Alien Invaders: coral pathogens

The following is a repost from the old Southern Fried Science WordPress blog. The original can be found here.

It was a story that could very easily have been written as science fiction. Gorgonian (sea fan) corals of the Florida coast were turning black and dying. The infectious culprit was something no one working on the reefs had encountered before. It was totally alien. The black rot spread across the Caribbean, decimating coral populations. By the time the contagion had been deduced, more than 50% of total sea fan tissue had been eradicated in the Florida Keys. It was one of the worst coral epidemics in recent history.

The culprit was indeed an alien, though certainly not extra-terrestrial. In fact, it was very terrestrial. Aspergillus sydowii, a globally distributed saprophytic soil fungus was the nightmare creature. Aspergillus causes a variety of diseases in humans and birds, but had not previously been recognized as a marine pathogen.

The coral epidemic lasted six years, beginning in 1997. Not surprisingly, it targeted larger, mature corals, severely reducing their biomass and impairing reproduction. Because of this predilection, the total population of gorgonia remained stable, thanks to the influx of juvenile corals from other sources. Eventually, the disease epidemic subsided, due largely to increased host resistance, but also due to the decline in large corals.

How Aspergillus was introduced into the ocean is no surprise. Fungi are prolific in their spore production and dispersal. For every cubic meter of air, there are more than 10,000 fungal spores. That’s a lot of opportunity to take hold. But there is a concept called the mycostatic effect in marine mycology. Simply put, most fungal spores do not germinate in the sea. The osmotic pressure of saltwater prevents most fungal spores from functioning. So, though prolific in numbers in the sea, fungi are not often seen as having a large ecologic contribution.

At least, that’s the conventional wisdom.

Recently other reports of fungal infection in the ocean have appeared. In the Fiji Basin anther black yeast, those this one uncharacterized, has infected mussels at a deep sea hydrothermal vent. These mussels grow black and eventually die. More then 60% of the mussels at one site were infected. How the disease spreads, and how it was transmitted to the deep sea remains a mystery.

Fungi were one of the first to colonize land, and I have a bit of a history of that event here. Only the most basal fungal forms are known to thrive in aqueous environments. So how do these more derived forms return to the sea?

The final chapter (or first chapter? I’m still getting a handle on this whole narative thing) is one that is a common thread in modern ecology. Fungal spores identical to those of the infectious Aspergillus were isolated in the Virgin Islands from layers laid down by dust storm events. These dust storm events originated on the African Coast and carried spores across the Atlantic to settle in the Caribbean. An organism of little notice (though very significant ecology) becomes a pathogen when introduced into a new environment.

We live in a global ecosystem. What happens to one place, no matter how seemingly remote, happens to us all.

~Southern Fried Scientist