SpeleoScope
Project Overview

We have recently discovered and preliminarily characterized dense, microbial mats up to several centimeters in thickness that line the narrow and twisting interior rock architecture of highly active hydrogen-sulfide and carbon monoxide emitting springs located in a carbonate cave (Cueva de Villa Luz) in Tabasco, Mexico (Boston, 1999, 2000, 2001; Hose et al., 2000; Northup and Lavoie, 2001; Lavoie et al., 1998; Hose and Pisarowicz, 1997, 1999). Subsequent to this, other team members have discovered additional examples of this phenomenon in temperate zone hydrogen sulfide caves in Wyoming (Soroka and Kleina, unpublished data). Similar springs exist in many caves although interior mats have not yet been reported, e.g. gypsum caves in Ukraine (D. Faguy, pers. comm.), sulphidic karstic caves in Romania (B. Onac, pers. comm.), and submarine sulphide caves in Italy (Mattison et al., 1998). Microbial mats in subterranean sulfide springs may be a globally distributed ecosystem type.

The Mexican cave spring mats are of unknown linear extent but are present at least to the reach of a sterile-suited and gloved human arm (Achenbach, 2000). Isotopic values of the gaseous sulfide emitted, elemental sulfur and gypsum deposits at the spring mouths, and sulfide emitted from the nearby volcano, El Chichonal, lead us to hypothesize that the springs may be lined with biological mat for a significant portion of their length. We base this on evidence of considerable biological reworking of the isotopically heavy volcanic source sulfide gas (d34S value of +4.6 ‰) to the isotopically much lighter sulfide values at emission points within the cave (d34S value of -11.7 ‰). Further isotopic fractionation has occurred in spring mouth deposits of elemental sulfur and gypsum (d34S values of -23.5 to 25.6 ‰).

The presence of active, dense organism assemblages containing isolates that we have shown mediate gypsum and sulfate mineralization by metabolic activity has lead us to tentatively conclude that the deposits are biologically mediated, at least in part. We have grown and characterized to metabolic class (and some to genus level) a variety of metabolically relevant organisms (sulfate reducing bacteria, elemental sulfur oxidizers, and hydrogen sulfide oxidizers) that we isolated from these deposits.

Volcanic origin of the original sulphide and other reduced gases has been preliminarily verified by gas chromatography of collected gaseous spring emissions. These were compared to known "fingerprints" of volcanic emissions by volcanologist Tobias Fischer (unpublished data) also at the University of New Mexico.

The mats contain identifiable green and purple sulfur bacteria and indeed, some of the mat is colored in visible light. However, these organisms are growing in the complete absence of light in the totally dark zone of this cave. Additionally, small (1-2mm long) blood red worms, as yet unidentified, graze the more distal parts of the mat in the zone that mixes with the normal oxygen-containing cave air. These mats are part of the overall biological enrichment that has occurred in this cave due to an abundant energy source (hydrogen sulfide) driving a chemolithoautotrophic food chain that supports huge numbers of higher organisms (fish, bats, invertebrates of many kinds) compared to the more usual oligotrophic environments of non-sulfide caves.

Many questions attend this discovery. How far into the springs do the mats extend? What is the concentration of sulphide and oxygen at various points within these springs? Does mat grow beyond the oxic zone? How does the nature of the biological community change with position vis a vis the spring mouth? Are the green and purple sulfur bacteria ubiquitous in the mats or only near to the exit? What is the biodiversity and taxonomic affiliation of the mat organisms and how do these vary over the extent of the mats? How does the gaseous and solid isotopic fractionation vary with distance upspring? Are there distinctive characteristics of biogenic mineral products that can differentiate them from their abiotically precipitated counterparts?

None of these questions are presently accessible to study because we simply cannot reach in to retrieve sample, or to perform various sensing and imaging tasks. We have devised an instrument concept for a flexible, threadable, thin probe that could be surface sterilized and fed into the winding rock micropassages that constitute the spring interior architecture. The probe in its simplest incarnation would be fitted with imaging capability via optical fiber technology and house gas sensing and ion selective microelectrodes bundled into the tip and protected by an abrasion barrier. This probe could be used to discover the extent of the mats by visual inspection in visible and fluorescent wavelengths and would be able to map cross gradients of critical microbial resources like hydrogen sulfide and oxygen. Temperature, pH, eH, and conductivity measurements are operationally very similar and can be incorporated in the same bundle. A more elaborate version would include the ability to sample fluid and gas. Further refinements would include aseptic microsampling of biological and mineralogical material.

Wider application of this instrument includes other difficult to access environments like hydrothermal surface springs, volcanic structures that have transient water features that may house microbial communities of the mat or biofilm variety, submarine cold methane seeps that may house microbial communities and have complex dissolved gas chemistry that will vary with distance from source to emission point, deep sea hydrothermal vents, and the newly discovered non-hydrothermal sulfide glacier springs reported from the arctic. Such an instrument would also be invaluable in many possible extraterrestrial environments where refugia of microbial life, organic compounds, or reduced gases may be found.