Susceptibility to additional stressors in metal-tolerant soil microbial communities from two pollution gradients

• We investigated the effect of secondary stressors on the microbial communities along two pollution gradients.
• Secondary stressors restructured the microbial communities similarly in all soils.
• Metal-tolerant communities are not more susceptible to additional stress.

Soil microbial communities exposed to long-term metal pollution have been shown to maintain their functionality by developing tolerance under field conditions. This study investigated how the cost of acquiring metal tolerance affected the responses of such communities to secondary stressors (stress-on-stress). We studied soil microbial communities living in soils polluted by metals to different degrees, sampled along two metal pollution gradients in Poland. We hypothesized that communities adapted to high concentrations of metals would be more susceptible to additional stressors that they had not previously encountered (benzo[a]pyrene and salt), compared to communities exposed to low concentrations of metals. In contrast, metal-adapted communities were expected to be more stable when exposed to stress factors within the same category of toxicity or to stress factors that periodically occur naturally, such as arsenic and flooding. In 60-day microcosm experiments, phospholipid fatty acid (PLFA) profiles were used to compare changes in microbial communities sampled from the field and exposed to four different additional stressors. Our results showed that, regardless of existing pollution, additional stress restructured the microbial communities similarly in all soils. However, salt and flooding stress had stronger effects on community structure than arsenic and benzo[a]pyrene. PLFAs indicative of fungi, Gram-negative and Gram-positive bacteria generally declined under stress relative to unspecific PLFAs (such as 16:0 and 18:0). This trend may be caused by the inhibition or suppression of sensitive microbial groups and selection of tolerant groups by the stressors applied and also because of the proliferation of fast-growing species under stress conditions. Overall, our study showed that metal-tolerant communities that have been selected in the field over many years are not more susceptible to additional stress than communities exposed to low concentrations of metals; thus, adaptation in these microbes has evolved without apparent costs.