Abstract
Abstract. Calcitic nanofibres are ubiquitous habits of secondary calcium carbonate (CaCO3) accumulations observed in calcareous vadose environments. Despite their widespread occurrence, the origin of these nanofeatures remains enigmatic. Three possible mechanisms fuel the debate: (i) purely physicochemical processes, (ii) mineralization of rod-shaped bacteria, and (iii) crystal precipitation on organic templates. Nanofibres can be either mineral (calcitic) or organic in nature. They are very often observed in association with needle fibre calcite (NFC), another typical secondary CaCO3 habit in terrestrial environments. This association has contributed to some confusion between both habits, however they are truly two distinct calcitic features and their recurrent association is likely to be an important fact to help understanding the origin of nanofibres. In this paper the different hypotheses that currently exist to explain the origin of calcitic nanofibres are critically reviewed. In addition to this, a new hypothesis for the origin of nanofibres is proposed based on the fact that current knowledge attributes a fungal origin to NFC. As this feature and nanofibres are recurrently observed together, a possible fungal origin for nanofibres which are associated with NFC is investigated. Sequential enzymatic digestion of the fungal cell wall of selected fungal species demonstrates that the fungal cell wall can be a source of organic nanofibres. The obtained organic nanofibres show a striking morphological resemblance when compared to their natural counterparts, emphasizing a fungal origin for part of the organic nanofibres observed in association with NFC. It is further hypothesized that these organic nanofibres may act as templates for calcite nucleation in a biologically influenced mineralization process, generating calcitic nanofibres. This highlights the possible involvement of fungi in CaCO3 biomineralization processes, a role still poorly documented. Moreover, on a global scale, the organomineralization of organic nanofibres into calcitic nanofibres might be an overlooked process deserving more attention to specify its impact on the biogeochemical cycles of both Ca and C.
Highlights
Calcitic nanofibres are features of secondary calcium carbonate (CaCO3) deposits commonly observed in various vadose environments
These features are almost always described in the presence of another typical feature of secondary CaCO3 deposits, needle fibre calcite (NFC), a monocrystalline habit thought to be fungal in origin (Callot et al, 1985a; Phillips and Self, 1987; Verrecchia and Verrecchia, 1994; Cailleau et al, 2009b; Bindschedler et al, 2012)
Regarding the three main theories currently discussed, i.e. physicochemical processes, biomineralization of rod-shaped nanosized bacteria, and nucleation mediated by organic templates, the latter seems to be the most realistic considering our present knowledge on the nature of calcitic nanofibres and the results discussed in the study presented in this paper
Summary
Calcitic nanofibres are features of secondary calcium carbonate (CaCO3) deposits commonly observed in various vadose environments (caves – as moonmilk, soils, aqueducts). Despite this ubiquity, the origin of calcitic nanofibres still remains unclear (Banerjee and Joshi, 2013) Three hypotheses for their formation have been proposed: (i) physicochemical processes (Jones and Ng, 1988; Jones and Khale, 1993; Borsato et al, 2000); (ii) biomineralization of rod-shaped nano-sized bacteria (Phillips and Self, 1987; Verrecchia and Verrecchia, 1994; Loisy et al, 1999; Gradzinski et al, 2012); and (iii) nucleation mediated by an organic template (Benzerara et al, 2003; Olszta et al, 2004; Cailleau et al, 2009a)
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