Acidophilic algae isolated from mine-impacted environments and their roles in sustaining heterotrophic acidophiles.

Ivan Ňancucheo,D. Barrie Johnson

doi:10.3389/fmicb.2012.00325

Abstract

Two acidophilic algae, identified as strains of Chlorella protothecoides var. acidicola and Euglena mutabilis, were isolated in pure culture from abandoned copper mines in Spain and Wales and grown in pH- and temperature-controlled bioreactors. The Chlorella isolate grew optimally at pH 2.5 and 30°C, with a corresponding culture doubling time of 9 h. The isolates displayed similar tolerance (10–50 mM) to four transition metals tested. Growth of the algae in liquid media was paralleled with increasing concentrations of dissolved organic carbon (DOC). Glycolic acid was identified as a significant component (12–14%) of total DOC. Protracted incubation resulted in concentrations of glycolic acid declining in both cases, and glycolic acid added to a culture of Chlorella incubated in the dark was taken up by the alga (~100% within 3 days). Two monosaccharides were identified in cell-free liquors of each algal isolate: fructose and glucose (Chlorella), and mannitol and glucose (Euglena). These were rapidly metabolized by acidophilic heterotrophic bacteria (Acidiphilium and Acidobacterium spp.) though only fructose was utilized by the more fastidious heterotroph “Acidocella aromatica.” The significance of algae in promoting the growth of iron- (and sulfate-) reducing heterotrophic acidophiles that are important in remediating mine-impacted waters (MIWs) is discussed.

Highlights

Mining of metals and coal can impair the environment in many ways
One of these grew as single or small groups of round to oval-shaped cells, ∼3–5 μm diameter, while the other occurred as aggregating worm-like cells, ∼30–50 μm long, that displayed gliding motility. The identities of these isolates were confirmed by analysis of their 18S rRNA genes (1678 nt and 1778 nt gene length products for the Chlorella- and Euglena-like isolates, respectively)
The smaller algae shared 99% gene similarity with Chlorella protothecoides var. acidicola www.frontiersin.org which was isolated from acidic water at Pisciarelli, Italy (AJ439399; Huss et al, 2002), while the other shared 99% gene similarity with a strain of Euglena mutabilis (ELC 1) isolated from Lake Caviahue, an acidic water body in Argentina (EU090196; Brankatschk et al, unpublished)

Summary

Introduction

One of the most widely documented is the generation of mine-impacted water bodies (MIWs; drainage streams and pit lakes) that are characteristically acidic (sometimes extremely so) and which contain elevated concentrations of iron and other transition metals, aluminium, sulfate, and sometime arsenic. These constitute an “extreme” environment, which is hostile to most life forms (reviewed in Johnson, 2009). Where solar energy is available, net carbon fixation by acidophilic microalgae may exceed that of chemolithotrophic bacteria, since the most abundant chemical energy source in the most acidic mine waters (ferrous iron) is a relatively poor electron donor in terms of free energy ( F298 of −73 kJ; Kelly, 1978)

Methods

Results

Conclusion