Spatial heterogeneity of photosynthesis and the effect of temperature-induced bleaching conditions in three species of corals

Heterogeneity in photosynthetic performance between polyp and coenosarc tissue in corals was shown using a new variable fluorescence imaging system (Imaging-PAM) with three species of coral, Acropora nobilis, Cyphastrea serailia and Pocillopora damicornis. In comparison to earlier studies with fibre-optic microprobes for fluorescence analysis, the Imaging-PAM enables greater accuracy by allowing different tissues to be better defined and by providing many more data points within a given time. Spatial variability of photosynthetic performance from the tip to the distal parts was revealed in one species of branching coral, A. nobilis. The effect of bleaching conditions (33degreesC vs. 27degreesC) was studied over a period of 8 h. Marked changes in fluorescence parameters were observed for all three species. Although a decline in Phi(PSII) (effective quantum yield) and Yi (the first effective quantum yield obtained from a rapid light curve) were observed, P. damicornis showed no visual signs of bleaching on the Imaging-PAM after this time. In A. nobilis and C. serailia, visual signs of bleaching over the 8 h period were accompanied by marked changes in F (light-adapted fluorescence yield), NPQ (non-photochemical quenching) and E-k (minimum saturating irradiance), as well as Phi(PSII) and Yi. These changes were most marked over the first 5 h. The most sensitive species was A. nobilis, which after 8 h at 33degreesC had reached a Phi(PSII) value of almost zero across its whole surface. Differential bleaching responses between polyps and coenosarc tissue were found in P. damicornis, but not in A. nobilis and C. serailia. NPQ increased with exposure time to 33degreesC in both the latter species, accompanied by a decreasing E-k, suggesting that the xanthophyll cycle is entrained as a mechanism for reducing the effects of the bleaching conditions.