Carbon in the form of CO 2, HCO 3 - and CO 3 -2, are oxidized forms of C, and tend to be the only forms present where O 2 is plentiful. In anoxic environments methanogens (Archaea) convert organic C and CO 2 into methane (CH 4 ). Methane is a gas and can bubble out of the water or it can be oxidized to CO 2 by methylotrophic bacteria.

Methanogens are not true bacteria, they belong to the Archaea Most methanogens can grow on CO 2 (or organic C)and H 2 as their sole energy source: Chemoautotrophs —chemical bond energy is their energy source they utilize CO 2 as their C source

C-transformations in aerobic and anaerobic environments Under anaerobic conditions organic molecules break down to methane instead of CO 2 —This process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria. They utilize the energy released from 2H 2 + Organic C (CH 2 O)→CH 4 + H 2 0 to build their biomass Oxidation state Where do we find methanogens?

We keep track of the e - transfer using Oxidation numbers (Ox#) For each e - transferred the Ox# changes by 1 2H 2 + O 2 2H 2 O Some rules for Oxidation numbers 1. In free elements Ox# =0 2. For ions with one atom Ox# = charge. eg H + Ox# of H + = 1 3. Ox# of O in most compounds is -2, 4. Ox# of H in most compounds is +1, 5. For a complex ion like SO 4 -2, the net Ox# = charge (Thus S=+6) How to assign Oxidation numbers

Carbon and Oxygen play a major role in biological energy transformations Oxidation means giving up electrons, and reduction means taking on electrons (─) The oxidation state of C in CO 2 is +4, in Carbohydrates is 0 How many electrons are taken up by each C during photosynthesis?

Chemical equation for the reduction of CO 2 by H 2

Energy Source Carbon Source Light Chemical CO 2 Organic Photo- autotroph Photo- heterotroph Chemo- autotroph Chemo- heterotroph Four nutritional categories Nutrition and Metabolic Diversity