Sunday 4 March 2012

The death of a hydrothermal vent community.

The East Pacific Rise runs from the Southern Ocean to the Gulf of California, where it passes into the subduction zone beneath the North American Plate, becoming the San Andreas Fault. A number of deepsea hydrothermal vent communities have been discovered along the East Pacific Rise, some of which have been studied extensively. One of these was the site known as Bio9, located at 9°50' North, which was discovered in 1991, and studied extensively over the next few years. The site thrived until 1995, but appeared to sicken and die over the next two years.

In a paper published in the journal Geochemical Transactions on 27 January 2012, Michael Hentscher and Wolfgang Bach of the Department of Geosciences at the University of Bremen discuss the history of the Bio9 hydrothermal vent community over the period 1991-1997, and theorize about how changes to the chemistry of the water emerging from the vent may have brought about the demise of the vent community.

Diagram showing the chemical cycle of a healthy hydrothermal vent community. From Hentscher & Bach (2012).

The Bio9 community was dominated by tubeworms of the genus Riftia when it was first discovered. These worms obtain nutrition from symbiotic bacteria that live within their bodies, bacteria that oxidize hydrogen sulphide from the mineral rich waters. These worms were thriving in 1991, but in 1994 the colonies appeared to have developed rusty red spots. In 1995 the colonies were largely covered by the rusty spots, and by 1997 the colonies had largely died off.

Healthy Riftia tubeworms. NOAA.

Hentscher & Bach theorize that the rusty material was colonies of red, iron-oxidizing bacteria, and that their growth reflected an increase in the amount of iron ions in the water emerging from the vents; they do not believe the bacteria directly harmed the tubeworms.

The death of the tubeworms between 1995 and 1997 would therefore not be directly related to the spread of the red rust in 1994-5. Hentscher & Bach suspect that this was due to a fall in the level of hydrogen sulphide within the vent water, resulting in the death of first the symbiotic bacteria within the worms, and then the worms themselves.

The chemicals emitted by terrestrial volcanic springs and fumaroles change over time, and there is no reason to suspect that deep-sea hydrothermal vents would behave any differently. If this is the case then the death of this vent community would be a part of the natural life-cycle of the tubeworms, and hydrothermal vent communities in other parts of the world, with different biological communities, probably suffer similar die-offs.