Origin of Sedimentary BHPs Along a Mississippi River–Gulf of Mexico Export Transect: Insights From Spatial and Density Distributions

Abstract

We investigated the origin of sedimentary bacteriohopanepolyols (BHPs) in low-density (<1.6 g cm −3), mesodensity (1.6-2.0 and 2.0-2.5 g cm −3) and high-density fractions (>2.5 g cm −3) as well as unfractionated bulk samples in a highly dynamic coastal setting along a transect spanning from the Mississippi Delta into the Gulf of Mexico (GOM). We observe selective partitioning of BHPs among density fractions both in total abundance and structural diversity. BHPs primarily accumulate in the low-density fraction at all sites. Good correlation with other particle properties [surface area-normalized organic carbon loadings (OC/SA), C/N ratios, 14 C values, and lignin phenol abundances] and the spatial distribution of absolute and relative BHP abundances suggests that a significant fraction of BHPs are terrestrially sourced and entrained in the OC pool that is stabilized on clay particles or associated with plant fragments. Only a small subset of BHPs seems to have a significant autochthonous origin at the station furthest offshore and they are associated with the low and mesodensity fractions. While provenance and hydrodynamic sorting of particles across the shelf seem to primarily determine BHP inventories along the transect, the samples also harbor an unusual diversity of aminofunctionalized BHPs. A subset of these BHPs appears to derive from aerobic methane oxidizing bacteria and are likely exported with particles from the coastal swamps in the Mississippi Delta. Other amino-functionalized BHPs seem to derive from aerobic ammonia oxidizing bacteria thriving in the nutrient-rich Mississippi River plume. We find no evidence for anaerobic ammonium oxidizing bacteria related to recurring seasonal hypoxia on the Louisiana Shelf.

Bacterial and archaeal lipids trace chemo(auto)trophy along the redoxcline in Vancouver Island fjords

“…Total lipids were extracted using Soxhlet extractors with methylene chloride:methanol (2:1, v:v). As previously noted by Schubotz et al., (2018), Soxhlet extraction rather than Bligh & Dyer was the only feasible way to extract doubled GF/F filters, and no obvious bias was evident for ether lipids (Schubotz et al., 2018) or BHPs (Kusch et al., 2019, 2021) using this technique. Total lipid extracts (TLEs) were stored at −20°C until analysis.…”

“…Prior to the detection of BHT‐x, BHT‐II ratios (BHT‐II/[BHT + BHT‐II]) were used to detect anammox rather than absolute BHT‐II concentrations since BHT‐II is also observed in oxic environments (Kusch et al., 2018, 2019; Matys et al., 2017; Sáenz et al., 2011). In Effingham Inlet and Deer Bay, the BHT isomer ratios (BHT isomer/[BHT + BHT isomer]) are significantly higher below the redoxcline (Figure 7b,c) and are within the range reported for water column SPM and core‐top sediments in the Arabian Sea by Sáenz et al., (2011).…”

“…The acetylated samples were filtered using 0.45 µm PTFE filters with methanol: propan‐2‐ol (3:2; v:v). BHPs were analyzed in the Organic Geochemistry Laboratory at the University of Colorado Boulder using a Thermo Ultimate 3000 HPLC – Q Exactive Focus Orbitrap MS system with APCI ion source operated using the chromatographic and MS conditions described by Kusch et al., (2019). BHPs were quantified against pregnane‐diacetate without correction for differences in response factor due to the absence of authentic BHP standards.…”

“…Brocadia spp.” enrichment cultures (Rush et al., 2014; Schwartz Narbonne et al., 2020) in addition to its δ 13 C‐depleted signatures in cultures and the environment (Hemingway et al., 2018; Lengger et al., 2019). However, trace amounts of BHT isomer have also been found in oxic marine environments (Kusch et al., 2018, 2019; Matys et al., 2017). This conundrum has now been resolved by a new GC method that revealed the presence of two BHT isomers (BHT‐II and BHT‐x) that otherwise co‐elute during standard HPLC analysis, of which BHT‐x is now considered to be the more robust anammox biomarker (Schwartz Narbonne et al., 2020).…”

A unique bacteriohopanetetrol stereoisomer of marine anammox

“…Matys et al, 2019;Talbot et al, 2003). A late-eluting BHT isomer has also been found in oxic or seasonally anoxic marine settings (Kusch et al, 2018(Kusch et al, , 2019Matys et al, 2019). In some of these cases, identification of the stereochemistry of the BHT isomer may reveal that it was not BHT-x, and thus not produced by 'Ca.…”