Background
The authors of this compelling paper note that "within the last 30 years, the number of coral diseases has increased from two to 18 (Sutherland et al., 2004)," and that "coinciding with the emergence of coral diseases, many coral reefs have experienced dramatic increases in abundance of benthic macroalgae (Hughes, 1994; McCook, 1999; Edmunds, 2002)," which latter organisms have often been considered to have merely taken advantage of declining live coral cover to expand their territory.  On the other hand, it is possible that the presence of macroalgae may be the triggering factor that ultimately leads to the death of nearby corals, and that macroalgae may therefore be creating their own opportunities for territorial expansion.

What was done
To investigate this possibility, Nugues et al. conducted a field experiment in which the green calcareous alga Halimeda opuntia was transplanted next to healthy colonies of Montastraea faveolata, one of the primary reef-building corals in the Caribbean, and in which the subsequent incidence of disease was compared with that of a control group of coral unexposed to the alga.  This study was conducted between December 2002 and February 2003 on the reef terrace at Carmabi Buoy Zero in Curacao, Netherlands Antilles, where 80 colonies of M. faveolata were tagged and randomly assigned to two treatments: algal transplant and control.

"To simulate natural conditions," in the words of the five scientists, "each plant was placed to overgrow a few centimeters of living coral tissue, allowing physical contact between the alga and the coral tissue."  The colonies were then monitored every week or biweekly for three months, while the living coral tissue of control colonies was kept free of all H. opuntia.

What was learned
Signs of disease appeared on coral colonies exposed to the alga within two weeks of the transplantation; and after one month, 55% of the colonies exposed to the algal transplants were affected.  "In contrast," write Nugues et al., "all control colonies remained healthy throughout the study period."  In addition, laboratory analysis confirmed that the diseased corals were afflicted with Aurantimonas coralicida, a marine ureolytic bacterium that induces white plague type II disease; and the pathogen was found in all tissue samples collected from diseased areas of the infected corals, but it was not present in any samples collected from healthy corals.  As a result, Nugues et al. concluded that "H. opuntia (or other organisms associated with the alga) serves as a natural reservoir for the pathogen."

What it means
Nugues et al. note that "elevated temperature, declining water quality and transport of aeolian dust from Saharan Africa have been proposed as potential causal agents of coral diseases (Hayes et al., 2001; Harvell et al., 2002; Bruno et al., 2003)."  Now, yet another agent can be added to the list of coral scourges, as they accurately state their experiment "demonstrates that physical contact with the macroalga H. opuntia can trigger white plague type II in the coral M. faveolata."

References
Bruno, J.F., Petes, L.E., Harvell, C.D. and Hettinger, A.  2003.  Nutrient enrichment can increase the severity of coral diseases.  Ecology Letters 6: 1056-1061.

Edmunds, P.J.  2002.  Long-term dynamics of coral reefs in St. John, US Virgin Islands.  Coral Reefs 21: 357-367.

Harvell, C.D., Mitchell, C.E., Ward, J.R., Altizer, S., Dobson, A.P., Ostfeld, R.S. et al.  2002.  Climate warming and disease risks for terrestrial and marine biota.  Science 296: 2158-2162.

Hayes, M.L., Bonaventura, J., Mitchell, T.P., Prospero, J.M., Shinn, E.A., Van Dolah, F. et al.  2001.  How are climate and marine biological outbreaks functionally linked?  Hydrobiologia 460: 213-220.

Hughes, T.P.  1994.  Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef.  Science 265: 1547-1551.

McCook, L.J.  1999.  Macroalgae, nutrients and phase shifts on coral reefs: scientific issues and management consequences for the Great Barrier Reef.  Coral Reefs 18: 357-367.

Sutherland, K.P., Porter, J.W. and Torres, C.  2004.  Disease and immunity in Caribbean and Indo-Pacific zooxanthellae corals.  Marine Ecology Progress Series 266: 273-302.