GAP and either pyruvate or dihydroxyacetone phosphate (DHAP). The pyruvate or DHAP is used towards biomass while the other 2 hexulose phosphate molecules and the molecule of GAP are used to regenerate the 3 molecules of ribulose-5-phosphate. Serine cycle Unlike the other assimilatory pathways, the serine cycle uses carboxylic acids and amino acids as intermediates instead of carbohydrates. First, 2 molecules of formaldehyde are added to 2 molecules of the amino acid glycine. This produces two molecules of the amino acid serine, the key intermediate of this pathway. These serine molecules eventually produce 2 molecules of 2-phosphoglycerate, with one C3 molecule going towards biomass and the other being used to regenerate glycine. Notably, the regeneration of glycine requires a molecule of CO2 as well, therefore the Serine pathway also differs from the other 3 pathways by its requirement of both formaldehyde and CO2. Yeasts Methylotrophic yeast metabolism differs from bacteria primarily on the basis of the enzymes used and the carbon assimilation pathway. Unlike bacteria which use bacterial MDH, methylotrophic yeasts oxidize methanol in their peroxisomes with a non-specific alcohol oxidase. This produces formaldehyde as well as hydrogen peroxide. Compartmentalization of this reaction in peroxisomes likely sequesters the hydrogen peroxide produced. Catalase is produced in the peroxisomes to deal with this harmful by-product. Dihydroxyacteone (DHA) cycle The dihydroxyacetone (DHA) pathway, also known as the xylulose monophosphate (XuMP) pathway, is found exclusively in yeast. This pathway assimilates three molecules of formaldehyde into 1 molecule of DHAP using 3 molecules of xylulose 5-phosphate as the key intermediate. DHA synthase acts as a transferase (transketolase) to transfer part of xylulose 5-phosphate to DHA. Then these 3 molecules of DHA are phosphorylated to DHAP by triokinase. Like the other cycles, 3 C3 molecules are produced with 1 molecule being directed for use as cell material. The other 2 molecules are used to regenerate xylulose 5-phosphate. Environmental Implications As key players in the carbon cycle, methylotrophs work to reduce global warming primarily through the uptake of methane and other greenhouse gases. In aqueous environments, methanogenic bacteria produce 40-50% of the world's methane. Symbiosis between methanogens and methanotrophic bacteria greatly decreases the amount of methane released into the atmosphere. This symbiosis is also important in the marine environment. Marine bacteria are very important to food webs and biogeochemical cycles, particularly in coastal surface waters but also in other key ecosystems such as hydrothermal vents. There is