The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in the agricultural, medical, and industrial sectors. Microalgae are proving to be valuable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bioproducts production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction in contaminants and facilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide variety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

The wide metabolic diversity of microalgae, their fast growth rates, and low-cost production make these organisms highly promising resources for a variety of biotechnological applications, addressing critical needs in industry, agriculture, and medicine. The use of microalgae in consortia with bacteria is proving valuable in several areas of biotechnology, including the treatment of various types of wastewater, the production of biofertilizers, and the extraction of various products from their biomass. The monoculture of the microalga Chlamydomonas has been a prominent research model for many years and has been extensively used in the study of photosynthesis, sulphur and phosphorus metabolism, nitrogen metabolism, respiration, and flagellar synthesis, among others. Recent research has increasingly recognised the potential of Chlamydomonas–bacteria consortia as a biotechnological tool for various applications. The detoxification of wastewater using Chlamydomonas and its bacterial consortia offers significant potential for sustainable reduction of contaminants, while facilitating resource recovery and the valorisation of microalgal biomass. The use of Chlamydomonas and its bacterial consortia as biofertilizers can offer several benefits, such as increasing crop yields, protecting crops, maintaining soil fertility and stability, contributing to CO₂ mitigation, and contributing to sustainable agricultural practises. Chlamydomonas–bacterial consortia play an important role in the production of high-value products, particularly in the production of biofuels and the enhancement of H₂ production. This review aims to provide a comprehensive understanding of the potential of Chlamydomonas monoculture and its bacterial consortia to identify current applications and to propose new research and development directions to maximise their potential.

A naturally occurring multispecies bacterial community composed of Bacillus cereus and two novel bacteria (Microbacterium forte sp. nov. and Stenotrophomonas goyi sp. nov.) has been identified from a contaminated culture of the microalga Chlamydomonas reinhardtii. When incubated in mannitol- and yeast extract-containing medium, this bacterial community can promote and sustain algal hydrogen production up to 313 mL H₂·L^-1 for 17 days and 163.5 mL H₂·L^-1 for 25 days in high-cell (76.7 μg·mL^-1 of initial chlorophyll) and low-cell density (10 μg·mL^-1 of initial chlorophyll) algal cultures, respectively. In low-cell density algal cultures, hydrogen production was compatible with algal growth (reaching up to 60 μg·mL^-1 of chlorophyll). Among the bacterial community, M. forte sp. nov. was the sole responsible for the improvement in hydrogen production. However, algal growth was not observed in the Chlamydomonas-M. forte sp. nov. consortium during hydrogen-producing conditions (hypoxia), suggesting that the presence of B. cereus and S. goyi sp. nov. could be crucial to support the algal growth during hypoxia. Still, under non‑hydrogen producing conditions (aerobiosis) the Chlamydomonas-M. forte sp. nov. consortium allowed algal growth (up to 40 μg·mL^-1 of chlorophyll) and long-term algal viability (>45 days). The genome sequence and growth tests of M. forte sp. nov. have revealed that this bacterium is auxotroph for biotin and thiamine and unable to use sulfate as sulfur source; it requires S-reduced forms such as cysteine and methionine. Cocultures of Chlamydomonas reinhardtii and M. forte sp. nov. established a mutualistic association: the alga complemented the nutrient deficiencies of the bacterium, while the bacterium released ammonium (0.19 mM·day^-1) and acetic acid (0.15 mM·day^-1) for the alga. This work offers a promising avenue for photohydrogen production concomitant with algal biomass generation using nutrients not suitable for mixotrophic algal growth.

The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in both the agricultural, medical, and industrial sectors. Microalgae are proving to be val-uable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bio-products production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction of contaminants and fa-cilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide va-riety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

Summary: The FixK ₂ protein plays a pivotal role in a complex regulatory network, which controls genes for microoxic, denitrifying, and symbiotic nitrogen‐fixing lifestyles in Bradyrhizobium diazoefficiens . Among the microoxic‐responsive FixK ₂ ‐activated genes are the fixNOQP operon, indispensable for respiration in symbiosis, and the nnrR regulatory gene needed for the nitric‐oxide dependent induction of the norCBQD genes encoding the denitrifying nitric oxide reductase. FixK ₂ is a CRP/FNR‐type transcription factor, which recognizes a 14 bp‐palindrome (FixK ₂ box) at the regulated promoters through three residues (L195, E196, and R200) within a C‐terminal helix‐turn‐helix motif. Here, we mapped the determinants for discriminatory FixK ₂ ‐mediated regulation. While R200 was essential for DNA binding and activity of FixK ₂ , L195 was involved in protein–DNA complex stability. Mutation at positions 1, 3, or 11 in the genuine FixK ₂ box at the fixNOQP promoter impaired transcription activation by FixK ₂ , which was residual when a second mutation affecting the box palindromy was introduced. The substitution of nucleotide 11 within the NnrR box at the norCBQD promoter allowed FixK ₂ ‐mediated activation in response to microoxia. Thus, position 11 within the FixK ₂ /NnrR boxes constitutes a key element that changes FixK ₂ targets specificity, and consequently, it might modulate B. diazoefficiens lifestyle as nitrogen fixer or as denitrifier.

The wide metabolic diversity of microalgae, their fast growth rates, and cost-effective production make these organisms highly promising resources for a variety of biotechnological applications, addressing critical needs in industry, agriculture, and medicine. The utilization of microalgae in consortia with bacteria is proving to be valuable in different biotechnological fields, including treating various types of wastewaters, producing biofertilizers, and extracting various products from their biomass. Monoculture of the microalgae Chlamydomonas has been a prominent research model for many years, extensively utilized in studying photosynthesis, sulfur and phosphorus metabolism, nitrogen metabolism, respiration, and flagella synthesis, among others. Recent re-search has increasingly recognized the potential of Chlamydomonas-bacteria consortia as a bio-technological tool for various applications. Bioremediation of wastewater using Chlamydomonas, and its bacterial consortia presents significant potential for the sustainable reduction of contam-inants, while also facilitating resource recovery and valorization of microalgal biomass. Using Chlamydomonas and its bacterial consortia as biofertilizers can offers several benefits, such as en-hancing crop yield, protecting crops, maintaining soil fertility and stability, aiding in CO2 miti-gation, and contributing to sustainable agriculture practices. Chlamydomonas-bacterial consortia play a significant role in the production of high-value products, particularly in biofuel and en-hancing H2 production. This review aims to achieve a comprehensive understanding of the po-tential of Chlamydomonas monoculture and its bacterial consortia, identifying current applications, and proposing new research and development directions to maximize its potential.