Modern microbial mats from hypersaline lakes in the Puna, Andean Range, Argentina

Abstract

High-altitude lakes found in the Puna and Andean regions at the northwest of Argentina are considered extreme aquatic habitats. Microbial mat ecosystems found in these lakes are of potential biotechnological interest because of the organomineralization compounds that they produce. We describe microbial mats collected from three locations in the Argentinean Puna (Socompa Lake, Tolar Grande and Diamante Lake) and discuss why similar microbial mat communities produce different microbialite structures with varied mineral composition. The Puna region represents a large basin that is fragmented into a system of minor interrelated basins demarcated by mountains. Its climate is arid, with annual precipitation less then 350 mm, and daily temperatures range from -10 to 20ºC in summer and -15 to 10ºC in winter. Due to its high altitude and low latitude geographical position, the Puna region is exposed to high solar irradiance (~165% of the value at the sea level), especially in the UV-B region, where the instantaneous flux can reach up to 17 W/m2 (approximately half the amount on equatorial Mars). At the bottom of the Puna basins, large lakes rich in minerals have formed. Due to irregular inter-annual precipitation, the lake limits are variable and, occasionally, the lakes become totally dry and are transformed into large salars. The lakes harbour microbial mats associated with mineral precipitation. Due to the extreme environmental conditions, such as residual volcanic activity associated with sulphur availability, hypersalinity, high UV irradiation, low O2 pressure and low nutrient availability, these modern stromatolite-like ecosystems constitute excellent models to study geochemical cycles and biogeochemical interactions during the early Earth. Chemical analyses revealed significant differences amongst the studied lakes. The Socompa Lake (pH 8.5) contains more nutrients (e.g., P=232 mg/L) and organic matter (19%), as well as large quantities of silicium (450 mg/L) and chlorophyll a (70 µg/L) due to diatoms. The Tolar Grande Lake (pH 7.2) contains large quantities of Mg (86 mg/L) and Ca (206 mg/L) but low concentrations of organic mater (1%), silicium (16 mg/L) and chlorophyll a (0.2 µg/L) due to diatoms. In the same way, Diamante Lake (pH 10) is also formed by abundant inorganic material but shows very abundant arsenic (As 234 mg/L) and phpsphorous (225 mg P/l, 700 mg PO4/L). Minerals associated with or precipitated within the studied microbial mats also differ. The mats from the Socompa Lake are composed mainly from aragonite, and can thus be considered as typical stromatolites. On the other hand, the mineral composition of the Tolar Grande (gypsum and halite) and Diamante mats (gaylussite and calcite) differ from those largely proposed for stromatolite biogenesis. These lakes are good laboratories to try to comprobe the hypotesys that similar microbial mat community produce different organosedimentary structures with different mineral composition depending of the physico-chemical environmental conditions.