Abstract
In the field of microbial biomineralization, much of the scientific attention is focused on processes carried out by prokaryotes, in particular bacteria, even though fungi are also known to be involved in biogeochemical cycles in numerous ways. They are traditionally recognized as key players in organic matter recycling, as nutrient suppliers via mineral weathering, as well as large producers of organic acids such as oxalic acid for instance, an activity leading to the genesis of various metal complexes such as metal-oxalate. Their implications in the transformation of various mineral and metallic compounds has been widely acknowledged during the last decade, however, currently, their contribution to the genesis of a common biomineral, calcite, needs to be more thoroughly documented. Calcite is observed in many ecosystems and plays an essential role in the biogeochemical cycles of both carbon (C) and calcium (Ca). It may be physicochemical or biogenic in origin and numerous organisms have been recognized to control or induce its biomineralization. While fungi have often been suspected of being involved in this process in terrestrial environments, only scarce information supports this hypothesis in natural settings. As a result, calcite biomineralization by microbes is still largely attributed to bacteria at present. However, in some terrestrial environments there are particular calcitic habits that have been described as being fungal in origin. In addition to this, several studies dealing with axenic cultures of fungi have demonstrated the ability of fungi to produce calcite. Examples of fungal biomineralization range from induced to organomineralization processes. More examples of calcite biomineralization related to direct fungal activity, or at least to their presence, have been described within the last decade. However, the peculiar mechanisms leading to calcite biomineralization by fungi remain incompletely understood and more research is necessary, posing new exciting questions linked to microbial biomineralization processes.
Highlights
Biomineralization is defined as the formation of a mineral as a result of biological activity [1]
Three levels of biological mediation over the formation of biominerals can be defined [2,3]: (i) biologically controlled mineralization (BCM), when crystal nucleation, growth and morphology are under the tight genomic control of an organism; (ii) biologically induced mineralization (BIM), when biological activity induces physicochemical changes in the environment resulting in mineral nucleation and growth, influencing mineral morphology; (iii) biologically influenced mineralization involving a biological matrix to initiate or enhance crystal nucleation and growth, with an influence on mineral morphology
Besides fungal metabolic activities that can influence alkalinity and Ca2+ concentrations potentially leading to induced CaCO3 biomineralization, fungi can participate through organomineralization
Summary
Biomineralization is defined as the formation of a mineral as a result of biological activity [1]. Three levels of biological mediation over the formation of biominerals can be defined [2,3]: (i) biologically controlled mineralization (BCM), when crystal nucleation, growth and morphology are under the tight genomic control of an organism; (ii) biologically induced mineralization (BIM), when biological activity induces physicochemical changes in the environment resulting in mineral nucleation and growth, influencing mineral morphology; (iii) biologically influenced mineralization (or organomineralization sensu stricto; [2]) involving a biological matrix to initiate or enhance crystal nucleation and growth, with an influence on mineral morphology In this last type of biomineralization, living organisms are not directly required and only the organic fraction matters as a template for nucleation. In an era of increased pressure to find sustainable solutions to our industrial way of life it has become more important than ever to utilize microbes as catalysts for some activities such as metal decontamination for example, and presses the need to more thoroughly characterize the diversity of biomineralization processes linked to microbes The aim of this contribution is to review the latest advances in the field of CaCO3 fungal biomineralization. The last part will be devoted to what the reasons might be for the lack of knowledge regarding the field of fungal biomineralization when compared to prokaryotic biomineralization
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