Bacteria in Weathered Basaltic Glass, Iceland

Abstract

Bacteria play an important role in rock weathering and yet their diversity and potential activity in the terrestrial rock weathering environment is poorly understood. Culture and culture-independent methods (16S rDNA) were used to investigate the populations of bacteria inhabiting a basaltic glass/palagonite subglacial (hyaloclastite) deposit subject to weathering in Iceland. The rock hosts a diverse microbial community. The 16S rDNA clones were dominated by Actinobacteria, Proteobacteria, Bacteroidetes and Acidobacteria. Representatives of Gemmatimonadetes and Verrucomicrobia were present. Isolation of organisms on basalt/palagonite yielded only two isolates, an actinobacterium and a Bacteroidetes, showing that the active species, at least in the time scale of laboratory cultivation, are a small proportion of the total diversity. Firmicutes and Actinobacteria were isolated when basalt/palagonite was supplemented with an organic source. Many of the isolates demonstrated tolerance to transition metals (Cr, Cu, Zn, Ni, Co) naturally present in the rock. The growth of the isolates was inhibited at typical pH values for Icelandic rain, which suggests that the increase in pH caused by the consumption of protons in rock weathering, for example by palagonite formation, may play a role in defining which organisms are active. Colonization experiments show that the filamentous growth habit of the actinobacterium isolated on basalt/palagonite allows it to actively invade and colonise the basaltic glass. The filamentous growth of some actinobacteria may be an important contributor to their role in systemic interstitial rock weathering in the natural environment.

Keywords:

• Actinobacteria
• basaltic glass
• Iceland
• volcanic
• weathering

This work was made possible and supported by the Royal Society and the Leverhulme Trust (Project No. F/00 269/N) to Charles Cockell. Felicity Knowles was supported by a Society for General Microbiology (SGM) Vacation Studentship. We thank Heather Davies for work on the TEM sections.

Notes

¹Spore formation was tested by growing isolates for 1 week in a modified Schaeffer sporulation medium (Schaeffer et al. 1965) consisting of 1.6 g of nutrient broth (Oxoid), 0.2 g of KCl, 0.5 g MgSO₄·7H₂O in 1L ddH₂O with the following ingredients added to the cooled medium (per 100 mL): 2 mL 1M Ca(NO₃)₂, 2 mL 0.1M MnCl₂·4H₂O, 2 mL FeSO₄·7H₂O, 2 mL 50% D-glucose monohydrate.

²We cannot rule out spore formation in other media for those that did not display spore formation in modified Schaeffer's medium.

³Growth response measured after 1 week. After 3 weeks isolates showed growth on pH 5.6 plates, suggesting delayed, but not arrested growth.

⁴Metal resistance measured as Minimum Inhibitory Concentration (MIC) in mM. ‘a’ means a MIC of < 0.125 mM.