High temperature ecosystems and their chemical interactions with their environment.

Phototrophic thermal ecosystems consist of microbial mats whose composition is largely determined by water temperature, dissolved oxygen, sulfide and pH. Mats exposed to sunlight consist of an upper zone of phototrophic bacteria and cyanobacteria and an undermat of heterotrophic bacteria. There is little or no net accumulation of reduced carbon and a quasi-equilibrium is established between the synthesis and oxidation of reduced carbon. The flux of carbon and other metabolites induces chemical change in the interstitial water which may assist the deposition of hydrothermal minerals. Uptake of carbon dioxide by phototrophs is favourable to calcium carbonate (travertine) deposition. Thermal systems also contain a range of chemolithotrophs and sulfate reducers potentially capable of depositing carbonate. Acid production by sulfate reducers may have the ability to precipitate silica from alkaline thermal waters but has not yet been demonstrated in vivo. Deposition of thermal ochre is also possible via bacterial oxidation of reduced iron and manganese. It appears that bacteria play a minor role in the deposition of hydrothermal minerals through chemical interaction. However, they may play a more important physical role by providing a large surface are suitable for mineral nucleation. If hydrothermal deposits occur on Mars, the distribution of travertine is likely to be restricted if there is a lack of pre-existing sedimentary carbonate. Less biologically interactive deposits of silica and ochre may predominate.