Evaluation of harmful heavy metal (Hg, Pb and Cd) reduction using Halomonas elongata and Tetragenococcus halophilus for protein hydrolysate product

Abstract

Background: Fermented fish is one form of natural protein hydrolysate has been reported to have many health benefits such as antihypertensive, antioxidative activity and anti-inflammatory peptides. However, as known that toxic heavy metal is widely spread and entered throughout the food chain particularly in tuna internal organ which is widely used to process fermented product or discharged to be fish meal. Consumption of fermented tuna viscera containing heavy metal is risky. Detoxifying and/ or reducing those toxic elements is more interesting particularly by using bacterial cells and their metabolic product. Halomonas elongata is moderately halophilic bacteria having heavy metal removal ability, normally found in hypersaline environments.  Tetragenococcus halophilus is halophilic lactic acid bacteria and probiotic actually found in fermented food such as fish sauce, shrimp paste and fermented fish. Some scientific data reported that using T. halophilus improved amino acid profiles and desirable volatile compounds as well as reduced biogenic amine content in fish sauce product. Therefore, it was hypothesized that using H. elongata and T. halophilus can reduce heavy metal content and improve organoleptic quality of fermented fish viscera product (Tai pla) in the next experiment.

Objective: This present work aimed to determine growth character of H. elongata and T. halophilus reared at various NaCl concentrations; 10, 15, 20 and 25% there after heavy metal reduction by using these microorganism reared at optimum NaCl concentration was evaluated. 

Methods: H. elongata and T. halophilus were reared in SNB (saline nutrient broth) and MRS-broth added with NaCl at concentration 10, 15, 20 and 25%, respectively. Each organism to obtain content at cultured in the media containg appropriate NaCl content was added with Hg, Pb and Cd at concentration, 0.5, 1, and 3 mg/L, respectively.
Thereafter supernatant of each condition incubated at 48 and 96 hr was taken to heavy metal analysis.

Results: The results showed that the higher NaCl content the slower of late log phase and stationary phase particularly in T. halophilus. This may due to T. halophilus did not produce special metabolite as exopolysaccharide which found in H. elongate. Regardless heavy metal concentration, the result revealed that CdCl₂ at 3 mg/kg caused more H. elongata cell death but did not occur in T. halophilus. In addition, removal of Hg, Pb and Cd was 12.70, 84.78 and 75.83%, respectively by rearing with H. elongata for 48 hr. Furthermore, the removal of Hg, Pb and Cd done by T. halophilus was 12.68, 91.27 and 95.12%, respectively at 96 hr of of incubation.

Conclusion: H. elongata and T. halophilus prefered SNB containing NaCl concentration between 10-20%. Higher NaCl concentration, 20-25% log phase was extended. Both
H. elongata and T. halophilus could remove all test heavy metals, however T. halophilus seemed to have more Pb and Cd removal capability compared with H. elongata. Using H. elongata and T. halophilus for next fermented tuna viscera is possible.

Keywords: Halomonas elongata, Tetragenococcus halophilus, Heavy metal, NaCl