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.
Highlights
Nitrogen (N) is an essential macronutrient that supports life in all living beings
Besides CYP55 and flavodiiron proteins (FLVs), the Chlamydomonas genome contains four genes encoding for hybrid cluster proteins (HCPs) [39], which are metalloproteins characterized by the presence of an iron-sulfur-oxygen cluster considered to be unique in biology [25]
N homeostasis in ecosystems relies on the balance between N fixation and N release to the atmosphere in gaseous forms
Summary
Nitrogen (N) is an essential macronutrient that supports life in all living beings. This nutrient is an elemental constituent of biomolecules, such as nucleic acids, proteins, chlorophylls, cofactors, and signal molecules, among others. The industrial N2 -fixation into ammonia (Haber-Bosch process) and the use of this ammonia as fertilizers has dramatically improved crop productivity but, at the same time, it has increased the reactive N load by more than 120%, which includes all biologically active, photochemically reactive, and radiatively active N compounds in the atmosphere and biosphere [1,2,9,10] This increase in the N load comprises a higher global amount of reduced inorganic forms of N (NH3 and NH4 + ), oxidized inorganic forms (as NOx, HNO3 , N2 O, and NO3 − ), and organic compounds (such as urea, amines, and proteins), in detriment to the unreactive N2 gas. We analyze the metabolic flexibility of microalgae to carry out different steps of the N cycle with special attention to the denitrification pathway
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