Methyl red dye decolourization by the photosynthetic bacteria, Rhodopseudomonas palustris and Afifella marina

Abstract

Anoxygenic photosynthetic bacteria are common inhabitants of wastewater: we found that Rhodopseudomonas palustris and Afifella marina in eutrophic conditions only partially degraded the azo dye (50 mmol m⁻³), Methyl Red, but completely degraded it under specially defined conditions. The azo dye is potentially a source of both carbon and fixed nitrogen. Rhodopseudomonas palustris and Afifella marina can live heterotrophically, photoheterotrophically or photoautotrophically under anoxic conditions where they can fix N₂ if no organic nitrogen or NH₃ is available. If organic carbon sources are available or if NH₃ is present, the cells again only partially catabolised Methyl Red. In the absence of no alternative organic carbon sources and no NH₃, the cells almost completely spectroscopically decolourised Methyl Red in 4 days. In sewage ponds the ready availability of alternative organic carbon and NH₃ would result in only partial removal of Methyl Red. Rhodopseudomonas cells responded to the availability of Methyl Red in N-free media, by increasing both Optimum irradiance and maximum ETR (E_opt 276.3 μmol quanta m⁻² s⁻¹; ETR_max 391.4 μmol e⁻ g⁻¹ BChl a s⁻¹) compared to control cells incubated in PM media with no organic carbon source and no fixed N-source (E_opt 115.2 μmol quanta m⁻² s⁻¹; ETR_max = 153.0 μmol e⁻ g⁻¹ BChl a s⁻¹. If no alternative C or N sources are available, Rhodopseudomonas embedded in alginate biobeads will completely and repeatedly break down Methyl Red. The marine Afifella readily broke down Methyl Red but again breakdown was only complete if alternative carbon and no fixed nitrogen sources were available. The toxicity of the breakdown products produced by photosynthetic bacteria from azo-dyes needs to be followed up. Photosynthetic bacterial-alginate biobeads have long lifetimes (Rhodopseudomonas ≈ 2 months, Afifella > 6 months) making them of great biotechnological potential.