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Modern hot spring fields, such as the volcanically heated examples around Rotorua, New Zealand, may seem inhospitable to living things, with the thermal fluids gushing from vents to outflow channels to ponds and marshes at temperatures ranging from boiling to tepid. But these waters actually contain an abundance of microbial life. Across this changing temperature gradient, the colours of each spring-fed outflow channel and pond shift from hues of yellows to oranges to greens and browns (see Figure 1). The colours are brightened by the UV-protectant pigments of the hot-spring microbes that are well-adapted to these ‘extreme environments.’ The ‘extremophile’ microbes occur as thin biofilms at the boiling vent source areas, and form thick mats in the more moderate temperature, shallow channels and pools in the middle and distal thermal discharge areas.


Figure 1: Hot spring discharge at Orakei Korako Geothermal Park, New Zealand. Steaming area is location of boiling vent; orange and brown colours indicate microbes (cyanobacteria) containing UV-protective pigments living in moderate (orange; ~40-55 ⁰C) to warm (brown; ~25-35 ⁰C) temperature spring waters. Dissolved silica in the hot spring waters will, upon cooling, deposit on any surface and build up a solid opaline silica deposit called sinter. The white areas are dried sinter that is not actively forming at present, owing to shifting of the thermal fluid discharge like a river channel would do.  The microbial mats flourish in the thermal fluids, and help build up the sinter “apron” deposit over time, as they become incorporated into the forming solid silica deposit.


Figure 2: Opaline silica precipitating from hot spring waters will coat any surface, including microbes. In this image, dark green-brown-orangish microbial filaments, in original upwards growth position and still containing their original pigments, are embedded in opaline silica (light grey to beige) that entombed the cyanobacteria in a fresh sinter deposit from the Tokaanu geothermal area, New Zealand. Note the white areas are empty space.

Those hot springs that discharge near-neutral to high-pH thermal fluids (pH 5-9+) at Earth’s surface contain dissolved silica that precipitates as a solid opaline deposit upon all surfaces bathed by the discharging thermal fluids, including microbial biofilms and mats (Figures 1 & 2).

Hot spring microbes aid in the growth of a type of sinter -- digitate, or "finger-like" structures -- which build up from very thin layers of silica and microbial filaments, to line spring pool margins and emerge from the floors of shallow outflow channels. In geological terms, these digitate sinters are stromatolites (Ruff and Farmer, 2016).


Figure 3: Digitate hydrothermal silica. Left: Digitate sinter containing microbial filaments (see Sriaporn et al., 2020) from Whangapaoa thermal spring, New Zealand.

Right: Digitate hydrothermal silica deposit, Columbia Hills, Mars (NASA/JPL/USGS).

Video 1: A CT scan of an opaline silica sinter nodule with fine-scale internal laminations defining a microstromatolite, from Mars Pool, Rotorua, New Zealand.
Video 2: A CT scan of an opaline silica sinter nodule with digitate branching structures, or microstromatolites from El Tatio, Chile. Note the fine-scale internal laminations and the boundary between a lower package of laminae and upper package of laminae.

The microbes help create the digitate sinter in Earth’s hot springs: occasionally, changes in physical environmental conditions (i.e. geology) will cause burial of digitate sinter in muddy sediment, or will encrust the growing digitate features with another very thin layer of opaline silica. The hot spring microbes respond to the shifting geological environment by growing upwards towards the sunlight they need for energy, building a new layer of filaments.

The shape of the digitate structures is influenced by the interaction of microbes with the geology of the area and its environmental conditions. In Rotorua’s varied hot springs, the digitate sinter shapes can range from thin and “needle-like”, to wide and “knobby”.


Figure 4: Layers of smooth silica alternating with layers of silica-encrusted microbial filaments, building up hot spring stromatolites. Champagne Pool, Waiotapu geothermal area (from Handley et al., 2008).


Figure 5: Life turning to stone -  the very beginning of the silicification process of microbial filaments is captured in a magnified image showing detail at the scale of

2 microns wide. Tiny spheres of opaline silica encrust the living microbial filaments sampled from a hot spring at Orakei Korako, New Zealand.

Studying modern hot springs allows scientists to peer into the past and gain a better understanding of the ancient hot springs where life on Earth - and perhaps on Mars - may have first formed. Locations such as these are called analogue environments, and provide planetary scientists with an opportunity to gain further insight into the off-world environments they study beyond the imaging and data returned by robotic missions.


Want to learn more?

You can dive into the science behind hot springs in these key papers:

Guido, D.M., Campbell, K.A., Foucher, F. and Westall, F., 2019. Life is everywhere in sinters: examples from Jurassic hot-spring environments of Argentine Patagonia. Geological Magazine, 156(9), pp.1631-1638.

Sriaporn, C., Campbell, K.A., Millan, M., Ruff, S.W., Van Kranendonk, M.J. and Handley, K.M., 2020. Stromatolitic digitate sinters form under wide‐ranging physicochemical conditions with diverse hot spring microbial communities. Geobiology, 18(5), pp.619-640.

Handley, K.M., Turner, S.J., Campbell, K.A. and Mountain, B.W., 2008. Silicifying biofilm exopolymers on a hot-spring microstromatolite: templating nanometer-thick laminae. Astrobiology, 8(4), pp.747-770.


Ruff, S.W. and Farmer, J.D., 2016. Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile. Nature communications, 7(1), p.13554.

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