The modern world is confronted with the pressing challenges of reducing carbon dioxide (CO₂) emissions as the leading promoter of climate change while also addressing the energy and food crisis. Microalgae, being photosynthetic organisms, possess the unique ability to utilize solar energy for the conversion of CO₂ into organic matter. Their ability to remove CO₂ from the atmosphere proves advantageous in mitigating climate change. Algae have demonstrated effectiveness in wastewater treatment (WWT) by removing pollutants. In agriculture, microalgae offer a green alternative to chemical fertilizers, benefiting crop growth. The ease and cost-effectiveness of sampling and culturing microalgae contribute to their popularity. This chapter examines the sustainable applications of microalgae, including WWT, bioproduct recovery, bioremediation, energy production, agriculture, and biodiversity conservation. The examination of the growing global microalgae market, particularly in relation to biotechnological applications, was also reviewed as an indicator of the immense potential offered by microalgae.

In wastewater treatment plants, the stability of the related microbiota is pivotal for a steady and appropriate operation in biological wastewater treatment (BWT). The interactions between phages, which are high in amounts and diversity, and their particular hosts are significant due to their specificity in this microbial community. As drivers of diversity, phages are capable of changing the microbial structure by affecting host cells. This study aimed to enhance the stability of the functional microbiota, a primary concern of BWT, by evaluating the influence of bioaugmentation with bacteriophage cocktails. The sequencing data revealed significant alterations in the entire microbiome, including microeukaryotes, induced by the bacteriophages. These alterations led to a reduction in opportunistic microorganisms while preserving the beneficial ones. The genus Proteobacteria was found unaffected by three antibiotics in the bioreactor due to its antibiotic-resistant members, leading to its further growth, while the antibiotic-sensitive genus decreased in quantity. Furthermore, the phages notably influenced the metabolic pathway such as nitrogen, carbohydrate, and amino acid metabolisms by eliminating opportunistic microbes and providing improved growth conditions to bacterial species that are essential for effective reactor performance and wastewater treatment.

Wastewater treatment plants (WWTPs) serve as habitats for diverse and densely populated bacterial communities, fostering intricate microbial interactions. Conventional treatment methods employed often fail to completely eliminate pathogens. Consequently, inadequate chemical treatments lead to the eventual release of waterborne bacterial pathogens into the environment through effluent water. Anaerobic digestion represents a biological treatment approach for organic waste and wastewater, providing cost-reduction benefits and enabling energy generation through biogas production from organic waste. However, the role of viruses-host interactions in anaerobic digestion and their effects on biological wastewater treatment (WWT) has been lacking and requires further research and attention. Bacteriophages (phages), viruses that target specific bacteria, are abundant within WWTPs and engage in diverse interactions with their host organisms. Also, there are reports indicating the presence of archaeal viruses capable of impacting crucial methanogenic organisms in anaerobic digestion, alongside phages. Despite their apparent lack of discernible metabolic functions, viral community have significant potential to influence WWT by shaping the structure of microbial communities, thereby impacting the efficiency of the processes. This chapter aims to explore the influence of reported viral communities, especially phages on shaping microbial communities; elucidate the dynamics and limitations of phage-host relationships; and evaluate their potential as biological tools for enhancing the anaerobic digestion process in WWT.