Microbial Diversity in Natural Environments: Focusing on Fundamental Questions
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Antonie van Leeuwenhoek DOI 10.1007/s10482-006-9090-x ORIGINAL PAPER Microbial diversity in natural environments: focusing on fundamental questions David M. Ward Received: 11 May 2006 / Accepted: 11 May 2006 Ó Springer Science+Business Media B.V. 2006 Abstract Interactions with Gijs Kuenen and communities, and for investigating how such units other Dutch scientists have led my lab to funda- coordinate the physiological activities within mental insights into the composition, structure guilds of the community. and function of a hot spring cyanobacterial mat community that should influence our thinking Keywords Microbial mats Æ Molecular analysis Æ about all microbial communities. By focusing on Species Æ Photosynthesis the distribution of molecular sequence variants of predominant mat phototrophs, we have discov- Abbreviations ered that small-scale sequence variation can be GNSB green nonsulfur or nonsulfur-like ecologically meaningful. By applying novel culti- bacteria vation approaches, we have been able to obtain DGGE denaturing gradient gel electrophoresis genetically relevant community members and thus to test the hypothesis that closely related sequence variants arose via adaptive evolutionary Introduction radiation. By applying the analytical tools of organic geochemistry we have gained insight into It is a pleasure to submit this contribution in the metabolisms of major phototrophic members honor of Gijs Kuenen, a long-time friend and of the mat community as well as interactions colleague in microbial ecology, as well as between phototrophic guilds. These observations other Dutch scientists, many of whom are challenge traditional paradigms about prokary- also associated with the Delft School of otic species and cause us to consider evolutionary Microbiology, who have had a major influence ecology theory as we develop genome-based on my research through the years. As I methods for high-resolution analysis of the spe- contemplate the impact Gijs has had on my cies-like fundamental units comprising microbial program, I think back to a backpacking trip we made together in the early 1990s to the Edith Baldy Basin in the high country of Montana. We were en route between Hidden Lake and D. M. Ward (&) Edith Lake, when Gijs was inquiring as to what Department of Land Resources and Environmental my plans were following our initial molecular Sciences, Montana State University, Bozeman, MT 59715-3120, USA analyses of well studied hot spring cyanobacte- e-mail: [email protected] rial mat communities. I don’t recall my specific 123 Antonie van Leeuwenhoek answer, but it was something along the lines of Methods continuing our analysis deeper and deeper into the molecular diversity of microorganisms of the The variety of methods used to obtain the results community, as if on a quest to discover all the presented in this review have been detailed in the community members through their molecular primary peer-reviewed papers cited below. signatures. I also don’t recall Gijs’ specific reaction, but it was something along the lines of how silly an idea that was. He encouraged me Results to focus on the predominant and easily detected members of the community, asking fundamental An example of the communities in question, questions about their diversity and ecology, microbial mats in alkaline siliceous hot springs including their contribution to community (Octopus Spring and Mushroom Spring,Yellow- function. In the intervening years, my students stone National Park) is shown in Fig. 1. Before and I have followed this advice and I believe it my students and I began studying this system as has led to much more important insights, as a model for microbial community ecology in Gijs predicted. In this paper, I would like to 1977, Tom Brock (my mentor), Richard highlight our progress, paying special attention Castenholz and their students had studied the to ways in which Gijs and other Dutch scientists system for a number of years (Brock 1978; were involved. Ward and Castenholz 2000). The mat seemed to Fig. 1 Landscape view of Octopus Spring, an alkaline habitat in the summer of 1991. The top green layer of a mat siliceous hot spring in Yellowstone National Park. sample collected at ~ 60°C (inset at lower left) is comprised Cyanobacterial mat is visible beneath the inset (upper of sausage-shaped Synechococcus and filamentous right) showing Alyson Ruff-Roberts, Gijs Kuenen, Eric Roseiflexus cells (inset to right of mat image) Kopczynski and Dave Ward conducting fieldwork in this 123 Antonie van Leeuwenhoek be a relatively stable and simple system, con- variants were observed, all distantly related structed by a single morphotype of unicellular to the S. lividus 16S rRNA