Magnetofossils as tracers of oxygenation change: a case study from the stratified Pettaquamscutt River Estuary

Magnetotactic bacteria (MB) are motile organisms commonly found around the oxic-anoxic-interface (OAI) in sediments and stratified water columns. Magnetite and greigite crystals synthesized by MB intracellularly, termed magnetosomes, can be preserved in sediments as magnetofossils. Changes in OAI thickness (due to changes in temperature, clathrate dissociation & methane oxidation, organic carbon supply/oxidation, or sedimentation rate) would produce proportional changes in MB population and sedimentary magnetofossil concentration. While potentially useful as an oxygenation proxy, magnetofossil quantification techniques and variables controlling their preservation in sediments need to be better understood. Most prior work focused on cultured magnetite-MB and sediment mixtures while studies of greigite-MB (found just below the OAI in the sulfidic hypolimnion) is lacking because axenic cultures do not exist. To address these issues, we study wild magnetite- and greigite-MB from the seasonally stratified Pettaquamscutt River Estuary Upper Basin (RI, USA) as a function of water depth, d. Transmission electron microscope imaging of 21 MB (377 magnetosomes) revealed a complexity in wild MB not found in cultures. From d=3.9 m-7.0 m, live-cell assays confirmed the presence of multiple MB morphotypes, both north- (majority) and south-seeking (minority), and a few magnetic protists. Based on a previous microscopy study just 1.4 km south of Upper Basin (Bazylinski et al., 1995), magnetite-MB are expected for d<5.0 m, mix magnetite- and greigite-MB for 5.0 m<d<6.0 m, and greigite-MB for d>6.0 m. Coercivity distributions for all depths are characterized by a small variance, reflecting uniformity in magnetosome size. Interestingly, despite changing from dominant magnetite to greigite-MB with increasing depth, the median coercivity remained largely unchanged. Median coercivity is therefore not diagnostic of magnetosome mineralogy. We also report ferromagnetic resonance spectroscopy (FMR) results. The first derivative of the absorption spectra for d<5.6 m typically present multiple low-field maxima, which is consistent with observations from magnetite-MB cultures. In contrast, only one maximum in the spectra was observed for 6.0 m<d<7.0 m (where greigite-MB dominates). While this raises the possibility of using FMR to differentiate between greigite and magnetite magnetosomes, abiogenic samples with a similar FMR specturm have also been observed. Additionally, in contrast with the sharply defined FMR parameters measured from magnetite-MB cultures, a wide range of parameters was found for the water column samples (asymmetry=0.8-1.3, alpha=0.27-0.34, geff=1.93-2.18, and linewidth=93-152 mT). Caution should therefore be taken when using FMR for magnetosome detection.