Microbial community structure and functioning along metal pollution gradients

Abstract

Toxic effects of heavy metals on soil microorganisms have been confirmed in a number of laboratory studies. However, most real-field studies do not allow for strong general conclusions due to a range of problems, such as pseudoreplication and confounding factors, which are almost impossible to control for with the most commonly used polluted versus unpolluted or random sampling designs. Effects of metal contamination on soil microbial community traits were measured along 2 pollution gradients in southern Poland. Employing an experimental regression design, using 2 separate gradients, the authors aimed to control for effects of soil properties and beta-diversity of microbial communities. General microbial activity was measured as soil basal respiration rate and substrate-induced respiration, while microbial functional and structural diversity were analyzed with community-level physiological profiles and phospholipid fatty acid patterns, respectively. Metal concentrations were normalized to their toxicity and integrated in a toxicity index (TI). Microbial activity (basal and substrate-induced respiration) decreased in both gradients with increasing TI. Community-level physiological profiles for fungi correlated positively with TI, but no impact of TI on the community-level physiological profiles of bacteria was observed. The phospholipid fatty acids a:15 and i:17 were positively correlated with TI, whereas 16:1ω9 and 18:2ω9 were negatively correlated with TI. The use of 2 gradients (Olkusz and Miasteczko Śląskie) allowed the authors to reveal a clear effect of pollution on general microbial structure and activities, even though they were not able to control completely for all confounding factors. Soil pH, organic matter content, and nutrient level appeared to be at least as important as TI in determining microbial community structure and activities.

Activity and functional diversity of microbial communities in long-term hydrocarbon and heavy metal contaminated soils

“…The readings were taken at 590 nm after inoculation and at 12 h intervals for 168 h using a microplate reader (Victor™ X5 Multilabel Plate Readers, PerkinElmer). The absorbance measurements for individual substrates were corrected as described by Azarbad et al (2013). The corrected absorbance values were used to calculate the following microbial indices: substrate richness (Rs), functional diversity based on the Shannon-Wiener biodiversity index (H') and evenness (I') (Klimek and Niklińska 2007).…”

“…There were also observed strong negative correlations between diversity indices and moisture (r=-0.72) and conductivity (r=-0.72, p<0.01). It seems that some metals and physicochemical factors are more important in controlling the soil bacterial diversity than hydrocarbons (Azarbad et al 2013). The number of utilized carbon sources, which is defi ned as the richness index, varied depending on the soil, from 26 to 31 out of 31 carbon sources available on the Biolog EcoPlates TM (Table 5).…”

Functional and compositional responses in soil microbial communities along two metal pollution gradients: does the level of historical pollution affect resistance against secondary stress?

“…Soil samples were stored at 4°C for 10 days before all chemical and microbiological analyses were performed. Characteristics of the sampling locations, including total and soluble metal levels, have been previously reported (Azarbad et al 2013(Azarbad et al , 2015. For the purpose of discussion, basic properties of the soil samples (e.g., pH, organic matter content, and water holding capacity) and levels of heavy metals used to calculate the toxicity indices (TI) have been included in Supplementary Table S1.…”

“…Both areas are highly contaminated since the 1970s by two zinc-and-lead smelters. The upper soil layers contain high levels of copper, lead, zinc, and other metals (e.g., up to 4300 mg kg −1 total Zn and 2900 mg kg −1 total Pb in Olkusz forest) placing them among the most polluted soils in Europe (Nowicki 1993;Azarbad et al 2013). The long-term exposure to metal contamination has negatively influenced microbial biomass and basal respiration rates in these two gradients (Azarbad et al 2013).…”

“…The upper soil layers contain high levels of copper, lead, zinc, and other metals (e.g., up to 4300 mg kg −1 total Zn and 2900 mg kg −1 total Pb in Olkusz forest) placing them among the most polluted soils in Europe (Nowicki 1993;Azarbad et al 2013). The long-term exposure to metal contamination has negatively influenced microbial biomass and basal respiration rates in these two gradients (Azarbad et al 2013). However, this has not resulted in large shifts in microbial community composition, diversity, and functional capabilities, as revealed by high-resolution molecular approaches such as Illumina sequencing and GeoChip hybridization (Azarbad et al 2015).…”