Nitrogen scavenging from amino acids and peptides in the model alga Chlamydomonas reinhardtii. The role of extracellular l-amino oxidase

Abstract Phytoplankton live under constantly changing environments in which concentrations of inorganic nitrogen can be limiting but organic nitrogen sources (urea, amino acids or peptides) are available. Understanding how algae, as primary producers, assimilate organic nitrogen is of eco-physiological importance although such studies are rare. Given genetic tractability, the green microalga Chlamydomonas reinhardtii is an excellent model system for elucidating amino acid assimilation by algal species. This alga can extracellularly deaminate most amino acids using a l -Amino acid Oxidase (LAO1), which generates ammonium that can then be taken up as a source of nitrogen. In this work, we have used lao1 mutant strains to investigate the impact of the absence of this enzyme on C. reinhardtii growth using amino acids and di-/tri-peptides as sole nitrogen sources. Our results show that LAO1 enzyme is crucial for growth on most proteinogenic amino acids and peptides by this alga. We present findings from an analysis of algal genomes that reveal a new evolutionary branch for algal l -amino acid oxidase genes ( ALAAO ) that includes Rhodophyta, Alveolata, Heterokonta, Haptophyta, and Dinophyta species. Interestingly, C. reinhardtii appears to be the only green algal that contains an ALAAO homolog, and we present a hypothesis about the possible origins of ALAAO genes in algae based on a comparative analysis of currently available and assembled algal genomes.