Hematite Crystallization in the Presence of Organic Matter: Impact on Crystal Properties and Bacterial Dissolution

Microbial dissimilatory iron reduction (DIR) is a widespread process in oxygen-poor sediments and waters, where it regulates Fe and C cycling and contributes to nutrient and trace metal distribution. This process is fairly well-studied in terms of iron substrate, microbial DIR strain, and water chemistry. However, far less is known about DIR rates and yields of iron substrates that are tightly associated with organic matter, even though most iron substrates that form in nature are partially or completely covered by organic matter. Here, we assessed the impact of alginate on hematite crystallization and subsequently assessed the stability of thus formed alginate–hematite precipitates in DIR experiments with Shewanella oneidensis MR-1. We found that during hematite synthesis via forced hydrolysis, the presence of alginate reduces hematite crystal and particle sizes and induces the formation of closely associated alginate–hematite composites. This is explained by alginate molecules acting as nucleation sites. Upon exposure of these composites to S. oneidensis MR-1, a ∼30–50% decrease (dependent upon the alginate content of composites) in the initial hematite reduction rate is observed in comparison to alginate-free, pure hematite. However, while DIR rates ceased after 48 h in the pure hematite system, reduction steadily progressed in the presence of the alginate–hematite composites, overall leading to a slightly higher DIR yield after 8 days. These trends are explained by alginate physically hindering direct contact between bacterial cells and hematite surfaces, thus lowering initial DIR rates. In turn, this lower rate potentially reduces quick passivation of cell and/or mineral surfaces by Fe(II) adsorption and/or surface precipitates as observed in the pure hematite system, thus enabling prolonged DIR reaction in the presence of alginate. Overall, this study highlights that a common natural organic molecule, such as alginate, can largely impact hematite formation in natural settings, leading to composites that show very different stabilities toward DIR compared to pure hematite. These are important considerations for predicting DIR processes and any associated element cycles in natural settings as well as for potential use of DIR for biotechnical applications