The influence of microbial mats on travertine precipitation in active hydrothermal systems (Central Italy)

The study of hydrothermal travertines contributes to the understanding of the interaction between physico-chemical processes and the role played by microbial mats and biofilms in influencing carbonate precipitation. Three active travertine sites were investigated in Central Italy to identify the types of carbonate precipitates and the associated microbial mats at varying physico-chemical parameters. Carbonate precipitated fabrics at the decimetre- to millimetre-scale and microbial mats vary with decreasing water temperature: a) at high temperature (55-44°C) calcite or aragonite crystals precipitate on microbial mats of sulphide oxidizing, sulphate reducing and anoxygenic phototrophic bacteria forming filamentous streamer fabrics, b) at intermediate temperature (44-40°C), rafts, coated gas bubbles and dendrites are associated with Spirulina cyanobacteria and other filamentous and rod-shaped cyanobacteria, c) low temperature (34-33°C) laminated crusts and oncoids in a terraced slope system are associated with diverse Oscillatoriales and Nostocales filamentous cyanobacteria, sparse Spirulina and diatoms. At the microscale, carbonate precipitates are similar in the three sites consisting of prismatic calcite (40-100 μm long, 20-40 μm wide) or acicular aragonite crystals organized in radial spherulites, overlying or embedded within biofilm EPS (Extracellular Polymeric Substances). Microsparite and sparite crystal size decreases with decreasing temperature and clotted peloidal micrite dominates at temperatures < 40°C, also encrusting filamentous microbes. Carbonates are associated with gypsum and Ca-phosphate crystals; EPS elemental composition is enriched in Si, Al, Mg, Ca, P, S and authigenic aluminium-silicates form aggregates on EPS. This study confirms that microbial communities in hydrothermal travertine settings vary as a function of temperature. Carbonate precipitate types at the microscale do not vary considerably, despite different microbial communities suggesting that travertine precipitation, driven by CO2 degassing, is influenced by biofilm EPS acting as template for crystal nucleation (EPS-mediated mineralization) and affecting the fabric types, independently from specific microbial metabolism.


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Travertines are terrestrial carbonates precipitated in hydrothermal settings (Pedley, 1990; without the contribution of microbial metabolism, mediated by organic compounds independently and become distally progressively calcified ( Figure   The maximum water temperature is 49.5°C from one orifice, while for the other two orifices 304 temperature is 46.1-46.5°C (Table 1). The values of pH are 6.5-6.6 at the vent and 7.5 nearly 24.5 m 305 from the orifices where temperature drops to 36.4°C. Alkalinity decreases from 31.7 meq/l to 21.6 306 meq/l 14 m along the channel. 307 At the rims of the vent orifices and in the first 9 m of the channel, white filamentous bundles 308 encrusted by carbonate form fans 1-2 cm in size ( Figure 3C-D) oriented with the water flow 309 direction. These carbonate-encrusted filaments can be observed also as fossil calcified streamer 310 deposits on the channel sides ( Figure 3E). Around the vent pool, travertines are coated by orange-311 pink microbial mat, while decimetre-size stagnant pools with a few millimetres water depth and the sides of the channel are covered by white, submillimetre-thick paper-thin rafts with underlying 313 adherent green microbial mats. From nearly 9 m from the vent, where temperature drops to 44-41°C 314 (Table 1), light green microbial mats drape the channel floor and are associated with millimetre-size 315 carbonate coated gas bubbles and rafts ( Figure 3E). At 20 to 25 m from the vent, the topographic 316 gradient increases and the light pink to green channel floor is characterized by centimetre-size 317 terraces and pools with coated gas bubbles and dendrites ( Figure 3F). The main vent and channel at 318 Bollore visited during the humid winter season (January) had a different appearance thriving with 319 green to brown/pink coloured microbial mats and abundant carbonate-encrusted filamentous 320 bundles ( Figure S2).

