Abstract: Nitrous oxide (N₂O) is an important greenhouse gas, which could be produced by bacteria, fungi, algae and higher plants. Bacteria produce N₂O through nitrification and denitrification. P450nor is the key enzyme involved in the production of N₂O in fungi. However, the mechanism of N₂O production in algae and higher plants has rarely been reported. In this study, algae, whose evolution status is between fungi and higher plants, was taken as a research object. From the uploaded data in GenBank, we obtained three different P450nor protein sequences from three algae species (Chlamydomonas reinhardtii, Gonium pectorale and Chlorella variabilis), which were systematically close to P450nor protein from fungi. Then, their functions and evolutionary relationships were examined. Those proteins are considered as soluble, non-secretory and hydrophilic ones in mitochondria. All of them included Pfam P450 conserved domain and three conserved motifs, suggesting that algae P450nor is highly conservative. Evolutionary analysis showed that the P450nor of algae and fungi has a high homology, implying that P450nor of algae and fungi have similar biological functions. Due to the special phylogenetic status of algae, our results would provide  molecular evidence for N₂O biosynthesis of algae and a clue to the imbalance of global N₂O budget.

Key words: microbial biomass carbon, Xishuangbanna., tropical forest, ant nesting, quotient of microbial biomass carbon