Interaction of metals and protons with anoxygenic phototrophic bacteria Rhodobacter blasticus

Towards a better understanding of acid–base properties and metal adsorption capacities of the first primary producers on Earth, the surface chemistry of non-sulfur anoxygenic phototrophic bacteria (APB) Rhodobacter blasticus f-7 was characterized using a combination of potentiometric acid–base titration methods and electrophoretic mobility measurements as a function of pH (3 to 11) and ionic strength (0.001 to 1.0 M). Surface titrations were performed using limited residence time reactors taking into account the cell-wall bound Ca and Mg from the culture media for net proton balance calculations. Electrophoretic mobilities of live APB cells were investigated in 0.001–0.5 M NaCl at pH of 1 to 11 and different Zn, Cd and Pb concentrations. Adsorption of Zn, Cd, Pb, Cu, Co, Ni, Sr, Al, Ga, Ge, Mo, and W was studied at 25 °C in 0.01 M NaNO3 as a function of pH and metal concentration in batch reactors. A competitive Langmuir sorption isotherm in conjunction with a linear programming optimization method (LPM) was used to fit experimental data and assess the number and nature (carboxylate, phosphoryl/phosphodiester and amine) of surface sites and adsorption reaction constants involved in the binding of trace metals to the Rhodobacter blasticus f-7 surface. We found that the total H/OH binding site number (60–120 μmol/gwet) for APB is comparable to that of cyanobacteria studied previously by the same technique (50–200 μmol/gwet). Similarly, LPM adsorption parameters for Zn, Cu, Pb and Cd for APB are in close agreement with those observed for the cyanobacteria. As such, results of the present study indicate similar affinity of both bacteria cells surfaces to divalent metal micronutrients, most likely due to the dominance of carboxylate and phosphorylate binding at rather high metal loading.