INNER WORKINGS Inner Workings: Endoliths hunker down and survive in extreme environments Amber Dance Baas-Becking’sidea.Hehasfoundthatthe Science Writer Antarctic endolith populations resemble those intheMojave.Thereason,hesuspects,isthat the maximum temperature in the Antarctic Henry Sun works where other biologists fear (1) and later Antarctica (2). Endoliths have summer, about 5–10 °C, matches the mini- to tread, or maybe they just don’twantto enabledSunandotherstostudyhowmi- ’ mum in the Mojave winter and spring. bother. Antarctica s McMurdo Dry Valleys crobes travel the Earth (3) and even how they Therefore, the two environments have featuretemperaturesaslowasminus68°C, might exist on other planets. overlapping niches that similar endoliths can windsupto200milesperhour,andlessthan Endoliths could, theoretically, live in colonize. That does not necessarily mean that 10% relative humidity. Early polar explorer warmer or wetter climes; they grow just fine the South Pole and United States deserts Robert Scott dubbed them the “Valley of in Petri dishes at room temperature in Sun’s maintain a constant endolith exchange pro- the Dead.” However, they are the favorite laboratory. However, surface organisms typi- gram, Sun says. However, over evolutionary field site of Sun, a microbiologist at the Des- cally outcompete them, Sun says, limiting timescales, they and other deserts do share ert Research Institute in Las Vegas, Nevada. endoliths’ effective range to cold, dry places organisms, probably via dust storms that wear When Sun cracks open a piece of sand- where no other creatures can manage. “They away rocks and send the endoliths flying. stone or other porous stone, he looks for a live in a cold environment not because they Sun’s research relies on fieldwork, where layer of green or gray just below the surface. It’s ‘like’ it, but because they have no choice,” hiking and observing are key activities. He a telltale sign of a community of rock-dwelling Sun explains. Hence, his field work takes packs light on the way in, returning home ’ microbes called endoliths. These algae, cyano- him to places like Antarctica or the parched with lots of samples. Sun s Antarctic wander- bacteria, or lichens survive in the only place Mojave Desert. ings have led directly to discoveries, like the where moisture won’t immediately evaporate. Biologists have long debated the hypothe- time he overturned black basalt pebbles and As an energy source, endoliths capture light sis of Dutch microbiologist Lourens Baas- saw brownish, greenish scum, indicating liv- that filters through the outer layer of rock. Becking, who proposed in 1934 that micro- ing organisms in liquid water underneath. A Sun became fascinated with endoliths after organisms could reach any corner of the microscope revealed cyanobacteria swim- ming in the chilly water. Sun also found that meeting the late Imre Friedman, who first globe, but only reproduce when they found endoliths are not most prevalent in the discovered the organisms in the Israeli desert the right habitat (4). Sun’s work supports warmest, north-facing rocks, as was previ- ously believed. Instead, endoliths show up most frequently in rocks that are free of snow cover, presumably because the organisms need a clear path to sunlight (5). The study of endoliths may also help inform the search for microbial life on Mars, where the chilly, arid surface mimics that of the Dry Valleys. The Mars Science Labora- tory Curiosity rover is currently analyzing both loose soils and samples drilled out of rock for evidence that the planet might have been habitable at some point. Based on their findings with endoliths, Sun and other researchers suggest that the rock, rather than soil, may provide the best bet for evidence of Martian life.

1 Friedmann EI (1971) Light and scanning electron microscopy of the endolithic desert algal habitat. Phycologia 10(4):411–428. 2 Friedmann EI, Ocampo O (1976) Endolithic blue-green algae in the dry valleys: Primary producers in the Antarctic desert ecosystem. Science 193(4259):1247–1249. 3 Smith DJ, et al. (2013) Intercontinental dispersal of bacteria and archaea by transpacific winds. Appl Environ Microbiol 79(4):1134–1139. A fractured piece of sandstone from Antarctica shows a line of gray-green endoliths living 4 Baas-Becking LGM (1934) Geobiologie; Of Inleiding tot de Milieukunde (WP Van Stockum & Zoon, The Hague, The Netherlands). just below the top surface. The organisms remove iron from their area, so it accumulates as 5 Sun HJ (2013) Endolithic microbial life in extreme cold climate: the orange layer below. 1 bar = 1 cm. Image courtesy of Henry Sun. Snow is required, but perhaps less is more. Biology 2(2):693–701.

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