Culturable Rhodobacter and Shewanella species are abundant in estuarine turbidity maxima of the Columbia River

Measurements of dissolved, ascorbate-reducible and total Mn by ICP-OES revealed significantly higher concentrations during estuarine turbidity maxima (ETM) events, compared to non events in the Columbia River. Most probable number (MPN) counts of Mn-oxidizing or Mn-reducing heterotrophs were not statistically different from that of other heterotrophs (10³ – 10⁴ cells/mL^-1) when grown in defined media, but counts of Mn oxidizers were significantly lower in nutrient-rich medium (13 cells/m^-1). MPN counts of Mn oxidizers were also significantly lower on Mn(III)-pyrophosphate and glycerol (21 cells/mL^-1). Large numbers of Rhodobacter spp. were cultured from dilutions of 10^-2 to 10^-5, and many of these were capable of Mn(III) oxidation. Up to ca. 30% of the colonies tested LBB positive, and all 77 of the successfully sequenced LBB positive colonies (of varying morphology) yielded sequences related to Rhodobadcter spp. qPCR indicated that a cluster of Rhodobacter isolates and closely related strains (95-99% identity) represented approximately 1-3% of the total Bacteria, consistent with clone library results. Copy numbers of SSU rRNA genes for either Rhodobacter spp. or Bacteria were four to eight fold greater during ETM events compared to non events. Strains of Shewanella sp. were retrieved from the highest dilutions (10^-5) of Mn reducers, and were also capable of Mn oxidation. The SSU rRNA gene sequences from these strains shared a high identity score (98%) with sequences obtained in clone libraries. Our results support previous findings that ETMs are zones with high microbial activity. Results indicated that Shewanella sp. and Rhodobacter spp. were present in environmentally relevant concentrations, and further demonstrated that a large proportion of culturable bacteria, including Shewanella sp. and Rhodobacter spp. were capable of Mn cycling in vitro.