Phosphorus (P) is an essential and limiting macronutrient that regulates marine primary productivity and affects the cycles of carbon and other major elements. The largest pathway for marine P removal is the in situ (or authigenic) formation of stable calcium (Ca) phosphate minerals such as apatite. However, under marine conditions, apatite formation is often kinetically inhibited. Polyphosphate (polyP) has been proposed to potentially mediate authigenic apatite formation. PolyP can be widely synthesized by most microorganisms in aquatic environments and has important physiological roles. PolyP release to local marine environment can be a potential orthophosphate (orthoP) source to induce the nucleation of Ca-phosphate minerals. However, despite the widespread presence of polyP and its relevance to authigenic apatite formation in marine environment, biotic and abiotic factors controlling polyP transformation and mineralization are not well understood, and the underlying mechanisms are poorly constrained. Our recent studies investigated enzyme and metal oxide mineral catalyzed polyP hydrolysis and subsequent precipitation of Ca-phosphate minerals. Two common microbial enzymes (acid and alkaline phosphatase) and three common groups of metal oxide minerals (Fe, Mn, and Al oxides) were reacted with linear polyP with different chain lengths and under varied solution conditions. Wet chemistry analyses were combined with advanced spectroscopic and microscopic techniques to reveal the reaction kinetics and molecular level mechanisms. Linear polyP can be degraded to orthophosphate and with significant promotion effect in the presence of Ca2+. Mineral type and structure, particle size, dissolved metal cations, and solution pH strongly influence the hydrolysis rate and extent. Under alkaline pH conditions, polyP hydrolysis leads to the precipitation of ACP in the presence of Ca2+, and the fraction of ACP increases as the pH value increases. The newly formed ACP eventually transforms to crystalline hydroxyapatite upon long-term aging. This study provides new insights into the roles of minerals in the transformation of polyP and the subsequent precipitation of Ca-phosphate minerals in marine sedimentary environments. Considering the ubiquity of metal oxide minerals in sediments, this research reveals the role of oxide minerals in P mineralization and burial in oceanic environments.

Mineral catalyzed polyphosphate hydrolysis and precipitation of calcium phosphate minerals

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