New Insights into the Lifestyle of the Cold-Loving SM1 Euryarchaeon: Natural Growth as a Monospecies Biofilm in the Subsurface

Abstract

In the surface waters of sulfidic springs near Regensburg, Bavaria, Germany, the SM1 euryarchaeon, together with filamentous bacteria, forms the recently described unique string-of-pearls community. In addition to naturally occurring string-of-pearls communities, the growth of these communities was also observed on polyethylene nets provided as an artificial attachment material in the streamlets of springs. In order to learn more about the distribution and origin of the SM1 euryarchaeon and its possible occurrence in the subsurface, polyethylene nets were incubated as deeply as possible in different spring holes. After a short residence time, slime-like, milky drops, almost completely composed of SM1 euryarchaeon, were attached to the nets, indicating that this organism grows independent of a partner in deeper earth layers. A newly designed in situ biofilm trapping system allowed the quantitative harvesting of organisms exhibiting this newly discovered lifestyle of the SM1 euryarchaeon for detailed biological studies. The discovery of naturally occurring archaeal biofilms extends our knowledge of the biology and ecological significance of archaea in their environments.Sulfidic springs are fairly common around the world and have been the focus of (micro)biological work for more than 150 years (29, 42). They were classified as areas of high bioactivity, as visible by the development of extended (white) mats representing vast populations of various genera of filamentous sulfur bacteria (23). As the main focus for our detailed investigations of microbial communities in nongeothermal environments, we have chosen a sulfurous marsh, the "Sippenauer Moor" near Regensburg, Germany (32). In this area, cold water (ϳ10°C) from deeper earth layers reaches the surface and emerges from the ground between rocks and tree roots, forming small streamlets that merge in a large pond. The intake of atmospheric oxygen causes the appearance of white microbial mats in the streamlets, indicating high bioactivity. In these sulfidic streamlets, growth of a unique microbial community occurs, with microbes forming a string-of-pearls-like, macroscopically visible structure: single, whitish pearls with diameters of up to 3 mm are connected to each other by thin, white-colored threads. In the pearl interior, the nonmethanogenic SM1 euryarchaeon is predominant, representing a deep phylogenetic branch within the 16S rRNA tree (32). The exteriors of the pearls and the connecting threads are mainly composed of a single phylotype, the filamentous sulfide-oxidizing bacterium Thiothrix sp. (21).During a recent microbial survey of cold sulfidic springs in Bavaria, Germany, a second type of microbial string-of-pearls community was discovered in the streamlet of the "Islinger Mühlbach" (Regensburg, Southern Germany) (31). The SM1 euryarchaeon was again predominant in the pearl interior, while its partner was represented by the filamentous so-called IMB1 ε-proteobacterium, most probably outcompeting Thiothrix at the low in situ oxygen concentrations. The ...

Tackling the minority: sulfate-reducing bacteria in an archaea-dominated subsurface biofilm

“…This second life-style of the SM1 Euryarchaeon differs also from other described microbial systems, in which archaea are involved in biofilm formation (Lapaglia and Hartzell, 1997;Tyson et al, 2004;Frols et al, 2008): first, the SM1 euryarchaeal biofilm represents the only known naturally occurring Archaea-dominated biofilm, revealing a purity of up to 95% based on microscopic counts (Henneberger et al, 2006). Second, the small archaeal cocci form porous colonies with defined distances between the single cells mediated by their unique cell surface appendages (Moissl et al, 2005;Henneberger et al, 2006). Third, bacteria in the biofilm are either randomly distributed or form dense microcolonies, and their varied morphological appearance hints at a broader genetic diversity.…”

“…Third, bacteria in the biofilm are either randomly distributed or form dense microcolonies, and their varied morphological appearance hints at a broader genetic diversity. Lastly, no other archaea have been detected within the biofilm, using fluorescence in situ hybridization (FISH) or conventional cloning strategies, suggesting that the SM1 euryarchaeal biofilm is a natural 'archaeal monospecies biofilm' (Henneberger et al, 2006). The Muehlbacher Schwefelquelle spring therefore represents an extraordinary window to an anoxic subsurface biotope of an unusual archaeon.…”

“…No nucleic acids, however, were found in the matrix surrounding the SM1 Euryarchaeon cells (Henneberger et al, 2006). The protein content is mainly owing to its extraordinary cell surface structures, called hami, which are highlycomplex, filamentous attachment tools with a nanosized grappling hook at their end (Moissl et al, 2005).…”

“…By placing an in situ trapping system in this subsurface setting, slime-like biofilm structures consisting almost exclusively of SM1 euryarchaeal cells can be caught from the water stream, in stark contrast to the abovementioned string-of-pearls community (Henneberger et al, 2006). This second life-style of the SM1 Euryarchaeon differs also from other described microbial systems, in which archaea are involved in biofilm formation (Lapaglia and Hartzell, 1997;Tyson et al, 2004;Frols et al, 2008): first, the SM1 euryarchaeal biofilm represents the only known naturally occurring Archaea-dominated biofilm, revealing a purity of up to 95% based on microscopic counts (Henneberger et al, 2006). Second, the small archaeal cocci form porous colonies with defined distances between the single cells mediated by their unique cell surface appendages (Moissl et al, 2005;Henneberger et al, 2006).…”

“…The SM1 Euryarchaeon is found in sulfidecontaining fresh and marine waters all over Europe (Rudolph et al, 2004), but only two sites (close to Regensburg, Bavaria, Germany) were studied extensively during the past 10 years: The Sippenauer Moor and the Muehlbacher Schwefelquelle (''Islinger Muehlbach''; Henneberger et al, 2006). Both of these sites are characterized by a main, sulfidic spring, emanating into a streamlet where whitish mats of sulfide-oxidizing bacteria cover the submerged surfaces.…”