MANGANESE FIBERS, TODOROKITE, AND THE MN-OXIDIZER CONNECTION

Stringy and fabric-like manganese deposits occur in Lechuguilla Cave, NM. Large amounts of this fluffy material fall from ceilings in some passages at a noticeable rate. Extremely low concentrations of organic carbon are available to support oligoheterotrophs. Organic carbon in bedrock (0.002--0.103%) and in decemented rock (0.006--0.014) is probably from original deposition. Much higher carbon values (0.05--1.63%) occur in the low-density manganese material, possibly the result of microbial chemoautotrophy.

SEM images reveal felted and filamentous textures. XRD analysis shows a predominance of todorokite, a hydrated manganese mineral, and the presence of iron oxides. Other unusual minerals are also associated with the todorokite (svanbergite (Sr-Al-sulfate-phosphate), gibbsite (Al-oxyhydroxide), and rare-earth phosphates). Manganese-oxidizing organisms and putative iron-oxidizing organisms have been isolated on selective enrichment media. XRD of some manganese-oxidizing isolates (months old) shows identifable todorokite lines. Analysis of younger cultures (weeks old) from the same material has shown only amorphous oxidized manganese. Initial oxidation of reduced manganese, provided to the bacteria in growth media, yields amorphous oxides. None are observed in sterile controls. Subsequently, crystallization occurs yielding todorokite, the predominant manganese mineral in the stringy manganese deposits. We tentatively conclude that this represents the progression of mineralization in culture, analogous to that occurring naturally in the cave. The transformation from amorphous oxides to todorokite could be 1) actively precipitated by the organisms, 2) a passive result of their presence (e.g. as a nucleation agent for crystallization), or 3) irrelevant to the process. On-going experiments to pinpoint the time course of development of todorokite in culture are underway coupled with TEM and metabolic uptake studies in situ.

Previously, microorganisms have been identified as responsible for much of the oxidized Mn observed in the terrestrial environment by accelerating the rate of Mn(II) oxidation. Instances of significant subsurface manganese oxidation may add to the global impact of microbial manganese oxidation.