The Negev Desert Analogue
Journal of Astrobiology and Space Science Reviews, 1, 369-377, 2019
The Negev Desert Analogue
Giora J. Kidron
Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel
Mars, Lichens, Cyanobacteria, Photosynthesis, Respiration
Lichens, Cyanobacteria and Stromatolites on Mars?
The search for life on Mars has been extensive and many theories have been advanced. Yet, no proof for surface (preferentially photoautotrophic) life has yet been found (Michalski et al., 2017), and efforts were directed towards possible life in the subsurface, which may experience better growth conditions (Rothschild, 1990; Mumma et al., 2009). Joseph, Dass, Rizzo, Cantasano and Bianciardi (2019), using high resolution photographs, argue for a homology between specimens/structures on Earth and those on Mars, and advocate for the possible existence of life on the Red Planet. Although acknowledging that morphology is not proof of life, Joseph et al. (2019) have argued for the existence of photoautotrophs on Mars, thus specimens which are very similar to crustose lichens (Fig. 1), fruticose lichens (Fig. 2), stromatolites and sediments possibly constructed by cyanobacteria (Figs. 15, 16).
As will be argued below, any suggestions advocating the current existence of photoautotrophic organisms on Mars, assuming of course a similar physiology to that known from Earth, cannot be disconnected from the abiotic conditions required to support their growth.
Mars is a Harsh Environment
Mars provides an extreme hostile environment for organisms (see review by Rummel et al., 2014). With distance to sun being ~1.5 times that of Earth, radiation, and photosynthetically active radiation (PAR) are respectively ~30% and ~55% lowerer and temperatures are extremely cold (Cousins and Crawford, 2011). Annual average temperatures range between ⁓ -60 to -110ºC in accordance with latitude, and temperatures >0ºC are extremely seldom and short. Even temperatures of ~20ºC, which were shown to facilitate growth of some bacteria, are seldom reached (Rummel et al., 2014). Atmospheric pressure is low (2-3 orders of magnitude lower than in Earth), while UV radiation is very high (~1000 times higher than on Earth; Cockell and Raven, 2004). The atmosphere is dry, and total precipitable water (i.e., the total amount of water within a certain surface area across the atmosphere which will potentially condense on an identical area on the surface) is 0.02-0.03 mm (i.e., ~1000 times lower than on Earth). Water, which is presents as ice is mainly confined to both poles. With the partial pressure being lower than that required to stabilize liquid water (Marchant and Head, 2007), water ice will quickly sublime to the adjacent dry atmosphere (Jakosky and Phillips, 2001; Martinez et al., 2017). As will be argued below, these conditions are apparently too harsh to enable photoautotrophy on Mars.
Water, Photosynthesis, Life on Mars?
For the following account, the Negev will be taken as an analogue for comparison. Although a hot desert, which does not adequately reflect the extremely cold conditions on Mars, I believe that basic physiological principals are probably shared by the different types of organisms, whether cyanobacteria, green algae, or lichens. My short account will not discuss possible subsurface life, as was also mentioned by the Joseph et al. (2019) as possible evidence of life. It will focus on the possible ability of photoautotrophs, as known from Earth, to grow under the current abiotic conditions on Mars, and it will be principally centered on water availability required for growth by each group of organisms. My account will focus on the water thresholds required for each type of organism but not less importantly, on the apparent water duration required to yield long-term carbon (C) gain by photoautotropic organisms.
Prokaryotes, bacteria and cyanobacteria, require liquid water for growth (Lange et al., 1986). With a film of water decreasing exponentially with the decrease in temperatures (De Vera et al., 2014), and subsequently impeding solute diffusion or cell motility (Rummel et al., 2014), the thickness of water film is crucial for cell functioning. According to Lange et al. (1992), the threshold for liquid water and hence the minimum water thickness required for cyanobacterial growth is 0.1 mm. Yet, even under optimal conditions, and assuming condensation of all precipitable water, water thickness on Mars will only reach 0.06 mm (Fouchet et al., 2007), i.e., too low to facilitate cyanobacterial growth. Moreover, since respiration is already reached at slightly lower values than 0.1 mm, and the necessary metabolic activity may delay the onset of net photosynthesis for ≥1 h even once the threshold (whether of light or water) for photosynthesis is reached (Lange et al., 1992), the duration during which water ≥0.1 mm persists is apparently too short to facilitate a positive C budget. Even fogs, interpreted as water ice, will immediately sublime upon sunrise (Rummel et al., 2014), making the conditions for bacteria and cyanobacteria growth highly unlikely. Nevertheless, high relative humidity (RH) may potentially facilitate chlorolichens, i.e., lichens with green algae as photobionts (hereafter lichens). Following the 'Negev Experience', this however is also unlikely under the current Martian conditions.
