Partnerships in good times continue for a long time. Good faith and loyalty among the partners is conveyed down the generations. The partnership will be caught in rough weather when a third factor favours one partner and encourages to prosper. This sometimes destroys the other partner! The same case is observed for the partners in nature who have lived for millions of years together and are disturbed by a newly arrived third factor.

Have you heard of nature's wonder called 'Coral reefs'? Well! The fascinating television serials, films, picturesque books that tell us about underwater life cannot skip our attention to this wonderful ecosystem. The colourful fishes moving in and around the coral reefs attract any one. The reefs provide protection and shelter for different species of fish. Without coral reefs, these fish would be left homeless! Not only do these fish increase the diversity of our world, but also reef fish and mollusks feed between 30 and 40 million people every year. They also make beautiful pets. Many people make their livelihood by catching and selling these animals. Coral reefs are also very important because they protect coasts from strong currents and waves by slowing down the water before it gets to the shore. That is why they are called barrier reefs - they provide a barrier between the ocean and the shore. But that is not enough.

Corals are very important in controlling carbon dioxide in the ocean water. To understand this one has to know how the corals grow. The baby coral (or coral polyp - a tiny animal that looks like an upside-down jellyfish) floats around in the water until it finds a hard place to attach itself. It then secretes external skeletons of calcium carbonate, in other words, turns carbon dioxide in the water into a limestone shell. The actual process of shell building is done by combining carbon dioxide (CO₂) and calcium (Ca) in the water to make calcium carbonate (CaCO₃) also known as limestone. Besides this, corals live a symbiotic life. Inside the sac of each coral polyp lives a one-celled algae called zooxanthellae. The algae also require carbon dioxide for their growth which they take from coral polyps and in return give off oxygen and other nutrients that the coral polyp needs to live. Thus they live in a mutual association which was conserved for millions of years. Without coral, the amount of carbon dioxide in the water would rise dramatically and affect all living beings on the Earth. Carbon dioxide is also consumed by all plants on the planet. Therefore it is important for life on the Earth. But too much of carbon dioxide destabilizes the natural equilibrium. Scientists at NIO have studied the cause of a disease called 'pink-line syndrome' (PLS) on the corals of Lakshadweep and arrived at the conclusion that the increased supply of carbon dioxide to the zooxanthellae is the root of this disease. Beautiful corals at the Kavaratti Island of Lakshadweep in the Arabian Sea were noticed to be getting affected with PLS. The PLS affected specimens, when studied, were found to be infested by a cyanobacterium by name Phormidium valderianum - the source of increased carbon dioxide, besides some fungal colonies.

A survey was carried out twice a year during 1996-99 from two locations - one each from north and south of the lagoon (Fig. 1) - to monitor symptoms of PLS and the extent of partial mortality of Porites lutea - a coral in that area.

While collecting samples during the survey, they noticed increase in the incidence of PLS in both the locations from 20% in 1996 to 80% in 1999 (Fig. 2). Throughout the period of survey, the incidence of PLS was higher in summer (April 1997, 1998, and 1999) when the seawater temperature is always relatively higher. The corals on the southern part of the lagoon in Kavaratti were affected more than those in the northern location of survey. This is because it is shallower than the northern part of the lagoon and also because the faster tidal currents in the northern part flush out the resident pathogen from the lagoon to the sea through a channel, thus lowering its chances of settling on coral colonies more frequently than the relatively stagnant waters in the southern part of the lagoon.

The scientists made some visual observations like deterioration of the coral skeletons. In the PLS-affected colonies, a cyanobacterial mat had covered the dead patches with the presence of degenerated coenosarcal tissue exposing the skeleton intermittently. Observations through the stereo zoom microscope showed how the wall of the corallite in the PLS-affected region had become thin and fragile (Fig. 3).

The decalcified PLS-affected specimens showed that the polyp tissue and not the skeleton had turned pink (Fig. 4). While fungi were constantly associated with the polyps in the healthy and PLS-affected specimens, the cyanobacteria were found only in the PLS-affected colonies.

The scientists also observed a significant difference between the density, Mitotic Index (MI) and the size of zooxanthellae of healthy and PLS-affected corals (Table 1). The reduced density of the zooxanthellae in the PLS-affected coral was attributed to the expulsion from the PLS-affected tissue (Fig. 5). Due to this expulsion, the MI is much lower than expected in the PLS-affected specimen

Table 1 Zooxanthellae status in healthy and PLS-affected Porites lutea

Laboratory experiments to observe the response of P. Lutea after inducting PLS using biotic and abiotic factors were also conducted. The biotic factors include effect of the associates that were noticed in the natural environment. During the survey, the fungi and the cyanobacterium was found to be associated with the coral. The effects of the fungi and of a cyanobacterium were tested separately on the healthy specimens of the coral. While the specimen associated with fungi did not show any symptoms of PLS, the other specimen to which the cyanobacterium - P. valderianum was inoculated did show positive symptoms of PLS. By the third day, all colonies of the coral inoculated with cyanobacterium, turned pink around the inoculum (Fig. 6) and within a week, the entire colony turned pink. At the end of two weeks, the entire colony was covered with the cyanobacterium.

It is known that the carbon concentrating mechanism (CCM) in various photosynthetic organisms including the cyanobacterium elevates the carbon dioxide concentration intracellularly and in some organisms the excess CO₂ is leaked out of the cell. To check whether such mechanism that increases the carbon dioxide concentration is the cause of PLS, another test was carried out. A micro environment was simulated by passing CO₂ gas in the coral cavities. CO₂ gas was generated by treating molluscan shells with 5% HCl. Molluscan shells, after washing with fresh water, were put in a 500 ml conical flask containing 5% HCl. The flask was closed using a rubber cork with a vent. Using a thin intravenous tube, CO₂ generated in the flask was passed on to the healthy colonies through the base of a pipette tip (Fig. 7).

After 2 weeks, the polyps adjacent to the elevated CO2 micro environment showed a pink line of 1 mm width in all the experimental coral colonies that morphologically resembled the pink colored tissue in PLS-affected specimens found in the lagoon.

The major symptoms that are associated with PLS are increase in zooxanthellae size, MI values and chl a concentration, decrease in zooxanthellae density accompanied by skeletal erosion, pink-line formation, and partial mortality. Considering these observed parameters and establishment of a successful induction of PLS using the cyanobacterium and gaseous CO₂ led scientists to hypothesize that the development of the PLS is related to one single mechanism, namely the cyanobacterial CCM.

If a small creature like ant enters the trunk of a giant elephant it makes him violent. So it is with this microscopic cyanobacterium. It is eroding coral colonies by inoculating the disease on a small scale. However, the risk is much larger. Scientists fear the increasing atmospheric CO₂ may erode the coral colonies - the ecosystem that we are supposed to pass on to the next generation.

To consult original article, refer :
Ravindran, J.; Raghukumar, C. Pink-line syndrome, a physiological crisis in the scleractinian coral Porites lutea. Mar. Biol.: 149(2); 2006; 347-356.