The unicellular fungal tool RhoTox for risk assessments in groundwater systems

•A tool for risk assessment in groundwater is proposed.•Based on cell viability of Rhodotorula minuta, a groundwater-specific species.•Sensitive to metals above background concentrations.

The recent inclusion of yeasts in environmental monitoring recognizes their ecological significance and sensitivity to toxicants. Here we present a robust and simple two-step toxicity assay and demonstrate the sensitivity of an ubiquitous groundwater yeast, Rhodotorula minuta, to a range of metals and metalloids. The test species was sensitive to copper with a 24 h EC50 of 35 µg Cu/L, followed in order of decreasing sensitivity by zinc, chromium (VI) and arsenic (EC50 4.40 mg As (III)/L). The strain demonstrated an unexpected tolerance to chromium (VI), having an EC50 value (3.45 mg Cr (VI)/L) similar to that of arsenic. The inclusion of a unicellular, microbial test-species into the suite of existing multicellular test species for toxicity evaluation is a key step towards strengthening the assessment of risk for groundwater ecosystems.