Implications in studies of environmental risk assessments: Does culture medium influence the results of toxicity tests of marine bacteria?

Two marine bacterial populations (Roseobacter sp. and Pseudomonas litoralis) were exposed to different concentrations of zinc (300, 625, 1250, 2000, 2500 and 5000 mg L−1) and cadmium (75, 250, 340, 500 and 1000 mg L−1) using two culture media (full nutrient Marine Broth 2216 “MB” and 1:10 (vol/vol) dilution with seawater of Marine Broth 2216 “MBSW”), in order to assess population responses depending on the culture medium and also potential adverse effects associated with these two metals. Different responses were found depending on the culture medium (Bacterial abundance (cells·mL−1), growth rates (μ, hours−1), and production of Extracellular Polysaccharides Substances (EPS) (μg glucose·cells−1). Results showed negative effects in both strains after the exposure to Zn treatments. Both strains showed highest metal sensitivity at low concentrations using both culture media. However, different results were found when exposing the bacterial populations to Cd treatments depending on the culture medium. Highest toxicity was observed using MB at low levels of Cd concentrations, whereas MBSW showed toxicity to bacteria at higher concentrations of Cd. Results not only showed adverse effects on Roseobacter sp. and Pseudomonas litoralis associated with the concentration of Zn and Cd, but also confirm that depending on the culture medium results can differ. This work suggests MBSW as an adequate culture medium to study metal toxicity bioassays in order to predict realistic effects on marine bacterial populations.