Pink-line syndrome, a physiological crisis in the scleractinian coral Porites lutea

Coral diseases are one of the major factors that alter coral cover and their diversity. We have earlier reported the “Pink-line syndrome” (PLS) in the scleractinian coral Porites lutea wherein a colored band appears between the dead and healthy tissue of a colony. About 20% of the P. lutea colonies were affected in Kavaratti of the Lakshadweep Islands in the Arabian Sea during April 1996 and the incidence increased fourfold within the next 4 years. Fungi were associated in both PLS-affected and healthy specimens, whereas the cyanobacterium Phormidium valderianum occurred exclusively in the PLS-affected specimens. There was an increased expression of a 29 kDa protein without any significant increase in total protein content in the PLS-affected colonies. A reduced number of zooxanthellae and an increase in zooxanthellae size, mitotic index, and chl a concentrations were some of the characteristics of the PLS-affected colonies. PLS induction experiments conducted using selected fungi and the cyanobacterium P. valderianum isolated from the affected colonies and abiotic factors, such as CO₂ enrichment and the effect of cyanobacterial photosynthesis inhibition, indicated that the CO₂ build-up around the host tissue caused the pink coloration. We hypothesize that these physiological changes disturb the mutualism between the zooxanthellae and the host. When the symbiosis is disturbed by the external CO₂, the host loses control over the zooxanthellae, causing their uncontrolled division. This process may lead to a break in photosynthate transfer to the host, thereby resulting in starvation and finally leading to partial mortality. We further hypothesize that these degenerative processes are triggered by the CO₂ produced by P. valderianum through its carbon concentration mechanism. In this context, any opportunistic cyanobacteria or other agents having potential to interfere with the physiology of the host or the symbiont can cause such a physiological disorder. The mechanism of PLS formation is an early warning to protect corals as the increasing atmospheric CO₂ could induce PLS-like physiological disorder in corals.