Microbial remediation with photosynthetic bacteria: a low-cost approach to reducing phoxim accumulation in aquaculture water

Abstract:

[Background] China is a major agricultural producer, with a large proportion of fishery, forestry, agriculture, and animal husbandry. Organic pesticides are widely used in animal husbandry and aquaculture, particularly the organophosphorus pesticides (OPs) which feature low toxicity and little residue. OP indirectly affects human health through biological enrichment, and the resulting reproductive toxicity cannot be ignored. Photosynthetic bacteria are environmentally friendly beneficial bacteria in water, some of which can degrade organic pesticides. [Objective] We isolated a photosynthetic strain (No. SPZ) from the Minghu Lake of Shanghai Ocean University and tested its tolerance to and degradation of phoxim, aiming at provide a strain for the removal of OPs in aquaculture water. [Methods] The strains used in this study were identified by 16S rRNA gene sequencing. The OD₆₆₀ of the isolated strain SPZ and the standard strain ST at different inoculum sizes was measured by UV spectrophotometry to determine the optimal inoculum size. The toxicity of phoxim to photosynthetic bacteria was determined based on the changing trends of OD₆₆₀ of the bacteria exposed to phoxim at different concentration levels. The phoxim-degrading ability of SPZ and ST was determined by high performance liquid chromatography (HPLC), and that of live and dead (from heating) SPZ was also detected (HPLC) to clarify the degradation mechanism. [Results] The 16S rRNA gene sequence analysis demonstrated that SPZ showed 99% similarity to Rhodopseudomonas and clustered with Rhodopseudomonas palustris ATCC 17001 (confidence: 100%). The optimal inoculation volume fractions of SPZ and ST were 10%. SPZ can tolerate 100 mg/L phoxim, and phoxim significantly inhibited the growth of the photosynthetic bacteria at the concentration > 100 mg/L. Phoxim was added to photosynthetic bacteria at logarithmic growth phase and the relative degradation of phoxim was 12.97%-26.69% (at 20.0 mg/L), 24.25%-32.85% (at 2.0 mg/L), and 16.66%-34.59% (at 0.2 mg/L) after 1 day, 46.63%-53.95% (at 20.0 mg/L), 24.78%-30.34% (at 2.0 mg/L), and 31.92%-39.25% (at 0.2 mg/L) after 3 days, and 93.65%-97.72% (at 20.0 mg/L), 67.69%-74.41% (at 2.0 mg/L), and 10.34%-24.27% (at 0.2 mg/L) after 5 days. [Conclusion] The common photosynthetic SPZ can effectively remove the phoxim in water. It has a broad application prospect in aquaculture and OPs-contaminated wastewater treatment.