Conspicuous gelatinous masses oscillating in the currents of high-velocity geothermal outflow channels (see video¹) in YNP have been referred to as ‘streamers’ and their composition perplexed early explorers, scientists and visitors alike. In a national park filled with megafauna, it is not often that a chemolithoautotrophic microbial community attracts the wonder of visitors and biologists from around the globe, but such is the case with streamer communities as they wiggle incessantly in the hot water currents of geothermal outflows. Despite their modest visual appearance, streamers host microbes that hold important clues to the evolution of life in low-oxygen environments.

Early microbiological work on streamer communities identified members of one of the most ancient bacterial groups (Aquificales) as the dominant microbes in these filamentous structures. The pink streamers in Octopus Spring growing above 80 ^oC were studied using 16S rRNA gene analysis in the early 1990s², and this pioneering work identified Thermocrinis spp. as the primary Aquificales present as well as several additional microbes referred to as EM3 and EM19. Later work^3,4,5  based on random shotgun (Sanger) sequencing of the Octopus Spring streamer communities confirmed the importance of Thermocrinis spp., as well as EM3 and EM19, 15 years after their original discovery. More sequencing, better microbial references, and better phylogenetic tools have now been used to identify EM3 and EM19 as aerobic heterotrophs of the early-evolved Pyropristinus and Calescamantes lineages, respectively. The fact that the same types of organisms have dominated these streamer communities for at least 30 years is testament to the consistent geochemical and temperature conditions observed over this time. It also illustrates that microbes from these specific groups are most competitive in these harsh environmental conditions.

After a decade of field observations of streamer communities in different pH and sulfide environments of YNP, we realized that metatranscriptomics would provide the potential sensitivity needed to understand whether oxygen reduction was occurring in high-pH streamer communities in the presence and/or absence of dissolved sulfide. Numerous alkaline siliceous geothermal springs also have high levels of dissolved sulfide. This geochemical treatment is provided by natural geophysical processes controlling subsurface boiling and gas distillation, and the resulting concentrations of gases including H₂S. Conch Spring is in the same region as Octopus Spring (Lower Geyser Basin) and has similar geochemistry but contains considerably higher levels of dissolved sulfide at discharge. Early efforts to obtain mRNA from Octopus Spring were successful, but it took numerous attempts and method development over several years to obtain mRNA from the sulfidic (and very ‘snotty’) streamers present in Conch Spring so that the two systems could be compared directly.

Replicate metagenomic data from these two streamer communities revealed several surprising results, but the transcriptomic data from each system was necessary to fully understand differences in ecophysiology between sulfidic and microaerobic conditions. Thermocrinis spp. (Aquificales), Pyrobaculum (Archaea) and Caldipriscus (Pyropristinus) are abundant populations in both habitat types, but other community members are remarkably different between the two systems.  Many have assumed that these geothermal systems are ‘anaerobic’ and devoid of oxygen due to sulfide and/or high temperature. Yet, most of the additional 10-plus microbial populations present in Octopus Spring are aerobic heterotrophs. Moreover, various oxygen reductases present in the 3 common populations are highly transcribed in both springs; and the high-affinity oxygen reductases are only transcribed in the presence of high sulfide (< 1 micromolar dissolved oxygen). This includes the bd ubiquinol oxygen reductases common in numerous bacteria as well as the cydAA’ cytochrome oxygen reductases shown to be important in archaea. These high-affinity oxygenase’s are active at nanomolar levels of oxygen, and explain the high transcription observed under sulfidic conditions of Conch Spring.

Results obtained from over a decade of field and laboratory investigations of these streamer communities support the importance of sulfur, arsenic and reduced carbon as electron donors, and low-levels of oxygen as an electron acceptor used in energy conservation. These findings show that early-evolved thermophiles utilize low-levels of oxygen for respiration under conditions often thought too low to support any microbes. The data also suggest that low-levels of oxygen common prior to the Great Oxidation Event may have been sufficient to support early aerobic metabolisms important in the evolution of both bacteria and archaea.

Of the many remarkable attributes of streamer communities, something quite powerful can be deduced simply by observing their realized niche. The streamer communities are large enough structures to be visible with the naked eye, and so it might go without saying that we can see them oscillating vigorously in the fast currents¹. However, this physical reality informs us that gas exchange is necessary for optimum growth, which is also further influenced by the rapid oscillation of large filamentous structures that create turbulence and likely increase rates of gas exchange with the atmosphere. Attempts to study the oxygen gradients that occur within a streamer community using oxygen microelectrodes were unsuccessful due to constant breakage of the fragile, small-diameter, glass electrodes. Future efforts to characterize the physical dynamics and rates of gas exchange will improve our understanding of factors controlling microbial growth rates in these high-velocity habitats.

¹ Video of streamer communities in Octopus Spring, Yellowstone National Park (YNP).

²Reysenbach AL, et al. Phylogenetic analysis of the hyperthermophilic pink filament community in Octopus Spring, Yellowstone National Park. Appl Environ Microbiol 60, 2113-2119 (1994).

³Takacs-Vesbach C, et al. Metagenome sequence analysis of filamentous microbial communities obtained from geochemically distinct geothermal channels reveals specialization of three Aquificales lineages. Front Microbiol 4, 84 (2013).

⁴Beam JP, et al. Ecophysiology of an uncultivated lineage of Aigarchaeota from an oxic, hot spring filamentous ‘streamer’ community. ISME J 10, 210-224 (2016).

⁵Colman DR, et al. Novel, Deep-branching heterotrophic bacterial populations recovered from thermal spring metagenomes. Front Microbiol 7, doi.org/10.3389/fmicb.2016.00304 (2016).