An Eco-Friendly Conversion of Aquaculture Suspended Solid Wastes Into High-Quality Fish Food by Improving Poly-β-Hydroxybutyrate Production

Abstract

The aquaculture industry is vital in providing a valuable protein food source for humans, but generates a huge amount of solid and dissolved wastes that pose great risks to the environment and aquaculture sustainability. Suspended solids (in short SS), one of the aquaculture wastes, are very difficult to be treated due to their high organic contents. The bioconversion from wastewater, food effluents, and activated sludge into poly-β-hydroxybutyrate (PHB) is a sustainable alternative to generate an additional income and could be highly attractive to the agricultural and environmental management firms. However, little is known about its potential application in aquaculture wastes. In the present study, we first determined that 7.2% of SS was PHB. Then, the production of PHB was increased two-fold by the optimal fermentation conditions of wheat bran and microbial cocktails at a C/N ratio of 12. Also, the PHB-enriched SS showed a higher total ammonia nitrogen removal rate. Importantly, we further demonstrated that the PHB-enriched SS as a feed could promote fish growth and up-regulate the expression of the immune-related genes. Our study developed an eco-friendly and simple approach to transforming problematic SS wastes into PHB-enriched high-quality food for omnivorous fish, which will increase the usage efficiency of SS and provide a cheaper diet for aquatic animals.

Keywords: accumulation optimization; aquaculture solid wastes; biopolymer; fish food; nitrogenous compounds; poly-β-hydroxybutyrate (PHB).

FIGURE 1

Graphical representation of the aquaculture suspended solid (SS) treatment process by adding carbons considering PHB accumulation.

FIGURE 2

Fourier transform infrared (FT-IR) spectra of PHB extract from SS. Red triangle and box with blue line mean bands of PHB.

FIGURE 3

Gas chromatogram of PHB extract from SS. Red triangle means the retention time.

FIGURE 4

Scanning electron microscopy observation of PHB extract from SS.

FIGURE 5

Bacterial diversity at the class level (A) and the genus level (B) in SS by MiSeq sequencing.

FIGURE 6

Parametric optimization for high-yield PHB production during SS treatment. (A), C/N ration; (B), carbon sources; (C), probiotics widely used in practical aquaculture. All the data are presented as mean ± SE. Values marked with different small letters indicates significant differences among groups (p < 0.05). The strain information used in this study is: Bacillus, Bacillus subtilis; Pseudomonas, Pseudomonas putida; Cocktail, cocktail of Pseudomonas and Lactobacillus.

FIGURE 7

Effects of PHB-IRenriched and untreated SS addition in culture water on the immune-related gene expression in the spleen and gills of gibel carp (Carassius auratus gibelio).