Role of urea in microbial metabolism in aquatic systems: a biochemical and molecular review

Urea synthesized commercially and formed naturally as a by-product of cellular metabolism is an important source of nitrogen (N) for primary producers in aquatic ecosystems. Although urea is usually present at ambient concentrations below 1 μM-N, it can contribute 50% or more of the total N used by planktonic communities. Urea may be produced intracellularly via purine catabolism and/or the urea cycle. In many bacteria and eukaryotes, urea in the cell can be broken down by urease into NH4 and CO2. In addition, some bacteria and eukaryotes use urea amidolyase (UALase) to decompose urea. The regulation of urea uptake appears to differ from the regulation of urease activity, and newly available genomic sequence data reveal that urea transporters in eukaryotic phytoplankton are distinct from those present in Cyanobacteria and heterotrophic bacteria with different energy sources and possibly different enzyme kinetics. The diverse metabolic pathways of urea transport, production, and decomposition may contribute to differences in the role that urea plays in the physiology and ecology of different species, and in the role that each species plays in the biogeochemistry of urea. This review summarizes what is known about urea sources and availability, use of urea as an organic N growth source, rates of urea uptake, enzymes involved in urea metabolism (i.e. urea transporters, urease, UALase), and the biochemical and molecular regulation of urea transport and metabolic enzymes, with an emphasis on the potential for genomic sequence data to continue to provide important new insights.