AUTHOR=Brodersen Kasper Elgetti , Kühl Michael 

TITLE=Effects of Epiphytes on the Seagrass Phyllosphere

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 9 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.821614

DOI=10.3389/fmars.2022.821614

ISSN=2296-7745

ABSTRACT=The seagrass phyllosphere consists of a mosaic of physico-chemical microgradients that modulate light harvesting, gas and nutrient exchange between the leaves and the surrounding water-column. The phyllosphere is thus of vital importance for seagrass growth and fitness. However, unfavourable environmental conditions such as hypoxia, increasing temperature and high nutrient inputs that are predicted to increase in severity in the Anthropocene, can render the leaf microenvironment into a hostile microhabitat that is challenging for the plants - especially if leaves are covered by epiphytes. Here, we summarize effects of epiphytes on seagrass leaves and discuss how they change the biogeochemical processes and chemical conditions in the seagrass phyllosphere. During night-time, water-column hypoxia can lead to anoxic conditions at the leaf/epiphyte interface, reducing diffusive O2 supply for plant respiration and transport to below-ground tissues. Furthermore, anoxia in epiphytic biofilms can also enable anaerobic processes that can lead to harmful nitric oxide production via denitrification. Such microenvironmental stress conditions at night-time are exacerbated by increasing temperatures. In light, epiphytes often results in lower leaf photosynthetic activity due to epiphyte-induced shading and a combination of O2 build-up and CO2 reduction in the phyllosphere owing to thicker total diffusional pathways, phyllosphere basification and epiphytic carbon fixation. Furthermore, absorbed light energy in epiphytic biofilms can also drive an increase in the leaf surface temperature relative to the surrounding seawater potentially aggravating heating events. In combination, these above-mentioned diurnal effects of epiphytes result in higher compensation irradiance of epiphyte-covered leaves and thus higher light requirements of seagrasses.