Biofloc technology adapted to regions with extreme salinity and temperature: A pending task in the field

Abstract

The aquaculture industry faces several global challenges, particularly concerning the environmental impacts of effluent discharge and the spread of viral and bacterial diseases. Diverse strategies have been implemented with different degrees of success. These include recirculating aquaculture systems (RAS, BioRAS), integrated multitrophic aquaculture (IMTA, Aquaponics and FlocPonics), and the use of microbial aggregates on submerged floating substrates (biofloc technology or BFT). Over the past two decades, BFT has emerged as a viable alternative for producing food from aquatic organisms, primarily due to its ability to recycle waste and significantly reduce water usage. Despite this, the technology is not yet used on a large scale around the world. Examples of commercial microbial-based systems are found in Vietnam using chemoautotrophic-based BFT and in Thailand using heterotrophic-based BFT (Emerenciano et al., 2022). The effectiveness of this technology has yet to be fully established in regions experiencing extreme environmental fluctuations, such as variations in temperature and salinity. This editorial provides insights into how BFT can be adapted and implemented under such conditions, including recommendations for assembling, managing, and optimizing microbial consortia that are suitable for coping with extreme environmental changes.

Aquaculture is an agro-industry whose contribution to human development has been evident throughout many decades, not only as a food source but also as a generator of foreign exchange, employment, and social welfare. It has even been considered a mitigator of overexploitation by fishing. However, despite its numerous benefits, diverse environmental impacts have been associated with the activity, primarily due to the discharge of effluents containing high concentrations of nitrogenous compounds, organic matter, antibiotics, and various chemical compounds. These effluents could degrade the environment, leading to unfavorable conditions for the surrounding ecosystems' flora, fauna, and microbiota (Martinez-Porchas & Martinez-Cordova, 2012).

BFT was developed in the 1970s but emerged strongly at the beginning of the 2000s as a solid strategy to overcome some of these problems. Based on the bioaugmentation of heterotrophic bacteria through bio preparation of the systems conditions, including a high carbon: nitrogen ratio, pond lining, reduced light intensity, high alkalinity, proper aeration, and solids removal protocols, the system produces edible microbial biomass for the cultured animals while recycling generated wastes (Khanjani et al., 2024). Despite this strategy gaining popularity and solving several aquaculture drawbacks, it was conceived to perform under optimal conditions in which regulating environmental variations is achievable. However, this is not the case for large farms, particularly those in arid, dry climates. In these regions, high temperatures and salinity can affect aquaculture. Salinity in open aquaculture systems increases due to evaporation, and temperatures peak during the day, forcing farmers to construct deeper ponds and/or perform large water exchanges. In addition, with global temperatures rising and precipitation patterns changing, coastal and other aquaculture areas could experience higher levels of salinity and temperature increases. Wanders et al. (2019) found an average global increase in riverine water temperature of 0.16°C each decade from 1960 to 2014, with more rapid warming toward 2014.

In this context, it is essential to adapt BFT for application under these conditions, as microorganisms are highly susceptible to changing environmental factors. Consequently, this document underscores the need for research to modify BFT protocols to accommodate high temperatures and salinity levels while outlining potential research avenues.

The microbial consortia of bioflocs are held together by the secretion of exopolysaccharides from bacteria, filamentous microorganisms, and electrostatic attraction. Recent studies have shown their usefulness for almost any stage of fish and crustacean culture, with positive results on the productive response, immunostimulation of cultured organisms, and water and sediment quality (Avnimelech, 2007, 2009; Burford et al., 2004). Biofloc is a versatile technology that can be adapted to different aquaculture systems. Although biofloc systems have been successfully tested in different parts of the world, this type of culture, as well as the research carried out in experimental systems for shellfish farming, is conducted in conditions different from the reality of farms located in warm climate zones with low rainfall (Krummenauer et al., 2011; Xu & Pan, 2012). In those areas, it is possible to reach 50 practical salinity units (UPS), which is almost 20 UPS more than the recommended salinity for most marine species. In this regard, the effect of salinity on BFT systems has been evaluated in the range of 10 to 30 PSU without reporting significant growth differences for cultured animals (Ray & Lotz, 2017). Higher salinity levels or temperatures have generally not been considered, most likely because such factors are not problematic in cases that include farms located in optimal environments. However, when suboptimal environmental conditions exist, adaptations are required. For instance, when marine bacteria are well adapted to a specific salinity range, higher suboptimal salinity can lead to the slow development or even lysis of salt-intolerant microbes, disrupting and altering the microbial community while inactivating or losing key functions, as has been described in marine microbes (Duc et al., 2023).

The above information applies to the aquaculture of marine animals; however, for freshwater organisms, the salinity variations also have an influence on the performance of bioflocs. De Alvarenga et al. (2018) confirmed this following an evaluation of growth performance, survival, gill lesions, and fillet composition of tilapia fingerlings (Oreochromis niloticus) reared for 70 days at different salinities (0, 4, 8, 12, and 16 g/L).