sequence, cyanobacterium, Synechococcus, and a single with variants falling into two clades (A and B morphotype of green nonsulfur bacterium clades in Fig. 2A). Similarly, several closely (GNSB), Chloroflexus. The cultivation from hot related GNSB-like 16S rRNA sequence variants spring mats such as these of S. lividus (Meeks were observed, all distantly related to the 16S and Castenholz 1971; Kallas and Castenholz rRNA sequence of C. aurantiacus (C-like 1982) and C. aurantiacus strains (Bauld and sequences in Fig. 2B). The GNSB sequences Brock, 1973; Pierson and Castenholz 1974) were later recognized as closer relatives of showing little genetic difference reinforced the Roseiflexus castenholzii, a bacteriochlorophyll notion of a simple community composition a-rich filamentous anoxygenic phototroph (Ferris et al. 1996b). By the early 1990s, we had (Nu¨ bel et al. 2002). This paper will focus on developed 16S rRNA-based cloning and the in-depth study of these phototrophic sequencing methods for molecular analysis of community members, which appeared to be the community and had already discovered that predominant based on the frequency of recov- the system was more complex than originally ery of sequence variants of these types and on thought (Ward et al. 1990, 1992). Several closely enrichments on highly diluted samples (see related Synechococcus 16S rRNA sequence below). Fig. 2 16S rRNA-based phylogenetic trees of (A) cyano- organisms predominant in situ (lower shaded regions) bacterial and (B) green nonsulfur-like bacterial diversity in and those readily cultivated (higher shaded regions). the Octopus Spring (OS, thick lines in A) or Mushroom Isolate C1 in (A) has a 16S rRNA sequence identical to Spring (MS, boxed entries in B) cyanobacterial mats that of S. lividus. [Ward et al. 1998 and Nu¨ bel et al. 2002] relative to pure-cultured organisms. Shading contrasts 123 Antonie van Leeuwenhoek Community composition and structure Shortly thereafter, my lab had the good fortune of collaborating with another Dutch scientist, Patterns lead to hypotheses about their causes Gerard Muyzer, who later joined Gijs’ department in Delft. Gerard had introduced to microbial We hypothesized that distribution along environ- ecology a new and powerful method, denaturing mental gradients might give clues to the signifi- gradient gel electrophoresis (DGGE), which sep- gance of closely related 16S rRNA sequence arates similarly sized molecules that differ in variants in the community. Our first approach to sequence (Muyzer et al. 1993). DGGE offers distribution analysis was oligonucleotide probing of greater resolution of closely related sequence the 16S rRNAs of both cultivated and uncultivated variants and is particularly useful for simulta- cyanobacteria (S. lividus and genotypes A, B) and neously observing multiple 16S rRNA sequences filamentous anoxygenic phototrophs (C. aurantia- in a single sample. Gerard visited my lab to teach cus and genotype C) that were known to us at the my student Mike Ferris DGGE technology. The time (Ruff-Roberts et al. 1994). Gijs was visiting resulting papers (Ferris et al. 1996a; Ferris and my lab in the summer of 1991, helping my student, Ward 1997) revealed the presence of even more Alyson Ruff-Roberts design and use such probes. closely related variants of Synechococcus (A¢,A¢¢ This work gave us the first insights that these and B¢)andRoseiflexus (C¢,C¢¢) 16S rRNA se- closely related genotypes (e.g, type-A and type-B quences (Figs. 2, 3B and 4A). Importantly, DGGE Synechococcus) were contributed by ecologi- analysis sharpened our focus on how these se- cally distinct community members, since distinct quence variants distribute along thermal (Ferris genotypes were found at different temperatures and Ward 1997) and vertical (Ramsing et al. 2000) (Fig. 3A) and sometimes at different pHs. gradients. It became clear that very closely related 16S rRNA variants (Fig. 4B) exhibited distinct ecological distributions (Fig. 4A). Such evolution- ary and ecological patterning lead us to hypothesize that, like many plant and animal species, adaptive evolutionary radiation might explain the origin of the diversity of closely related, yet ecologically distinct populations we were seeing (Ward et al. 1998)(Fig. 4C). Cultivating the as yet uncultivated To test the hypothesis that these closely related 16S rRNA sequence variants were from repre- sentatives of populations with different ecological adaptations, it was necessary to first cultivate these novel Synechococcus and Roseiflexus strains. Mi- crobiologists often use the word ‘‘uncultivatable’’ to describe the microbes recognized by molecular analysis, but not yet in culture. I prefer the phrase Fig. 3 Patterns suggesting adaptation of Octopus Spring ‘‘as yet uncultivated’’ to reflect the optimism cyanobacteria to different temperatures. (A)