Chlamydomonas reinhardtii, an Algal Model in the Nitrogen Cycle

Abstract

Nitrogen (N) is an essential constituent of all living organisms and the main limiting macronutrient. Even when dinitrogen gas is the most abundant form of N, it can only be used by fixing bacteria but is inaccessible to most organisms, algae among them. Algae preferentially use ammonium (NH4+) and nitrate (NO3−) for growth, and the reactions for their conversion into amino acids (N assimilation) constitute an important part of the nitrogen cycle by primary producers. Recently, it was claimed that algae are also involved in denitrification, because of the production of nitric oxide (NO), a signal molecule, which is also a substrate of NO reductases to produce nitrous oxide (N2O), a potent greenhouse gas. This review is focused on the microalga Chlamydomonas reinhardtii as an algal model and its participation in different reactions of the N cycle. Emphasis will be paid to new actors, such as putative genes involved in NO and N2O production and their occurrence in other algae genomes. Furthermore, algae/bacteria mutualism will be considered in terms of expanding the N cycle to ammonification and N fixation, which are based on the exchange of carbon and nitrogen between the two organisms.

Assessment of Nitrate Removal Capacity of Two Selected Eukaryotic Green Microalgae

“…MACC-360 were able to remove 100% of nitrate from TAP-M5 by the 3rd day. This nitrate removal can be explained by previous studies, which have shown nitrate assimilation reactions in Chlamydomonas reinhardtii ; it has been stated that nitrate is first reduced to nitrite in the cytoplasm by nitrate reductase, followed by its transfer to chloroplast, where it is further reduced to ammonia by nitrite reductase and this ammonia is then incorporated in carbon skeletons by the Glutamine Synthetase/Glutamine Oxoglutarate Aminotransferase (GS/GOGAT) cycle, which is responsible for the synthesis of glutamate [ 35 , 36 , 37 ]. For the nitrate reduction in the cytoplasm, first, nitrate needs to enter inside microalgae cells, a process which is carried out by nitrate transporters.…”

Nitrous Oxide Emissions from Nitrite Are Highly Dependent on Nitrate Reductase in the Microalga Chlamydomonas reinhardtii

“…This residual NO 2 − assimilation is enough to avoid NO 2 − dissimilation to N 2 O. In addition, NIT2 also controls other steps in NO 3 − assimilation, including NO 3 − /NO 2 − transporters [ 36 , 40 ] and NO metabolism-related proteins such as AOX1 [ 51 ], THB1 and THB2 [ 38 , 39 ], and probably CYP55, which increases in response to NO 3 − [ 29 ]. Moreover, a putative NO 3 − -dependent regulation of the N 2 O production, mediated by NIT2, is also supported by the significant increase in the N 2 O emission rate observed in M3 cells incubated in NO 3 − compared to those incubated in NO 2 − ( Figure 2 c).…”

“…Recently, two works have identified the flavodiiron proteins (FLVs) and cytochrome P450 (CYP55) as NO reductases (NORs) in Chlamydomonas [ 26 , 27 ]. The Cr FLVs belong to a singular family of O 2 and NO reductases that are ubiquitous in oxygenic photoautotrophs, including cyanobacteria, the rhizarian Paulinella chromatophora , green algae, mosses, lycophytes, and gymnosperms, but are absent in angiosperms [ 28 , 29 ]. The Chlamydomonas CYP55 is a cytochrome p450 NO reductase closely related to fungal p450 nor members, which are considered biomarkers for N 2 O production [ 14 ].…”

“…Moreover, the Chlamydomonas NR can also donate electrons to the truncated hemoglobin THB1 to scavenge NO and produce NO 3 − by deoxygenation [ 38 , 39 ]. Thus, NR has a central role comprising the recently named NO 3 − -NO 3 − cycle [ 29 ].…”

Special Issue Editorial: Plant Nitrogen Assimilation and Metabolism

“…The living organisms present in nature have an enourmous versatility to metabolize most nitrogenous compounds. The paper by Bellido-Pedraza et al (2020) [ 1 ] reviews the N cycle in the model alga Chlamydomonas reinhardii [ 2 , 3 ] and its participation in different reactions of the N cycle [ 4 ]. The authors made particular emphasis of their description of new putative genes encoding enzymes involved in N-transforming reactions as NO and N 2 O production and their occurrence in other algae genomes.…”