Differential effects of Ca2+ and Mn2+ on cyanobacterial mineralization and organic matter preservation

Abstract

Cyanobacteria serve as significant contributors to sedimentary organic matter, and their development and preservation play a role in the quality of hydrocarbon source rocks. While their role as primary producers and environmental impacts on productivity are well-documented, mechanistic effects of metal ions (Ca²⁺, Mn²⁺) on cyanobacterial growth, mineralization, and organic matter preservation remain poorly understood. This study used the filamentous cyanobacterium Leptolyngbya boryana to investigate these issues through controlled experiments in: Ca²⁺ systems (0.02–0.05 mol/L Ca²⁺), Mn²⁺ systems (0.002–0.005 mol/L Mn²⁺) and Ca²⁺-Mn²⁺ mixed systems. The results show that: exclusive calcite formation in Ca²⁺ systems, rhodochrosite in Mn²⁺ systems, Mn-calcite, rhodochrosite and kutnohorite in experimental mixed systems and Mn-calcite and kutnohorite in the control group. Specifically, biomass shows: stimulation at low concentrations but inhibition at high concentrations in Ca²⁺ systems, initial enhancement followed by suppression in Mn²⁺ systems, general inhibition in mixed systems. Batch 6 (Mn²⁺ system, 0.003 M) shows optimal growth with dry biomass of 88.9 mg, chlorophyll-a (13.37 mg/L), extracellular polymeric polysaccharide and protein contents increased by 2.41 and 3.19 times versus control. Organic matter in rhodochrosite aggregates exhibits maximal thermal decomposition loss (7.61 %), total organic carbon content (2.49 %) and hydrocarbon potential (S₂ = 9.16 mg/g). Effectively encapsulation of cyanobacterial organic matter in mineral aggregates, combined with initial productivity and sequestration patterns, enhances hydrocarbon potential. These findings elucidate critical roles of Mn²⁺ in cyanobacterial productivity, biomineralization, and organic matter preservation, advancing the efficiency of carbonate mineral coating in preserving labile organic matter.