Monday 19 March 2012

Two new hydrothermal vent communities from the southern Central Indian Ridge.

Deep sea hydrothermal vents are unique ecosystems where the food chain is based not upon the photosynthetic activity of plants or algae, but rather of chemotrophic bacteria that gain their energy from the chemicals dissolved in the water issuing from the vents. These vents are found in areas of volcanic activity such as mid-ocean ridges where new seafloor is being created. At such locations water seeps through pores in the rock and comes into contact with volcanic magma. On the surface this would result in an explosive release of steam, but in the high pressure environment of the deep seas this is not possible, so the water is super-heated escapes from the vents at temperatures of hundreds of degrees centigrade, but still as a liquid. This super-heated water is typically rich in minerals absorbed from the magma, which feeds the ecosystems living at the vents. If this includes a lot of Iron Sulphide compounds then it will be dark and opaque; vents from which such water issues are known as 'black smokers'.

In a paper published in the journal PLoS One on 14 March 2012, a team of scientists led by Kentaro Nakamura of the Precambrian Ecosystem Laboratory at the Japan Agency for Marine-Earth Science and Technology, describe the discovery of two new hydrothermal vent communities on the southern Central Indian Ridge, east of Mauritius. The discoveries were made during a voyage by the Research Vessel Yokosuka, and visited by the submersible Shinkai 6500. The two sites have been named Dodo and Solitaire, after the extinct flightless birds of Mauritius and Rodruiguez.

The location of the newly discovered hydrothermal vent systems. Nakamura et al. (2012).

The more northerly of these, dubbed the Dodo Hydrothermal Field, is located on a basalt field named the Dodo Great Lava Plain. The site was about 15 m in diameter, and covered in small black smoker type vents. This was the hotter of the two sites, with a maximum recorded temperature of 356°C. The water at this site was unusually rich in dissolved Chlorine.

(A) Small black smokers on the Dodo Hydrothermal Vent Field. (B) Lava filed near the Dodo Hydrothermal Vent Field, showing brown staining. Nakamura et al. (2012).

The Dodo Field lacked a rich or diverse fauna. A large area around the vent had brown staining on the lava, and scattered across this, but densest near the vents, were Bythograeid Crabs, Austinograea rodriguezensis. Also present were numbers of the Alvinocaridid Shrimp Rimicaris kairei, which is the dominant animal at some previously discovered Indian Ocean hydrothermal vents. A few gastropods and sea anemones were also seen.

(A) Bythograeid Crabs, Austinograea rodriguezensis and Alvinocaridid Shrimps around a black smoker at the Dodo Hydrothermal Vent Field. (B) Bythograeid Crabs, Austinograea rodriguezensis, Alvinocaridid Shrimps and Sea Anemones around small vents on the Dodo Hydrothermal Vent Field. Nakamura et al. (2012).

Nakamura et al. considered the small chimney sizes and low faunal diversity at Solitaire Field to be indicative of a young field that had only recently started to vent hydrothermally rich water; thus this system could in theory support a diverse faunal assemblage, it is simply to young to have been colonized yet.

The Solitaire Field covers an area roughly 50 m by 50 m at the base of a cliff at the eastern end of a plateaux (called the Roger Plateaux). The site has three clusters of vent chimneys, all under five meters in height, only one of which produces black smoker type outflow. This was also the hottest site on the field, at 296°C, cooler than at the Dodo Vent Field.

Black smoker chimneys at the Solitaire Field. The white encrustation is a colony of the Alvinocaridid ShrimpRimicaris kairei. Nakamura et al. (2012).

The Solitaire Field is much richer biologically than the Dodo Field, both in terms of the number of species present and overall biomass. As well as Bythograeid Crabs and Alvinocaridid Shrimps there was a diverse fauna of gastropods, mussels, worms, barnacles, fish and larger crustaceans.

(C) Colony of the Alvinocaridid ShrimpRimicaris kairei on a vent chimney in the Solitaire Field. (D) Faunal assemblage around a vent in the Solitaire Field. (E) Colony of Scaly-foot Gastropods, Bythograeid Crabs and Alvinellid Polychaetes (yellow circle) in the Solitaire Field. (F) Alviniconcha gastropods and Bathymodiolus mussels in the Solitaire Field. (G) Phymorhynchus gastropods in the Solitaire Field. (H) Neolepas barnacles in the Solitaire Field; the fist time these have been recorded at an Indian Ocean hydrothermal vent. Nakamura et al. (2012).

One particularly interesting discovery at the Solitaire Vent Field is the presence of a pale colony of the Scaly-footed Gastropod. These have previously been recorded at the Kairei Vent Field, in the south-central Indian Ocean, to the east of the Solitaire Vent Field. The Kairei Gastropods secrete Iron Sulphide compounds in their shells and foot scales (shelly plates covering the upper part of the foot), giving them a black colour; the Solitaire Gastropods do not do this and are very pale, but DNA studies show them to be identical genetically. Nakamura et al. experimented to see if the Iron Sulphide minerals contributed to the strength of the shell in the Kairei Vent Field Gastropods, but found they had a weaker shell structure than the Solitaire Gastropods. For some reason they did not consider the possibility that this secretion of Iron Sulphide minerals may have been a way of removing excess Iron Sulphide from the soft tissues of the Gastropods.

Black, Iron Sulphide secreting Scaly-footed Gastropod from the Kairei Vent Field (top), and pale, non-Iron Sulphide secreting Scaly-footed Gastropod from the Solitaire Field (bottom). Scale bar is 10 mm. Nakamura et al. (2012).