Cyanobacterial exopolymer properties differentiate microbial carbonate fabrics

Abstract

Although environmental changes and evolution of life are potentially recorded via microbial carbonates, including laminated stromatolites and clotted thrombolites, factors controlling their fabric are still a matter of controversy. Herein, we report that the exopolymer properties of different cyanobacterial taxa primarily control the microbial carbonates fabrics in modern examples. This study shows that the calcite encrustation of filamentous Phormidium sp. secreting acidic exopolymers forms the laminated fabric of stromatolites, whereas the encrustation of coccoid Coelosphaeriopsis sp. secreting acidic exopolymers and poor calcification of filamentous Leptolyngbya sp. secreting non-acidic exopolymers form peloids and fenestral structures, respectively, i.e. the clotted fabric of thrombolites. Based on these findings, we suggest that the rise and decline of cyanobacteria possessing different exopolymer properties caused the expansion of thrombolites around the Proterozoic/Cambrian boundary.

Relative influence of biotic and abiotic processes on travertine fabrics, Satono‐yu hot spring, Japan

“…However, O. amphibia secreting an acidic EPS sheath is heavily calcified, while the other identified cyanobacteria do not calcify in the studied hot spring environments. This agrees with the recent suggestion from tufa deposits that the acidic and non‐acidic cyanobacterial EPS, respectively, induce and inhibit the CaCO 3 nucleation to control the calcification (Shiraishi et al ., ). Therefore, the observations at Satono‐yu demonstrate that EPS acidity is critical for determining the nucleation locus even at significantly high p CO 2 ( ca 22 to 28 matm), DIC concentration ( ca 35 to 38 mmol l −1 ) and saturation state ( ca 20‐fold to 34‐fold), even though such conditions are generally related to negligible microbial influence (Knoll et al ., ; Arp et al ., ; Riding, ).…”

“…Distribution patterns of acidic EPS, phototrophs and minerals at the travertine surface were observed via CLSM using lectin binding analyses (LBA). Fixed samples containing the surface part were treated for 20 min with 50 ng ll À1 of FITCconjugated lectin from Limulus polyphemus (Cosmo Bio, Tokyo, Japan), which has a binding specificity to acidic sugars found within cyanobacterial EPS (Shiraishi et al, 2017). Next, samples were rinsed with a washing buffer [88 mM NaCl, 20 mM Tris (pH 8Á0), 0Á01% (w/v) SDS (sodium dodecyl sulphate)] and submerged in distilled water.…”

“…Since cyanobacterial colonization at the travertine surface is inhibited under high flow velocity, the difference in the growth pattern may explain the dominance of P. laminosum at site 3. Indeed, cyanobacterial growth patterns appear to strongly control their adhesion to tufa deposits, much more than their EPS acidity (Shiraishi et al, 2017). The microbial composition changes described above affect the travertine fabric.…”

“…Although CO 2 degassing and a concomitant increase in the CaCO 3 saturation state is a fundamental driving force of travertine deposition, it cannot solely explain the difference between sparitic and micritic fabrics because the saturation state positively correlates with the rates of both crystal growth (leading to sparitic fabric) and crystal nucleation (leading to micritic fabric) (e.g. Stumm & Morgan, 1996;De Yoreo & Vekilov, 2003). Instead, the present study highlights the potential importance of microbial processes leading to the micritic fabric, which is suppressed under high flow velocity.…”

“…The key concepts underlying this model include: (i) the crystallization process comprises crystal nucleation and growth, and both the nucleation and growth rates are proportional to mineral supersaturation at the crystallization site (e.g. Stumm & Morgan, 1996;De Yoreo & Vekilov, 2003); (ii) the CaCO 3 mineral nucleation rate on cyanobacteria is also proportional to their EPS acidity, and acidic and non-acidic EPS are suitable and unsuitable for nucleation, respectively (Shiraishi et al, 2017); and (iii) the daily cycle of cyanobacterial migration forms the lamination, similar to other travertine sites (e.g. Takashima & Kano, 2008;Okumura et al, 2011Okumura et al, , 2012Okumura et al, , 2013a.…”