One of NASA's key reaearch goals concerns the question of life on Mars. There is considerable geological evidence that there once was liquid water on the surface of Mars. However, through time the planet evolved to become cold and dry. From a microbial perspective, the key biological stress is not the low temperatures, but the arid climate. To understand how life can survive in dry conditions we have been engaged in studies of microbial life in extreme dry regions of the Earth. A thorough understanding of how microbial life survive in dry environments on Earth will allow us to develop models of how life might have survived in dry conditions on Mars, what the environmental limit for life would have been as Mars became progressively more dry, and finally where to search for fossil evidence of past microbial life.
The main objective of our work in Death Valley National Park (DVNP), March 5-7, 1997, was to identify locations where hypolithic algae exist and to compare the algae in the dry conditions at locations within the Park to those in other places (e.g., Antarctica, the Gobi, Negev, and Atacama deserts). Our preliminary work in the region around Barstow showed the presence of hypolithic algae (i.e., algae that live on the bottom surfaces of rocks, at the rock-soil interface). These findings motivated our research in DVNP in order to understand hypolithic algal systems in a more natural setting.
Our initial research on hypolithic algae in DVNP consisted of site investigation and sample collection. We collected rock samples for microscopic analysis and comparison to similar communities in our collection from other deserts of the Earth. Our collection from DVNP consists of 20 rock samples exhibiting bands and surfaces of hypolithic algae; 17 of these samples are from the base of the alluvial fan at Johnson Canyon and three samples are from a location approximately 1.6 km (1 mile) from the road cut at West End Road. These samples will be returned to the Park at the end of the investigation.
GEOGRAPHY AND GEOLOGY OF MAIN SAMPLING SITE
The location at which most of the rock samples were collected, Johnson Canyon, is a gravel fan along the east foot of the Panamint Range (Fig. 1).
Figure 1. Regional view of Death Valley. Panamint Range is located immediately west of the Valley (from Hunt, 1975).
The Johnson Canyon Fan is about 5-6 miles long and range in elevation from below sea level to more than 2,200 feet above sea level (Fig. 2). The source areas of the fan consist mostly of Precambrian (more than 600 million years ago) and Paleozoic (about 600 to 225 million years ago) sedimentary rocks, with small amounts of granite and volcanics (Hunt, 1975). Figure 3 shows a regional map of Johnson Canyon.
JOHNSON CANYON FAN
Figure 2. Johnson Canyon Fan (from Hunt, 1975). (Sample location marked).
Figure 3. Regional map of Death Valley, showing Johnson Canyon (from Kirk, 1976). (Sample location marked).
The collected rock samples range in size from approximately 4 cm to 15 cm, the majority being in the 4 cm to 10 cm range. All the rocks are quartzite (a metamorphic rock consisting mainly of quartz and formed by recrystallization of sandstone), with the exception of one chalcedony (a cryptocrystalline variety of quartz), collected on the roadside of West End Road. The biological significance of these samples is manifested in their characteristic green bands, 3 mm to 10 mm wide, on the sides of the rocks, or as green bottom surfaces (Fig. 4).
The bands and surfaces represent hypolithic algae, (i.e., algae that grow on the lower surfaces of translucent stones, partially embedded in the soil; Fig. 5). The apparent reason algae retreat to and exist in this habitat is the increased level of protection from intolerable levels of irradiation, high temperatures, and arid surface conditions, as described by Vogel (1955). Most importantly, the hypolithic niche is favorable because it provides sufficient moisture to sustain metabolic activity and growth in an arid environment. Until quite recently dewfall was believed to be the main source of water under stones (e.g., Friedmann et al., 1967). However, recent studies suggest that most of this water can be attributed to rainfall (Allen et al., 1997a, b). In any case, the rocks hold moisture after a wetting event longer than soil uncovered by rocks. As discussed by (Friedmann et al., 1967), microbial life is possible under those rocks which are translucent and able to transmit light sufficient for photosynthesis.
Figure 4. Quartzite from Johnson Canyon fan. Sample shows green bands of hypolithic algae. Rock is 9.5 cm wide.
Figure 5. Diagram of hypolithic microbial growth in deserts (after Friedmann and Galun, 1974)
Microscopic studies of the hypolithic algae collected at the locations listed above suggest the organisms are Microcoleus and Chroococcidiopsis species (Fig. 6). Previously, one sp. of Microcoleus (M. chtonoplastes) was found in the Gobi desert of Mongolia and another (M. vaginatus) in the Negev, Israel (Allen et al., 1997a, b, and references therein).
Figure 6. Chroococciodiopsis sp. , strain (069) S21b, in the microscope.
Additional extensive studies of hypolithic algae have been conducted in the following hot deserts: South Africa (Vogel, 1995), Gobi (Allen et al., 1997 and Novichkova-Ivanova, 1988); Antarctica (Broady 1981, 1989)); Negev (Berner and Evenari, 1978; Friedmann et al., 1967). In the cold deserts of Antarctica hypoliths have been studied by Kukushima (1959) and by Cameron (1972), and Thompson (1979).
Hypolithic microbial growth is common in deserts where the soil is covered by desert pavement, as discussed by Friedmann and Galun (1974) . This surface feature is common in Death Valley and represents a residual concentration of wind-polished stones that are left behind where wind has removed the smaller particles. Desert pavement also constitutes a dominant surface feature of other deserts (e.g., the Gobi; Friedmann and Galun, 1974) where similar hypolithic algae exist.
This study of hypolithic algae in Death Valley National Park reveal an abundance of hypolithic algae of Chroococcidiopsis and Microcoleus species at Johnson Canyon and West End Road. Most of the algae exist on quartzite rocks, which are translucent and therefore allow photosynthesis to occur. These species are also characteristic of other hot and cold deserts, as discussed above. The presence of Chroococcidiopsis species has particular interest and relevance to the question of life on Mars because it represents a primitive type of cyanobacteria, probably similar to the earliest forms of cyanobacteria on Earth (Friedmann and Galun, 1974).
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