Microalgae have demonstrated a large potential in biotechnology as a source of various macromolecules (proteins, carbohydrates, and lipids) and high-added value products (pigments, poly-unsaturated fatty acids, peptides, exo-polysaccharides, etc.). The production of biomass at a large scale becomes more economically feasible when it is part of a biorefinery designed within the circular economy concept. Thus, the aim of this critical review is to highlight and discuss challenges and future trends related to the multi-product microalgae-based biorefineries, including both phototrophic and mixotrophic cultures treating wastewater and the recovery of biomass as a source of valuable macromolecules and high-added and low-value products (biofertilizers and biostimulants). The therapeutic properties of some microalgae-bioactive compounds are also discussed. Novel trends such as the screening of species for antimicrobial compounds, the production of bioplastics using wastewater, the circular economy strategy, and the need for more Life Cycle Assessment studies (LCA) are suggested as some of the future research lines.

Inoculation of soils with cyanobacteria is proposed as a sustainable biotechnological technique for restoration of degraded areas in drylands due to the important role that cyanobacteria and their exopolysaccharides (EPS) play in the environment. So far, few studies have analyzed the macromolecular and chemical characteristics of the polysaccharidic matrix in induced cyanobacterial biocrusts and the scarce existing studies have mainly focused on sandy soil textures. However, the characteristics of the cyanobacterial polysaccharidic matrix may greatly depend on soil type. The objective of this study was to examine the macromolecular distribution and monosaccharidic composition of the polysaccharidic matrix induced by inoculation of two cyanobacterial species common in arid environments, Phormidium ambiguum (non N-fixing) and Scytonema javanicum (N-fixing) in different soil types. S. javanicum promoted a higher release in the soil of the more soluble and less condensed EPS fraction (i.e., the loosely bound EPS fraction, LB-EPS), while P. ambiguum showed a higher release of the less soluble and more condensed EPS fraction (i.e., the tightly bound EPS fraction, TB-EPS). LB-EPSs were mainly composed of low MW molecules (< 50 kDa), while TB-EPSs were mainly composed of high MW molecules (1100–2000 kDa). The two EPS fractions showed a complex monosaccharidic composition (from 11 to 12 different types of monosaccharides), with glucose as the most abundant monosaccharide, in particular in the poorer soils characterized by lower organic C contents. In more C-rich soils, high abundances of galactose, mannose, and xylose were also found. Low abundance of uronic acids and hydrophobic monosaccharides, such as fucose and rhamnose, was found in the EPS extracted from the inoculated soils. Our results point to the influence of soil type on the macromolecular distribution and monosaccharide composition of the polysaccharidic matrix in induced biocrusts, which is likely to affect biocrust development and their role in soil structure and nutrient cycling in restored dryland soils.