First insights to the microbial communities in the plant process water of the multi-metal Kevitsa mine

Metallurgical processes demand large quantities of water. However, in many locations, water is becoming scarce and process water recycling is needed. Closing water loops can be challenging due to build-up of flotation chemicals, metal ions and microorganisms in the recycled water affecting the flotation performance. Here, we have characterized the microbial communities over a 2-month period in different locations of the multi-metal Kevitsa mine in Northern Finland, by microbiome sequencing, enumeration of bacteria, archaea and fungi by qPCR, and cultivation. The microbial communities showed high diversity, but were dominated by Alpha- and Gammaproteobacteria. In addition, various fungal taxa were detected, whereas the archaeal taxa were only sparsely detected from the sequence data. The number of bacterial 16S rRNA gene copies in Process water and Ni thickener overflow varied between 0.5-3.3 × 10⁵ mL^-1, whereas the Flotation tailings showed two orders of magnitude lower amounts. Fungi were present at 3.0 × 10²-8.1 × 10⁴ 5.8S rRNA gene copies mL^-1 in all samples, while the number of archaea fluctuated between 8.8 × 10¹-3.2 × 10⁵ 16S rRNA gene copies mL^-1. The number of all microbial groups were generally lower in September than in August. When tested on 8 different cultivation media, the microorganisms generally responded positively to organic carbon, and were also shown to oxidize thiosulfate, which may indicate that build-up of organic flotation chemicals and sulfur species from the ore may cause the microbial numbers to increase. This study is part of the H2020 ITERAMS project (Grant agreement# 730480), which strives to improve the recycling of water and minimize the environmental impact of mines.