Aquaculture is the fastest growing animal food production sector worldwide and is becoming the main source of aquatic animal foodstuff for human consumption. However, the aquaculture sector has been strongly criticized for its environmental impacts. It can cause discharge and accumulation of residual nutrients in the areas surrounding the production farms. This is because, of the total nutrients supplied to production ponds, only 30% are converted into product, while the rest is usually discharged into the environment to maintain water quality in aquaculture culture systems, thereby altering the physic-chemical characteristics of the receiving water. In contrast, this same accumulation of nutrients is gaining importance within the agricultural sector, as it has been reported that the main nutrients required by plants for their development are found in this aquaculture waste. The purpose of this review article is to indicate the different aquaculture production systems, the waste they generate, as well as the negative effects of their discharge into the environment. Biofiltration and bioremediation processes are mentioned as alternatives for aquaculture waste management. Furthermore, the state of the art in the treatment and utilization of aquaculture waste as a mineral source for agricultural nutrition through biodigestion and biomineralization processes is described. Finally, aquaponics is referred to as a biological production approach that, through efficient use of water and recycling of accumulated organic nutrients in aquaculture systems, can contribute to addressing the goals of sustainable aquaculture development.

In the last decade, the publications presenting novel physical and chemical aspects of gadolinium-based oxide (Gd₂O₃) and oxysulfide (Gd₂O₂S) particles in the micro- or nano-scale have increased, mainly stimulated by the exciting applications of these materials in the biomedical field. Their optical properties, related to down and upconversion phenomena and the ability to functionalize their surface, make them attractive for developing new probes for selective targeting and emergent bioimaging techniques, either for biomolecule labeling or theranostics. Moreover, recent reports have shown interesting optical behavior of these systems influenced by the synthesis methods, dopant amount and type, particle shape and size, and surface functionality. Hence, this review presents a compilation of the latest works focused on evaluating the optical properties of Gd₂O₃ and Gd₂O₂S particles as a function of their physicochemical and morphological properties; and also on their novel applications as MRI contrast agents and drug delivery nanovehicles, discussed along with their administration routes, biodistribution, cytotoxicity, and clearance mechanisms. Perspectives for this field are also identified and discussed.