This　work　investigated　the　improvement　in　biomass　yield　and　production　and　organics　reduction　of　Rubrivivax　gelatinosus　(R.　gelatinosus)　by　adding　magnesium　(Mg2+)　for　sewage　purification　under　natural　light　micro-oxygen　conditions.　The　results　showed　that　with　the　optimal　Mg2+　dosage　(10mg/L),　biomass　production　(4125.6mg/L)　was　enhanced　by　59%.　The　biomass　yield　was　improved　by　43.3%.　COD　and　protein　removal　increased　more　than　90%.　Hydraulic　retention　time　was　decreased　by　25%.　Moreover,　5-10mg/L　Mg2+　improved　R.　gelatinosus　yield　in　wastewater　to　medium　levels.　Analyses　revealed　that　different　Mg2+　concentrations　had　different　mechanisms　for　impacting　biomass　production　and　COD　removal.　This　was　because　there　was　an　upper　limit　(15mg/L)　and　an　optimal　value　(6mg/L)　of　intracellular　Mg2+　concentrations　for　R.　gelatinosus.　Appropriate　Mg2+　concentrations　enhanced　ATP　production,　which　improved　the　secretion　and　activity　of　protease,　indicating　that　more　protein　and　COD　were　removed　and　more　biomass　accumulated.　However,　excessive　Mg2+　was　harmful　for　biomass　accumulation　and　COD　removal　because　excess　Mg2+　decreased　ATP　production.　©2014　Elsevier　B.V.