the day. The diverse biomass levels at each site suggest that growth is restricted, but the survival capacity of T. umbrina allows for slow colonisation of highly hostile substrates.