Question

Describe what is similar and what is different about metabolic pathways used by a lithoheterotroph versus a lithoautotroph using ALL of the following terms: electron donor, electron acceptor, reduced, oxidized, organic carbon, and inorganic carbon.

Answer 1

Photolithotrophs

Photolithotrophs obtain energy from light and therefore use inorganic electron donors only to fuel biosynthetic reactions (e. g., carbon dioxide fixation in lithoautotrophs).

Lithoheterotrophs versus lithoautotrophs

Lithotrophic bacteria cannot use, of course, their inorganic energy source as a carbon source for the synthesis of their cells, they choose one of three options:

·         Lithoheterotrophs do not have the possibility to fix carbon dioxide and must consume additional organic compounds in order to break them apart and use their carbon. Only a few bacteria are fully heterolithotrophic.

·         Lithoautotrophs are able to use carbon dioxide from the air as carbon source, the same wayplants do.

Chemolithotrophs versus photolithotrophs

In addition to this division, lithotrophs differ in the initial energy source which initiates ATP production:

·         Chemolithotrophs use the above-mentioned inorganic compounds for aerobic or anaerobic respiration. The energy produced by the oxidation of these compounds is enough for ATP production, some of the electrons derived from the inorganic donors also need to be channeled into biosynthesis. Mostly, additional energy has to be invested to transform these reducing equivalents to the forms and redox potentials needed (mostly NADH or NADPH), which occurs by reverse electron transfer reactions.

·         Photolithotrophs use light as energy source. These bacteria are photosynthetic;photolithotrophic bacteria are found in the purple bacteria (e. g., Chromatiaceae), green bacteria (Chlorobiaceae and Chloroflexi) and Cyanobacteria. Purple and green bacteria oxidize sulfide, sulfur, sulfite, iron or hydrogen. Cyanobacteria extract reducing equivalents from water, i.e., they oxidise water to oxygen. The electrons obtained from the electron donors are not used for ATP production (as long as there is light); they are used in biosynthetic reactions. Some photolithotrophs shift over to chemolithotrophic metabolism in the dark.