Research at the University of Arkansas presented in June 2015 suggested that some methanogens could survive in Mars's low pressure. Rebecca Mickol found that in her laboratory, four species of methanogens survived low-pressure conditions that were similar to a subsurface liquid aquifer on Mars. The four species that she tested were "Methanothermobacter wolfeii", "Methanosarcina barkeri", "Methanobacterium formicicum", and "Methanococcus maripaludis". Methanogens do not require oxygen or organic nutrients, are non-photosynthetic, use hydrogen as their energy source and carbon dioxide (CO2) as their carbon source, so they could exist in subsurface environments on Mars.