The Primary Colonisers of Microbial Mats at Low Temperature Hydrothermal Vents at Loihi Seamount.

Areas of marine geothermal activity, such as hydrothermal vents provide a habitat rich in nutrients. This results in hotspots of many species of organisms, and can account for some of the most productive ecosystems on Earth. The depth at which these vents are formed far exceed the reaches of light to aid photosynthetic processes, therefore the organisms present at these vents rely on chemosynthesis. Chemoautotrophic micro organisms account for the primary production, forming the base of many food webs at these vent systems. The chemoautotrophs derive their nutrients from sulphur, iron or magnesium, which are often products from the vent processes. They can be present as microbial mats, symbiotes, free living, or they can be found in the tubes of the vents themselves.

The hydrothermal vents of Loihi seamount, located 21 miles East of Hawaii were used in this study. Loihi seamount is seismically active, and the youngest of the island arc formations of the Hawaiian Islands. The original vents which were discovered there were named Pele’s Vents. They showed high concentrations of dissolved carbon dioxide resulting in the low pH of 4.2. Fe(II) concentrations were also high (a magnitude of 6 times higher than that of the surrounding water), but the dissolved oxygen, magnesium and sulphate were depleted.

Following an eruption in 1996, Pele’s Vents became extinct and Pele’s Pit, a caldera, took its place (a caldera is a crater formed from the collapse of a volcano once the magma chambers below can no longer support its weight). The vents in this caldera and in the areas surrounding it (Naha and Ula Nui Vents) were the sites used in this study (figure 1). Depths of the vents varied from 1116m to 4988m, and temperatures ranged from 2°C to 165°C.

Figure 1: Map of Pele’s Pit, and the locations where the microbial growth chambers were deployed.

To conduct this study, microbial growth chambers (MGCs) were constructed using plexiglass cylinders which were enclosed at both ends to prevent the entry of macrofauna. The interior of the cylinders were covered in a fine mesh of silica wool (8μm diameter), which would be used as a substrate for micro organisms. They were then deployed at seven sample sites (figure 1) using submersibles and ROV’s, and kept there for short term (4-10 days) or long term periods (1-6 years). In total, 41 MGCs were deployed and recovered; 28 short term and 13 long term over a ten year period. Once recovered, the MGCs were opened using aseptic techniques, the silica wool was removed, placed into a sample bag, and frozen immediately at -80°C until ready for processing. When necessary, the samples were thawed on ice, the microbes present were isolated, DNA extraction took place, and then a PCR was conducted to amplify the DNA.

Cluster analysis showed that short term MGCs showed three clusters which were designated 'Short-term Clusters 1, 2 and 3'. Eighteen out of 28 of the short term MGCs were primarily colonised by ζ-proteobacteria, this is Short- term Cluster 1. Short-term Cluster 2 contained eight MGC communities, and showed two dominant populations from the ε-proteobacteria, or ζ-proteobacteria. Finally, Short-term Cluster 3 contained 2 MGC communities dominated by a single ε-proteobacteria population. The average temperatures of the vent effluents were 40°C, 51°C and 71°C. 

All vent sites had iron-rich microbial mats due to the Fe(II) enriched waters from the vents. The cooler temperature vents were predominantly colonized by iron-oxidising ζ-proteobacteria, and the warmer vents by sulphur-cycling ε-proteobacteria. Vents with an intermediate temperature were colonized by both ε-proteobacteria and ζ-proteobacteria. Therefore, the ε-proteobacteria and ζ-proteobacteria are the main colonisers of the hydrothermal vents in the Loihu seamount, Hawaii.

I did enjoy reading this paper, and it was the first to study and show that ζ-proteobacteria are the dominant colonisers of low temperature hydrothermal vents. However, I personally found that some of the methods used weren’t very well described, and so I found it a bit difficult to interpret them into my own words.

Rassa, A. C., McAllister, S. M., Safran, S. A. & Moyer, C. L. (2009) Zeta-Proteobacteria dominate the colonization and formation of microbial mats in low-temperature hydrothermal vents at Loihi Seamount, Hawaii. Geomicrobiology Journal. 26: 623-638.