A study published in the journal Communications Biology, from the Nature publishing group, reveals the unusual high concentrations of methane in surface waters of the volcanic soda lake Sonachi (Kenya), in the African Rift Valley. The metabolic plasticity and the high productivity of the prokaryotic community of the lake ─ cyanobacteria aggregates (phototrophs), bacteria (heterotrophs), and archaea (methanogens)─ could explain the high concentration of methane, a gas with a powerful greenhouse effect that presents values higher than those in the other lakes worldwide.
The study is led by experts from the Faculty of Biology of the University of Barcelona, the National Research Council of Italy and the University of Florence, and counts on the participation of research groups from ten international institutions.
Soda lakes in the African Rift Valley
The Great Rift Valley, the great tectonic crack of the African continent, has several tectonic or volcanic endorreic lakes where the lake can only leave the lacustrine system via evaporation. In these habitats under extreme and particular conditions, intense evaporation creates a high concentration of salt: in most of the cases, sodium carbonate, a highly soluble compound that generates a high alkalinity.
The lake Sonachi is a small saline-alkaline meromictic lake and its columna mixes shallow and deep waters partially. Located in the western side of the Naivasha lake, it is in the inside of a volcanic crater and it has a specific alkaline chemistry that attracted the interest of the international scientific community for some time.
“The soda lakes in the Rift are productive ecosystems thanks to the proliferation of cyanobacteria, such as spirulina. Parallelly, the high alkalinity turns them into extreme environments and it leads to an enormous scientific interest to discover the metabolism of the microbial communities”, notes the lecturer Andrea Butturini, co-author of the study and member of the Department of Evolutionary Biology, Ecology and Environmental Sciences.
“These extreme environments are natural habitats of interest for the study of the origins of life, astrobiology, the production of biofuel, biotechnology, etc.”, he continues. “Paradoxically, there are almost no integrating works that study in the original place and under natural conditions the composition and functioning of these lake communities as a whole. Consequently, there are still many unknown aspects of the carbon cycle of soda lakes”.
Saline-alkaline lakes, bacterial activity and methane
The article published in Community Biology is one of the first studies to relate the presence of gases (dihydrogen, methane and carbon dioxide) with the availability of the dissolved organic matter and the high microbial biodiversity in the soda lake water column, from the surface to the sediment-filled depth.
The study reveals that methane concentrations in the Sonachi lake are two or three orders of magnitude higher than those in boreal lakes, the most studied lacustrine systems. In the scientific literature, such high concentrations of methane that near to a lake’s surface had been never described before.
“Clearly, the most shocking result was to find high concentrations of methane in oxygenated waters near the surface of the lake, which derive from the high productivity of photosynthetic planktonic communities, mainly cyanobacteria”, highlights Butturini. “In this part of the water column, what we would expect to detect is a methane consumption and a lesser concentration of this gas”.
In most of the African soda lakes, the microbial planktonic community is articulated around the cyanobacteria, procaryote that are abundant in these lacustrine habitats. The particular biodiversity of the microbial community in the Sonachi lake could be a factor of the equation to explain the high methanogenesis described in the study.
“In the lake Sonachi, cyanobacteria unite a microbial community with a high genetic and functional diversity: autotroph bacteria (oxygenic and anoxygenic), fermentative, heteotroph, methanotroph (methane consumers), sulphur reducers and methanogens (hydrogenotrophic, acetotrophic and methylotrophic). Moreover, all these bacteria with such a versatile metabolism come together as aggregates that measure a few centimetres”, notes Butturini. “We do not know yet whether this pattern is replicated in other soda lakes in the Rift area. Therefore, we hope to tackle this mystery in future research studies”.
At a microhabitat scale, anoxic conditions (without oxygen) could be established in these microbial aggregates could, which facilitate the proliferation of the methanogen archaea (in theory, strict anaerobic), also thanks to the organic leachates that release cyanobacteria. “The Rift soda lakes are very productive due to the high concentration of cyanobacteria. This is why we state that the high bacterial productivity catalyses in some way the production of methane near the lake surface, a hypothesis to be further explored in the future”, notes Butturini.
Expanding the horizon with tropical and equatorial lakes
Methane, with an atmospheric concentration on the rise, generates a greenhouse effect higher to that of carbon dioxide and other compounds. In this context, the natural continental water systems ─reservoirs, lakes, shallow flood-prone areas, peatlands, etc. ─ are important sources of methane and carbon dioxide emissions to the atmosphere.
“Understanding the importance of continental water systems in the emissions of methane, carbon dioxide and nitrogen dioxide at a global scale is an active and innovative field of research in limnology and biogeochemistry. However, it is undeniable that our knowledge is largely based on natural systems of mild and boreal areas and that there are not many studies on tropical and equatorial systems”, stresses the expert.
“In this framework, our study reveals we should expand our horizons and other remote systems which are not much studied yet. The surprises can be huge. This has motivated us to establish a collaboration with the University of Nairobi in order to organize, in 2022, a course on limnology of extreme environments for students in that institution”, concludes Andrea Butturini.
Fazi, F.; Amalfitano, S.; Venturi, S.; Pacini, N.; , Vázquez, E.; Olaka, L. A.; Tassi, F.; Crognale, S.; Herzsprung, P.; Lechtenfeld, O. J.; Cabassi, J.; Capecchiacci, F.; Rossetti, S.; Yakimov, M. M.; Vaselli, O.; Harper, D. M.; Butturini, A. «High concentrations of dissolved biogenic methane associated with cyanobacterial blooms in East African lake surface water». Communications Biology, July 2021. Doi: 10.1038/s42003-021-02365-x