Prokaryotic heterotrophic production (PHP) in marine sediments is a crucial variable for quantifying the contribution of prokaryotes to the carbon cycling in marine ecosystems. Prokaryotic heterotrophic production has been successfully measured in sediments with radioactive tracers since 1981 (Moriarty and Pollard 1981; Kirchman et al. 1986) and is still utilized (Manini et al. 2004; Danovaro et al. 2008, 2009). Biomass production in aquatic systems is now commonly estimated from the rate of incorporation of radioactively labeled leucine (i.e., 3H-Leu) into prokaryotic proteins during short-term incubations (Kemp et al. 1993). Biomass production is calculated from the rate of protein synthesis because proteins comprise a large, fairly constant fraction of prokaryotic biomass. This procedure is used to estimate a conversion factor (cells or g C per mol of leucine incorporation) that converts leucine incorporation into production of biomass.