Urea is a crucial nitrogen nutrient source for algae with the potential to stimulate harmful algal blooms, but the molecular machinery underpinning urea uptake and assimilation by algae is not fully understood. Urease (URE) is commonly regarded as the responsible enzyme, but the urea amidolyase (UAD) system, albeit known to exist, has hardly been studied. Here, the species distribution, expression patterns, and functional roles of UAD system are examined, which comprises subunits DUR1 and DUR2. We find a widespread occurrence of UAD, spanning six major phytoplankton lineages, and seemly evolved independently from URE and lineage-specific loss. Quantitative analyses of marine planktonic metagenomes and metatranscriptomes revealed that UAD system is as prevalent as URE but appears higher expression capacity in phytoplankton than in microbes. Besides, a significant regulation of UAD by environmental nitrogen is uncovered in both global ocean and local dinoflagellate-dominant bloom events, potentially suggesting that UAD occupy a key position in nitrogen metabolism of marine phytoplankton. In UAD system, DUR2 subunit showed significant lower expression level than DUR1 and ureC, but it is essential for urea utilization, which was confirmed by completely growth restriction after DUR2 inactivation of Chlamydomonas reinhardtii under urea alone conditions. These findings underscore the importance to consider both URE and UAD enzyme systems when urea utilization by algae is assessed and reveal the crucial role of DUR2 in urea utilization in C. reinhardtii and potentially in many other algae.
 

urea, DUR1, DUR2, CRISPR, phytoplankton, nitrogen cycle