Heterotrophic microorganisms assimilate only low molecular weight organic compounds, which can be transported directly across the cell membrane. Such substrates include C2 compounds (e.g. acetate, ethanol), C3 compounds (e.g. pyruvate), C4 compounds (e.g. succinate), and C5 and C6 compounds (e.g. sugars, amino acids). In contrast, high molecular weight organic compounds such as polysaccharides and proteins can be utilized by bacteria only after extracellular cleavage, or hydrolysis, which can then be transported into the bacterial cell. Some variable and perhaps large fraction of the LMW-DOM is derived from the breakdown of high molecular weight DOM (HMW-DOM) by extracellular enzymes.
p.20; in ....
Sherr E., Sherr B.; 2000; Marine Microbes an Overview; in Kirchman D.L. (ed); Microbial Ecology of the Oceans; Willey-Liss Inc.; New York USA;