|
One global environmental problem that has not received much coverage in the popular press is coral bleaching, which is happening on a global scale. Corals are losing their colour at an alarming rate, current surveys suggesting that 52-90% of coral reefs have been affected. This is a modern problem too, because there is no record of this occurring previously in thousands of years of geological record. To understand what is happening, it is important to remember what gives corals their colour. Coral itself is colourless and translucent, being formed from calcium ions acquired from seawater. The colour is introduced by symbiotic single-cell algae, known as zooxanthellae, which live within the coral tissue and provide its energy via photosynthesis. If these pigmented algae are expelled, the colour disappears. Long-term expulsion can eventually lead to death of the coral since its energy source has gone but bleached reefs have been known to recover too. A host of factors have been considered responsible for coral bleaching, including the increase in surface temperature of the sea due to global warming, changes in salinity, stronger solar irradiance, and the presence of certain pollutants in the water. One of the most ubiquitous pollutants introduced by humans has caught the attention of a group of researchers in California, who conjectured that it might be playing a role in the coral conundrum. That pollutant is caffeine. The average global consumption of caffeine has been estimated at 70 mg/person/day and up to 3% of that is excreted intact. Wastewater treatment plants are unable to remove all of the caffeine so it enters the rivers. In fact, it is so prevalent that it is used as a tracer for the presence of wastewater effluent in natural waters, including the oceans. The research group, consisting of Kelly Pollack, Kimberly Balazs and Oladele Ogunseitan from the University of California, Irvine, estimated that 35 kg/day of caffeine enters the Pacific Ocean from the Orange County Sanitation District alone. The team decided to examine the effects of caffeine on the physiology of four species of coral algae covering three widely distributed different clades (groups), to see if there were any clues pointing to how they were affected. The algae were cultivated in natural seawater before being treated with caffeine over 3-5 weeks. In all cases, caffeine suppressed algal growth. The algal proteins were extracted for separation by 2D gel electrophoresis. The gels were stained and the protein spot intensities were compared with those on gels from algae grown in the absence of caffeine. Proteins found to be significantly up-or down-regulated were removed for digestion with trypsin and identification by liquid chromatography-tandem mass spectrometry. A broad range of proteins within the same clade and between different strains and clades were affected by caffeine, including some involved in photosynthesis, glycolysis and stress response. The most common proteins in the group were the heat shock proteins, Hsp60, Hsp70 and Hsp90 which were up-regulated 2-3 fold in one clade and down-regulated 9-fold in another. Heat shock proteins help to fold newly synthesised proteins, refold misfolded proteins, control regulatory proteins, and transport proteins across membranes. They are already known to be induced in coral under stress and the researchers postulated that caffeine "exacerbates the effects of other stressors that induce similar responses such as elevated temperatures and acidified oceans in a synergistic way to increase the pace and intensity of coral bleaching." They note, however, that the caffeine concentrations used in their study, 50 µg/L to 100 mg/L, are several orders of magnitude greater than those found in seawater but claim that the sensitivity of the stress response pathways to caffeine warrants further investigation. In particular, the synergistic effects of chronic exposure to caffeine and other stressors should be considered. Caffeine in sewage wastewater that is discharged into natural waters and transported to the oceans might be one of the principal causes of coral bleaching, due to its effects on algae, but more work should be carried out to clarify the role of heat shock proteins in the algae and coral. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
|
Coral bleaching showing typical whitening and "fluoro" pigments which also become evident. |
