Biofloc technology (BFT) is considered the new "blue revolution" in aquaculture. Such technique is based on in situ microorganism production which plays three major roles: (i) maintenance of water quality, by the uptake of nitrogen compounds generating in situ microbial protein; (ii) nutrition, increasing culture feasibility by reducing feed conversion ratio (FCR) and a decrease of feed costs; and (iii) competition with pathogens. The aggregates (bioflocs) are a rich protein-lipid natural source of food available in situ 24 hours per day due to a complex interaction between organic matter, physical substrate, and large range of microorganisms. This natural productivity plays an important role recycling nutrients and maintaining the water quality. The present chapter will discuss some insights of the role of microorganisms in BFT, main water quality parameters, the importance of the correct carbon-to-nitrogen ratio in the culture media, its calculations, and different types, as well as metagenomics of microorganisms and future perspectives.

The continuous development of world aquaculture demands new strategies and alternatives aimed to achieve sustainability. The use or microorganisms in aquaculture has greatly evolved during the last two decades. From being considered as a potential threat, during the last years, they have been used as probiotics and inclusively as food source for fish and crustacean. The microbial‐based systems represent one of the most viable strategies to achieve a sustainable aquaculture. In short, these systems are based on the promotion of microbial proliferation, either autotrophic or heterotrophic microorganisms; these microbes are expected to use, recycle and transform the excess of nutrients from faeces, dead organisms, unconsumed food and diverse metabolites into biomass, which would be further consumed by the cultured organisms. Successful results on using microbial‐based systems have been documented around the world; however, there are key aspects to consider and yet to experiment before a system could be implemented. Some of those aspects are analysed in this manuscript, while new advances in the use of microbial‐based systems and recommendations are also presented.

Aquaculture has been considered as an option to cope with the world food demand. However, criticisms have arisen around aquaculture, most of them related to the destruction of ecosystems such as mangrove forest to construct aquaculture farms, as well as the environmental impacts of the effluents on the receiving ecosystems. The inherent benefits of aquaculture such as massive food production and economical profits have led the scientific community to seek for diverse strategies to minimize the negative impacts, rather than just prohibiting the activity. Aquaculture is a possible panacea, but at present is also responsible for diverse problems related with the environmental health; however the new strategies proposed during the last decade have proven that it is possible to achieve a sustainable aquaculture, but such strategies should be supported and proclaimed by the different federal environmental agencies from all countries. Additionally there is an urgent need to improve legislation and regulation for aquaculture. Only under such scenario, aquaculture will be a sustainable practice.

The use and study of microbes in aquaculture has become a common practice in the last decade. Metagenomics is a relative recent genomics subdiscipline that has emerged as a promising scientific tool to analyse the complex genomes contained within microbial communities. However, despite the potential of metagenomics, its use is not yet common in some agro-industrial disciplines such as aquaculture. In this review, we analyse some of the potential uses of metagenomics in aquaculture to highlight the microbial diversity and dynamics of the culture systems. This review addresses some potential uses of metagenomics in the study of microbial diversity, microbial roles in microcosms, antibiotic resistance genes, novel and potential pathogens, microbial communities forming bioflocs, probiotics and other applications.

Shrimp polyculture is not yet a common practice among farmers; however, this activity represents an important alternative to solving and ⁄ or minimizing some of the problems that shrimp aquaculture has faced in the past two decades (environmental pollution, diseases and decreasing prices). In this context, many benefits have been achieved with some polyculture practices. Several species from diverse trophic levels have the potential to be co-cultured with shrimps. A good knowledge of the species that are candidates for polyculture and an adequately designed culture system are the most important points to consider when co-culturing shrimp with other species. The present paper is a review of the past, present and future of shrimp polyculture with other organisms.

The use of probiotics is a common practice of current shrimp aquaculture. Despite the immunophysiological responses that have been measured in shrimp exposed to probiotics, no information is currently available on the effect of this practice on the intestinal microbiota. The objective of this work was to evaluate the effect of a probiotic mixture on the intestinal microbiota of shrimp cultured under farm conditions. A culture-independent method based on high-throughput-sequencing (16S rRNA) was used to examine intestinal bacterial communities. A traditional system (without probiotics) was used as the reference. Targeted metagenomics analysis revealed that the probiotic mixture was based on bacteria in the phyla Proteobacteria and Firmicutes. A total of 23 species of bacteria were detected in the probiotic mixture; of these, 11 were detected in the intestine of shrimp reared in both systems, and 12 were novel for the system. Eight of the novel species were detected in shrimp cultured with the probiotic mixture; however, none of these novel species were related to marine or inclusively aquacultural environments, and only one (Bacillus subtilis) was recognized as probiotic for shrimp. The use of the probiotic mixture modified the bacterial profile of the shrimp intestine; however, most of the bacteria incorporated into the intestine were nonindigenous to the marine environment with no previous evidence of probiotic effects on any marine organism. The use of this probiotic mixture may represent a risk of causing environmental imbalances, particularly because farms using these types of probiotic mixtures discharge their effluents directly into the ocean without prior treatment.

The 16S rRNA gene has been used as master key for studying prokaryotic diversity in almost every environment. Despite the claim of several researchers to have the best universal primers, the reality is that no primer has been demonstrated to be truly universal. This suggests that conserved regions of the gene may not be as conserved as expected. The aim of this study was to evaluate the conservation degree of the so-called conserved regions flanking the hypervariable regions of the 16S rRNA gene. Data contained in SILVA database (release 123) were used for the study. Primers reported as matches of each conserved region were assembled to form contigs; sequences sizing 12 nucleotides (12-mers) were extracted from these contigs and searched into the entire set of SILVA sequences. Frequency analysis shown that extreme regions, 1 and 10, registered the lowest frequencies. 12-mer frequencies revealed segments of contigs that were not as conserved as expected (≤90%). Fragments corresponding to the primer contigs 3, 4, 5b and 6a were recovered from all sequences in SILVA database. Nucleotide frequency analysis in each consensus demonstrated that only a small fraction of these so-called conserved regions is truly conserved in non-redundant sequences. It could be concluded that conserved regions of the 16S rRNA gene exhibit considerable variation that has to be considered when using this gene as biomarker.

Micro‐organisms are an essential component of natural marine ecosystems, but also play important roles in anthropogenically modified ecosystems, including aquaculture. Although metagenomics is currently used to explore microbial communities, its application has not been as extensive in aquaculture. Although some taxonomic profiles and phylogenetic studies have been deciphered using biomarker genes, the functional potential of microbial communities associated with aquaculture systems is still unknown in most cases. Predicting functional profiles through 16S rRNA gene‐based metagenomics analysis is perhaps the most promising tool to elucidate the metabolic capabilities of microbial communities because there is no need to perform shotgun sequencing to have an idea of these capabilities. Moreover, robust bioinformatics background is not required to assess this information, and so the same data (16S rRNA sequences) are used to estimate both taxonomic and functional profiles, therefore providing deeper insights into these kinds of communities. In this review, we suggest the need to major use of novel bioinformatics tools that construct functional profiles from metagenomic 16S rRNA data, as strategy to obtain a preliminary approach about metabolic capacities of microbes that coexist in aquaculture systems.