Correlation between structural diversity and catabolic versatility of metal-affected prototrophic bacteria in soil

Abstract

Application of sewage sludge to an agricultural field resulted in contamination of Zn between 92 and 345 mg kg −1 of soil and other metals. The corresponding value of the control soil was 48 mg Zn kg −1. Metal effects on the structural diversity and the catabolic versatility of bacteria capable of growing in the absence of growth factors (prototrophs) were studied 6 yr after sludge application was discontinued. The number of strain clusters distinguished by amplified ribosomal DNA restriction analysis (ARDRA) was reduced by 39% when comparing isolates from the control and the most contaminated soil. Concomittantly, the average number of aromatic acids utilised per isolate from among 21 substrates tested decreased from 12.28 to 5.23. This loss in catabolic versatility was greater in Gram-negative (68%) than in Gram-positive bacteria (49%). Catabolic versatility, due to increasing metal stress, also decreased within ARDRA-clusters from different soils. As a consequence of metal stress, the proportion of frequent catabolic capabilities, partly related to energetically good aromatic C sources, increased, while the reverse was true for rare capabilities. Due to bioenergetic reasons discussed, it is supposed that the catabolically less versatile strains are more tolerant to metals than are the more versatile ones. The difference in the catabolic versatility between Gram-negative and Gram-positive bacteria and the greater loss of this property in the former may explain why, in metal contaminated soils, Gram-negatives are selected at the expense of Gram-positive bacteria.