A single-celled organism was isolated several feet below the surface of this anoxic, acidic (pH 3.6) hot spring (90degC). TEM shows the cell has a diffuse nucleoid structure and has an S-layer outside of a single monolayer cell membrane; there is no peptidoglycan layer. The organism oxidizes sulfur compounds to generate energy and fixes CO2.

Which of the following is the most likely classification for this organism based on its metabolism?

(A) Chemoorganoheterotroph
(B) Chemoorganoautotroph
(C) Chemolithoheterotroph
(D) Chemolithoautotroph
(E) Photoorganoheterotroph
(F) Photoorganoautotroph
(G) Photolithoheterotroph

Because the organism fixes CO2, the cell is autotroph. Heterotrophs need carbon sources to consume, releasing CO2. Heterotrophs also require oxygen as part of the phosphorylation chain, the process that effectively extracts energy from the breaking down of compounds.

Because the organism lives beneath the surface, it is likely it doesn't depend on light. This makes the organism a chemoautotroph cell, meaning that produces its energy from chemical reactions. This is supported by the organism living on acidic conditions without an extra layer outside the cell membrane: the cell membrane is required to facilitate the flow of protons during the production of ATP. The oxidation of sulfur compounds indicates dependance on inorganic molecules, making this a chemolithoautotroph. The prefix "-organo-" means the organism requires organic sources of carbon.

Based on the data provided, the presence of an S-layer and the nucleoid structure, it is clear that the cell found is an archaea, making this layer important for maintaining the structure of the membrane against the low pH and estabilising the osmotic pressure. This combination of factors support a chemolithoautotroph metabolism, quite common in archaeas.