Unbalanced growth of Acinetobacter CS 13 in glucose-limited chemostats caused by sublethal CdCl₂ concentrations

Abstract

The marine cadmium-sensitive Acinetobacter CS 13 was grown in glucose-limited chemostats to study the chronic toxicity of CdCl₂. Addition of 25, 50 or 100 μg Cd²⁺ l^-1 caused disturbances of balanced growth. At the dilution rate (D) of 0.05 h^-1 the washout rate (A) became constant at 84, 78 or 66 h after addition of 25, 50 or 100 μg Cd²⁺ l^-1 The washout rates were calculated to be respectively 0.037 h^-1, 0.039 h^-1 and 0.077 h^-1. At D=0.1 h^-1, A became constant after 48, 42 and 36 h, and were the same as D for all three Cd concentrations. Acridine orange staining revealed that 99% of the cells fluoresced green (viable cells) and 1% red or orange (dead cells). However, under Cd stress the percentage of red and orange fluorescing cells increased up to 20%. At D=0.1 h^-1, viable and direct cell counts always gave the same result. At D=0.05 h^-1, however, viable cell counts were lower than direct cell counts after 2–3 d of Cd exposure. It is assumed that at D=0.05 h^-1, a part of the cells appearing green was already Cd injured, but the true yellowish green colour was not identified. Decreasing cell densities were accompanied by increasing glucose concentrations. Cells accumulated Cd (0–25 μg Cd²⁺ l^-1) in relation to exposure time and Cd concentration in the medium, but maximum Cd contents were the same, increasing from 5.5 or 5.7 μg g^-1 dry wt in the control to 15 μg g^-1 after 2–3 d (D=0.05 h^-1) or 16 μg g^-1 after 1–2 d (D=0.1 h^-1).