*5.2. Acidic and Alkaline Habitats*

Acidic wetlands have a major impact on the global carbon and water cycles. With high acidity (pH 3.5 to 5.0), low temperatures, and extremely low concentrations of mineral nutrients (5 to 50 mg

per liter), wetlands are moderate to extreme habitats. Their microbial diversity remains poorly understood, because only microbial populations involved in CH4 cycling, i.e., methanotrophic bacteria and methanogenic archaea, have attracted considerable research interest. Other members of the microbial communities in acidic *Sphagnum* peatlands remain largely unknown [87].

The pH range for growth of the majority of myxobacteria is rather narrow, approximately 6.5–8.5. Therefore, they are common in soils of pH 6–8 (neutral to slightly alkaline pH). However, acidic or alkaline habitats also seem to be suitable for myxobacteria [10]. Even in 1977, Hook isolated ten species from waters of an alkaline bog and adjacent soils [88] like *Archangium*, *Corallococcus*, *Melittangium, Myxococcus*, and *Sorangium* (former *Polyangium*). *Corallococcus coralloides*, (formerly *Myxococcus coralloides*) was dominant in the terrestrial samples. With pH between 6.0 and 8.7, the investigated habitats were between slightly acidic and slightly alkaline. In 1979, Rückert also described *C. coralloides* as the predominant species in soils of pH 4.1–4.9 and as dominant as *M. fulvus* in soils of pH 3.0–3.5 [89]. In alpine acidic soils *C. coralloides* was the third-most dominant species behind two *Myxococcus* species. But, Rückert also noted that the overall myxobacterial diversity in acidic soils (pH 3.5–4.9) was less than in slightly acidic or neutral environments (pH 5.0–7.8). In 1984, Dawid isolated *Myxococcus xanthus, M. virescens* and *Polyangium* sp., but no cellulolytic species from undisturbed *Sphagnum* bogs of the Hohen Venn, Belgium [3].

Mohr et al. studied myxobacteria in peat bog and fen with cultivation and cultivation-independent methods [2]. Therefore, 38 moor samples of soil, water, plant residues, mud, and feces-material (Figure 7a–c) were screened using standard as well as moor-adjusted cultivation conditions (low pH, low temperature, moor-water for preparation of agar plates), screening numerous replicates over several years. The pH of moor samples analyzed in this study was between 4.0 and 7.0 and therefore comparable to those from the other studies. But almost exclusively species of the genus *Corallococcus* could be isolated from acidic soils of the Harz-region (Figure 7d,e). A *Sorangium* strain was detected on a raw culture plate with filter (Figure 7f), but could not be purified. In addition, the community composition of acidic high moor and fen revealed by cultivation-independent 16S rRNA clone library analysis gave a rather different picture of the myxobacterial diversity.

**Figure 7.** (**a**) Brockenfeld high moor; (**b**) fen Am Sandbeek; (**c**) Brockenfeld high moor scarp. Isolated *Corallococcus* sp. strains from moors (**d**) strain B19, (**e**) strain B2t-1. (**f**) *Sorangium cellulosum* (orange) on a raw culture plate (Stan 21 with filter) inoculated with soil material from moor. Pictures are from Mohr et al. (2017) and modified [2].

Phylogenetic analyses of clone sequences revealed a high diversity of undescribed myxobacteria in high moor and fen. Many sequences represent totally unknown taxa. However, numerous clones were closely related to sequences from other cultivation-independent studies of eubacterial diversity in which samples from peat swamp, wetlands peat bog, *Sphagnum* moss, pine forest, acidic fen soil, and forest soil were analysed (Figure 8). As mentioned above, cultivation exclusively revealed strains from the genus *Corallococcus* (but from almost all analysed samples).

**Figure 8.** Part of a distance tree showing some myxobacterial type strains, some representative clone sequences and one representative culture sequence from our study [2] as well as sequences from uncultivated myxobacteria from other studies. Red: clones from Brockenfeld high moor; blue: clones from fen Am Sandbeek; green: representative culture from Brockenfeld high moor. Accession numbers are given in brackets. Origin of sequences from uncultured myxobacteria are mentioned. Bar, 0.1 substitutions per nucleotide position.

To my knowledge, no publications about (bioactive) secondary metabolites from myxobacteria isolated from acidic or alkaline habitats are available. However, we screened 21 *Corallococcus* spp.-strains from the moor study for production of bioactive metabolites. Raw extracts of all strains showed high activity against Gram positives (*Micrococcus luteus*, *Staphylococcus aureus*, *Bacillus subtilis*, and *Mycobacterium* sp.), the yeasts *Saccharomyces pombe* and *Rhodotorula glutinis*, as well as against the filamentous fungi *Mucor hiemalis*, but no activity against Gram negatives. HPLC analyses of the raw extracts revealed three dominant peaks. By HPLC-fractionation of bioactive extracts and subsequent HPLC-MS analyses the already known substances dibenzylpyrazine, myxothiazol A, and myxothiazol Z/A-methylester, respectively, were identified (data not published). In summary, the moor habitat is a promising source and of high interest with regard to the cultivation of prospective new bioactive secondary metabolite-producing myxobacteria.
