The thick, mucilaginous biofilms that coat the insides of carbonate chimneys in the Lost City hydrothermal field are dominated by a single type of archaeon, according to their 16S genes. But this is a case where 16S doesn’t tell the whole story – not by a long shot. A study in mBio this week takes a close look at these biofilms and finds these archaea, which are practically identical at first blush, are probably highly differentiated.
The chemical engine behind these archaea and the rest of the rich microbial life of the Lost City is the process of serpentinization: the mineral olivine in the rocks reacts with seawater to make (among other things) hydrogen, methane, and larger hydrocarbons. This process, and locations like the Lost City may also have been the cradle for the earliest life on the planet, so teasing apart the different ecological roles of the biofilm members and their cooperative interactions could lend insights into how multicellular life developed.
In the Lost City, one result of serpentinization is the tall carbonate chimneys lined with thick biofilms that live off the bounty. Prior work on these communities revealed they are dominated by one uncultivated type of archaea known as Lost City Methanosarcinales (LCMS). Brazelton et al. thought it curious that a single species should dominate this habitat, considering that hydrothermal activity at the vents has been ongoing in the area for at least 100,000 years.
Transmission electron microscopy of the biofilm revealed multiple cell morphologies within the biofilm, which is not too surprising, given the fact that bacteria comprise as many as 20% of biofilm cells. But some of the cells contained intracellular membranes that could facilitate methane oxidation, and the most abundant bacteria in the chimneys are a group of sulfur-oxidizers, which are not likely to be methane oxidizers.
Experiments in the lab showed the biofilm both oxidizes and produces methane – and both processes were stimulated by hydrogen, indicating the two reactions are part of a syntrophic interaction. Metagenomic sequences from the biofilm yielded sequence that indicate acetate may play a role in the methane-cycling syntrophy, and a broad range of nitrogen fixation genes were identified, many of which were probably acquired via lateral gene transfer.