The Negev Desert Anologue vs Life on Mars
The Negev Desert is characterized by an abundance of dew (200 daily dew events a year amounting to 33 mm). Average daily amounts of 0.1‒0.3 mm were measured across the Negev Highlands (Kidron, 1999), having precipitable water of cerca 25 mm (Tuller, 1968), i.e., three orders of magnitude higher than on Mars. In the moderately high altitudes of the Negev Highlands (~500 m above msl), the confinement of endolithic cyanobacteria (hereafter cyanobacteria) to south-facing rock outcrops was explained by the high surface temperatures of these outcrops that prevent vapor condensation during the night. Average non-rainfall water (NRW), as condensed on cloths directly attached to the south-facing rock surfaces, yielded 0.02-0.03 mm only (Kidron et al., 2014), substantially lower than the 0.1 mm threshold required for net photosynthesis of cyanobacteria (Lange et al., 1992). The threshold of net photosynthesis for lichens was however half as much, 0.05 mm only, which corresponded to 80% (20% thallus weight) relative humidity (Lange, 1969; Kappen et al., 1979). This was verified when the moisture content of the lichen thalli (Ramalina maciformis) necessary for net photosynthesis of the lichen (Lange, 1969), was converted to millimeters of NRW (Kidron et al., 2014). The data imply that NRW at the south-facing bedrocks would not facilitate net photosynthesis of lichens and hence lichen growth. With rain, dew and fog serving as water sources, and with NRW at the south-facing rock outcrops being below the necessary threshold for cyanobacteria and lichen growth, rain remained as the sole source of water for these habitats, implying that (a) these outcrops can be regarded as the most xeric habitats for photoautrophs withiih the Negev Highlands and (b) that cyanobacteria solely rely on rainwater for growth (Kidron et al., 2014).
This was evident following a comparison with cobble-dwelling communities, thus crustose (mainly endolithic) lichens, which cover cobbles that are scattered along the south-facing slopes. While practically absent at the south-facing rock outcrops, efficient radiative cooling of the cobbles result in vapor condensation, which facilitate in turn lichen growth. These endolithic lichens are accompanied by the more mesic epilithic lichens at the sun-shaded north-facing slopes, explained by the higher RH and longer availability of NRW at these slopes.
Net photosynthesis of chlorolichens takes also place at high relative humidity (Lange, 1969). Whether following vapor condensation, i.e., dew (which results in high net photosynthesis) or following only high relative humidity (which yields lower amounts), they both require a minimal time duration under daytime (light) conditions to compensate for the dark respiration during the preceding night. This was shown by Kappen et al. (1979) for the Negev lichens and also for lichens under temperate conditions (Csintalan et al., 2000). On Earth, a minimum of two hours of daytime photosynthesis was found necessary under the Earth's PAR in order to compensate for the C loss during the preceding dark respiration (Csintalan et al., 2000). Given the lower PAR, and the very high UV levels on Mars (which will require partial shielding, further reducing the PAR), and given the fact that compensation of the dark respiration takes longer under low PAR (Lange et al., 1992), longer daytime activity will be required on Mars in order to compensate for the nocturnal C loss.
Yet, all findings point at an extremely low relative humidity (<5%) during the day, with a medium relative humidity (up to 70%) during the night (Martinez et al., 2017). Although RH of 70% is required for lichen activation (respiration), 80% are required for net photosynthesis (Lange, 1969). Thus the threshold for net photosynthesis would not be reached. Moreover, even when performing photosynthesis, growth will be possible only if the annual net carbon budget will be positive, i.e., only when the total amount of C gain by photosynthesis will surpass the total amount of C loss during nocturnal respiration. A very steep decrease in RH that takes place with the onset of daylight on Mars, even within 1 h (reaching ~5% already at 10:00 am; Harri et al., 2014), will result in extremely short water availability, too short to allow for a long enough time to compensate for the nocturnal C loss. This is even more so for fruticose lichens.
In comparison to crustose lichens, which cover almost all calcareous rocks in the Negev Highlands, 3-7 cm-tall fruticose lichens (like Ramalina maciformis) are especially abundant at the high-altitude mountains (900-1000 m above msl), which receive the highest dew and fog amounts in the Negev (Kidron, 1999). At this location, high relative humidity supports the lush cover of the fruticose lichens (as also reflected by their abundance in other dewy and foggy deserts), which may therefore be considered as the most mesic lichens of the Negev. Requiring a relatively thick film of air with high relative humidity that persists for >2 h, the current findings exclude any possibility for such water availability on Mars.
ConclusionsThe current account focused on the current surface conditions on Mars. It does not discuss past environmental conditions and the current possible existence of subsurface life on Mars, which should be thoroughly explored and as for now, should be seriously considered (especially the possible occurrence of biologically-mediated methane). As for the surface, scarcity of high relative humidity and liquid water may not only be reflected in the low available amount of water, which is well below the threshold required for activation of the above-mentioned organisms, but also in extremely short duration of water availability. For the photoautotrophs, this will result in a negative carbon budget, excluding their current possible growth on Mars.
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