Abundance and structural diversity of bacteriohopanepolyols in suspended particulate matter along a river to ocean transect

BHP abundance and structural diversity along a river to ocean transect

Sáenz, J. P.; Eglinton, T. I.; Summons, R. E. (2011) Organic Geochemistry, 42(7), 774–780. DOI: 10.1016/j.orggeochem.2011.05.006

Summary

We measured the concentration and structural diversity of Bacteriohopanepolyols (BHPs) in suspended particulate matter (SPM) along a river-to-ocean transect off the Pacific coast of Panama. Riverine SPM contained BHPs diagnostic of soil input and had 10-fold higher BHP concentration than marine SPM. The sharp contrast in structural diversity — 11 BHP structures at the river station versus only BHT in open ocean blue water — demonstrates that terrigenous organic matter exported to marine sediments could provide a significant, previously underappreciated contribution to the marine sedimentary hopanoid inventory.

Key Findings

• Total BHP concentration in riverine SPM (~1,111 ng/mg TLE at Station 1) was roughly 10× greater than in marine blue water SPM (~81–98 ng/mg TLE at Stations 5–6).
• Eleven BHP structures were detected at the river station; only BHT was detectable in blue water — a sharp drop in structural diversity across the transect.
• Adenosylhopane and its putative analogue (m/z 761), both considered diagnostic of soil-derived organic matter, were present at riverine through coastal stations but absent from blue water, confirming a terrigenous BHP source.
• Aminotetrol and aminopentol, associated with methanotrophic and sulfate-reducing bacteria, were detected only at the upriver station, suggesting a local sedimentary source.
• BHT concentration correlated linearly with salinity and TLE across the freshwater-to-saltwater transition (Stations 1–4), indicating conservative mixing of riverine and marine BHP pools.
• 2-MethylBHT was present at Stations 1–4, with highest relative abundance in coastal green water — raising the possibility that non-marine sources of 2-methylhopanoids contribute to the marine sedimentary record.
• If riverine particulate organic carbon constitutes even 10% of marine sediment organic matter, terrigenous BHPs would equal the abundance of marine-derived BHPs in coastal sediments.

Methods

• Six stations sampled along a transect from a river to open ocean near the Liquid Jungle Laboratory, Panama. SPM collected from near-surface water by peristaltic pump onto GFF filters (0.3 μm pore size).
• Lipid extraction by modified Bligh and Dyer; BHP analysis by HPLC-APCI/MSⁿ on Finnigan Surveyor LC / LTQ-MS.
• BHT and BHpentol quantification via external BHT standard; PD internal standard. TLE concentrations adjusted to ~10 mg/mL to ensure comparable detection limits.

Significance

We established that terrigenous organic matter is a major, quantitatively significant source of hopanoids to coastal marine sediments. The finding complicates interpretations of the marine sedimentary hopanoid record by demonstrating that BHP molecular fingerprints in coastal sediments reflect a mixture of marine and terrestrial sources. The presence of 2-methylhopanoids in riverine as well as coastal settings challenges their use as exclusive markers for marine cyanobacteria.