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

Abstract

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 NH 4 + and CO 2 . 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.

Expanded metabolic versatility of ubiquitous nitrite-oxidizing bacteria from the genus Nitrospira

“…2A). Full nitrification was also observed in a second coincubation experiment with an initial urea concentration of only 50 μM, which is closer to the micromolar levels of urea found in natural habitats (29) (Fig. S2C).…”

“…Urea is an important dissolved organic N compound in marine (coastal and open water) and freshwater ecosystems, where it is produced by heterotrophic bacteria and also released by phytoplankton, microfauna, and macrofauna (29). Because commercially synthesized urea plays a major role as plant fertilizer, urea is globally widespread in agricultural soils.…”

Role for urea in nitrification by polar marine Archaea

“…N. limnia (12) have ureases, showing that the potential to degrade urea is not ubiquitous among marine Thaumarchaeota. Indeed, ureases are rare in the Archaea domain (53). Nevertheless, we show that ureases are widespread in polar Thaumarchaeota and, interestingly, in the Arctic metagenome, Thaumarchaeota was the phylum with the highest abundance of urease genes (Fig.…”

Crenarchaeal heterotrophy in salt marsh sediments

“…Likewise, it was surprising that bacteria did not take up any carbon from urea at low concentrations, whereas the crenarchaea did. There is ample evidence of bacteria possessing urease genes (Collier et al, 2009;Solomon et al, 2010) and reports of Campylobacter nitrofigilis isolated from Spartina with urease activity along the United States eastern seaboard (McClung et al, 1983). While no bacterial activity was detected at 30 mM on urea, there was bacterial activity at 150 mM urea.…”