Tropical Andean Ecosystems as Models for Extrasolar and Solar Planetary Habitats
Abstract
Surface and subsurface biota in the Andean high mountain are near the limits of photoautotrophic and chemolithotrophic life in tropical region. Therefore, the relationship between N2, O2, and CO2 partial pressures, nitrogen fixation at soils, chlorophyll content, biological productivity (/max), and surface and subsurface thermal variation at the Andean high mountain was evaluated to determine the main factors affecting the growth of organisms living in this region. Climatic data were obtained from 9 meteorological stations placed between 2,448 and 4,772 m at the Sierra Nevada de Mérida, Venezuela. These data were related to the metabolic activity (viz. ammonification, nitrification and CO2 assimilation) investigated in several vascular plants and in soil microorganisms. The obtained results suggest that the lower development of the plants correlated to the altitude could be a direct consequence of the low O2 and CO2 levels recorded in the higher altitudes, which reduce the photosynthetic activity, biological productivity, and the production of NO-3 at the soils. Hence, chlorophyll content was inversely correlated to the increment of the altitude (2.3 x 10(exp -2) mg Chl (a+b) g(exp -1) dry weight m(exp -1)). Even chemolithotrophic bacteria cannot carry out the fixation process by nitrification at the snow zone (> 4,700 m) because this process require aerobic conditions while the high concentrations of NH3found in the snow zone correspond to the anaerobic nature of bacterial ammonification. Extrapolating the obtained data to an exobiological context it is suggested that extrasolar or solar environments with a PO2 probably < 125 mbar should be excluded on the occurrence of complex photosynthesis-based life forms due to the aerobic feature of bacterial nitrification. Altitudinal distribution of mean NO-3 values revealed the existence of a "critical nitrogen-fixing zone" localized at timberline (∼ 3,250 m). This parameter could be utilized to define the critical nitrogen-fixing boundary in the habitable zone (HZ).
Minimum PCO2 suitable for the occurrence of C3-like advanced photosynthesis-based life forms in these bodies should have values ≥ 17 x 10(exp -2) mbar corresponding to a mean temperature ≥ 277 K and to a biological productivity rate Π/Πmax ∼ 0.27 whereas that minimum PCO2 suitable for the occurrence of C4-like advanced photosynthesis-based life forms in those environments should not exceed values ∼ 14 x 10-2 mbar. Further, a timberline-like critical photosynthetic boundary was defined for latitudes near 0 ° from Earth-like planets having a mean surface temperature of ∼ 11 °C, Π/Πmax ∼ 0.64, and a PCO2 of ∼ 21 x 10(exp -2) mbar. On the other hand, high concentrations of atmospheric N2 and NO-3 at soils from extrasolar or solar bodies placed in the HZ are not necessarily diagnostic features of life able to fix atmospheric N2 by ammonification and nitrification. The former process require of H2O whereas the later process require the presence of O2 and probably of a PO2 > 125 mbar to maintain complex photosynthesis-based life. However, reactions between N2 and H2O in the atmosphere surrounding lightning discharges can provide an important source of NH3 and NOx even when O2 is a minor atmospheric constituent, and therefore could allow the availability of fixed nitrogen for the evolution of life on Earth-like planets.- Publication:
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Origins of Life and Evolution of the Biosphere
- Pub Date:
- June 2003
- DOI:
- 10.1023/A:1023960415501
- Bibcode:
- 2003OLEB...32..503M
- Keywords:
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- Microbes;
- Biomes