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A second vent is present at the Bollore mound ( Figure 3A and Figure   and Figure S3). The travertine slope apron consists of a waterfall (5 m high) and a terraced slope 332 system that distally merges with a river ( Figure 4A-B). Travertines precipitate along the channel, at 333 the waterfall and form the terraced slope (20 m long) with metre-scale sub-horizontal pools 334 separated by rounded rims and sub-vertical walls 0.1-1.5 m high ( Figure 4B). Water temperature is 335 nearly 33-33.8°C in the pools; pH values are 7.8-7.9 and alkalinity 9 meq/l ( Table 1). The channel 336 levee and the pool rims are coated by olive green millimetre-thick microbial mats ( Figure 4C). The 337 pool walls are draped by dark to light green filamentous mats encrusted by carbonate ( Figure 4D).
Areas of the terraced slope temporarily not flooded by thermal water are sites of vegetation growth, 339 mostly reeds, which are encrusted by carbonate at renewed flows. The pool floor includes 340 terrigenous mud to sand-size detrital sediment, carbonate coated plant fragments and millimetre-to 341 centimetre-size carbonate coated grains (oncoids; Figure 4E-F). The outer surface of these oncoids 342 appears irregular and pitted by green microbial mats ( Figure 4F). some filaments appear birefringent in crossed polarizers and are enriched in Si, Al, Ca, Mg, Na and K or Ca and S measured via EDX. Rare gypsum crystals were also identified as well as detrital 365 pyroxenes from volcanic rocks. 366 The carbonate crusts (20-100 µm thick) at the channel margin consist of microsparite/sparite 367 spherulites made of prismatic spindle-shaped crystals (10-50 µm long, 5-20 µm wide) that 368 precipitate within EPS, embedding prevalent filamentous, spiral-shaped microbes and rare rod-  Figure S5) from which filamentous microbes are emerging as they were entombed in the crystals.

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In the distal channel ( Figure 2F), the carbonate precipitates are similar to the proximal part 375 in terms of crystal shape, size and organization in radial spherulites with often micrite nuclei, 50-376 300 µm in diameter ( Figure 6D and Figure S5). Calcite spherulites precipitate within EPS  (diameters 0.2-1 µm), some spiral-shaped, and sparse, 1-2 µm size, rod-shaped microbes ( Figure   394 7C-D). Some calcite crystals are coated by a grumous organic film, which appears to be transitional 395 to and/or overlain by EPS and shows square-shaped moulds ( Figure 7C). This organic material    Figure 10A-B) and nuclei of peloids and organic matter, 5-20 µm in diameter. Spherulites can 443 occur embedded in clotted peloidal micrite or surrounded by erect filamentous microbes or 444 suspended in EPS. Filamentous microbes can depart radially from the calcite rosettes ( Figure 10A) 445 and emerge from hollows within the crystals as they were entombed during crystal growth ( Figure   446 10B and Figure S8, S9, S10). Precipitated carbonate occurs also in the form of micrite with 447 nanometre scale particles forming aggregates. Filamentous microbes with a diameter of 1 µm are 448 dominant, associated with thicker segmented filamentous forms (4-5 µm in diameter) with an outer 449 sheath ( Figure S8, S9, S10), spiral-shaped, segmented and chain-like filamentous microbes ( Figure   450 10C), rod-shaped microbes and pennate diatoms. Micritic carbonate precipitates encrust the 451 microbial filaments forming a carbonate tube coating them ( Figure 10D). In addition to calcite,  The carbon and oxygen stable isotope data of the three investigated travertine sites are 474 plotted in Figure 11 and summarised in Table S1. The three travertine localities plot in distinct

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There is no definite explanation for the formation of Ca-phosphate in the three investigated 563 sites. Ca-phosphate might be a fixation artefact because PBS (Phosphate Buffer Solution) was used.

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However, Ca-phosphate crystals are observed draped by EPS and microbes and might represent 565 authigenic precipitates ( Figure S5H and Figure S10C). Gypsum crystals might be related to the 566 abundant presence of H 2 S in thermal water that is oxidized to elemental sulphur and sulphate either 567 due to mixing with atmospheric oxygen or due to microbial activity (e.g., sulphide oxidizing             The following are available online at www.mdpi.com/xxx/s1, Figures S1-S12 , Table S1.                     with abundant carbonate-coated gas bubbles. The two images where taken during two following 1657 days: to notice that the two dead worms (black arrows) are not coated by carbonate in Figure S2D 1658 whereas are carbonate-coated in the image in Figure S2E