Optimization Of PHB Fermentation Process And Its Application In Marine Fish Culture

Commercial mairne fish culture are mostly conducted in cages and recirculation aquaculture systems.However,poor management in intensive aquiculture usually leads to the outbreak of aquatic plant disease.Application of chemicals and antibiotics often result in the environmental pollution,increased chemical residues and antibiotic resistance in the aquatic animlas.Poly-β-hydroxybuytrate(PHB)is the lipid-soluble particles,which accumulates in prokaryotic cells in condition of excessive carbon or less nitrogen,phosphrous and other elements.Studies have shown that the dietary PHB enhance the growth and resistance to bacterial pathogens of aquatic animals.Thus PHB is a potential biological control agent for aquaculture.In this paper,PHB accumuating Halomonas strain isolated from brine was used.The optimal growth and PHB accumulating conditions through flask culture,parallel fermentation tank(1 L×4)and 5-L fermentation tank.The parameters include temperature,pH,DO control in the process of fermentation,fermentation duration and carbon additon.The optimum fermentation was as follows:salinity 30 g/L,30 g/L glucose concentration,pH 7.5 and DO 10%.During 42-48 h fermentation,batch feeding procedure was applied:in total of 30 g/L glucose,2 g/L NH4Cl and 0.5 g/L KH2PO4 were added in 0-24 h fermentation peroid;but only 30 g/L glucose was supplemented in later fermentation period.Under such conditions,the PHB content in Halomonas cells reached 65%CDW and the cell dry weight reached 27 g/L.The same conditions were further optimized in 50-L and 500-L fermentor with slight modification.Under such conditions,the PHB content in Halomonas cells reached 65%CDW and the cell dry weight reached 23.05 g/L in 44 h fermentation.The above optimized conditions was finally applied to 5000-L fermentator,which resulted in maximum PHB accumulation of 78%CDW and cell dry weight of 31.75 g/L in 44 h fermentation.The Halomonas-PHB was applied to marine fish Cynoglossus semilaevis culture.The concentrated Halomonas-PHB was supplemented into the commercial feed with final PHB content of 1%and 3%,respectively,and the commercial feed was used as a control.The experiment was conducted in nine 20-m3 tanks equipped with recirculation system for 3 months.The results showed that supplementation of Halomonas-PHB(especially with 3%PHB)significantly improve the survival of Cynoglossus semilaevis juveniles.This paper provided technical data for Halomonas-PHB production through large-scale fermentation,which enable Halomonas-PHB being applied in intensive marine fish farming as a new biological control agent.