Impairment of symbiont photosynthesis increases host cell proliferation in the epidermis of the sea anemone Aiptasia pallida

Corals exposed to environmental stresses need to engage appropriate physiological strategies to survive. Here we examined tissue modifications following algal dysfunction. Aiptasia pallida was exposed during one week to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the herbicide called Diuron. DCMU treatment produced a drastic loss in photosynthetic efficiency and a subsequent diminution of algae density over the following days. Cell proliferation evaluated by measuring the number of cells labeled with a thymidine analogue (EdU) revealed a significant increase in EdU+ cells in the epidermis after one week of DCMU incubation and in the gastrodermis at four weeks. TUNEL histology showed that the extent of cell death was however similar in the epidermis of control and treated specimens. In addition we noticed a significant effect of DCMU treatment on the density of epidermal mucocytes after one, two and four weeks. These results show that inhibition of Symbiodinium photosynthesis in the absence of any known direct effect of DCMU on host cells can induce an increase of epidermal host cell proliferation in both the epidermis and the gastrodermis. While new host gastrodermal cells are likely to promote tissue regeneration in order to recruit new algae, the new host epidermal cells may contribute to tissue adaptation following a decrease in energy income. Some of these new epidermal cells, such as mucocytes, may contribute to an eventual increase of the host heterotrophic ability until restoration of algal autotrophic contribution.