Hopanoid Biomarkers

Hopanoids and their degradation products (hopanes) are among the most abundant molecular fossils in sedimentary rocks. The oldest uncontested records date to ~1.64 Ga; earlier reports (>2.5 Ga) have been challenged on the basis of potential contamination (French et al., 2015, PNAS). Interpreting this record requires understanding the sources, environmental distribution, and diagenetic fate of hopanoids in modern environments — a challenge that our biogeochemical surveys addressed directly.

BHP Distribution in the Modern Ocean

Suboxic/anoxic environments. We found that BHP abundance and structural diversity peak at suboxic and anoxic depths in the marine water column, based on surveys of the Arabian Sea OMZ, Peru Margin OMZ, and Cariaco Basin (Sáenz et al., 2011, Organic Geochemistry). This indicates that anaerobic bacteria are important sources of hopanoids in the marine water column.

River-to-ocean gradients. Terrigenous SPM (rivers, estuaries) contains ~10× higher BHP concentrations and far greater structural diversity than marine blue water SPM (Sáenz et al., 2011, Organic Geochemistry). This demonstrates that terrestrial organic matter is a significant, potentially dominant source of hopanoids in coastal marine sediments.

Diagnostic Markers

BHT II (suboxia/anoxia biomarker). An isomer of bacteriohopanetetrol consistently associated with suboxic and anoxic conditions in all three marine pelagic environments we surveyed. Absent from oxic and terrigenous settings, BHT II is a promising candidate for reconstructing past water column oxygen depletion (Sáenz et al., 2011, Organic Geochemistry).

Adenosylhopane (soil marker). Ubiquitous and abundant in soils; not detected in open ocean blue water samples, and its relative abundance decreases along the river-to-ocean transect, consistent with a terrigenous source (Sáenz et al., 2011, Organic Geochemistry).

2-Methylhopanoids (debated cyanobacterial marker). Traditionally interpreted as cyanobacterial biomarkers. However, we found 2-methylBHT in both terrigenous and marine coastal settings but absent from all marine cyanobacterial pure cultures surveyed — creating a paradox for their use as exclusive marine cyanobacterial markers (Sáenz et al., 2012, Geobiology; Sáenz et al., 2011, Organic Geochemistry).

Ladderane lipids (anammox marker). Though not hopanoids, ladderane lipids serve as unambiguous biomarkers for anaerobic ammonium-oxidizing (anammox) bacteria. We used both intact ladderane phospholipids (viable cells) and ladderane fatty acids (fossil biomass) to map anammox distribution in the Waquoit Bay subterranean estuary (Sáenz et al., 2012, Marine Chemistry).

Collaborative Environmental Surveys

Several collaborative studies extended the environmental biomarker picture:

• Pearson et al. (2009, Organic Geochemistry) surveyed BHP and sqhC gene diversity across a tropical land-sea gradient in San Salvador Island, Bahamas, revealing terrigenous dominance of marine hopanoid inputs.
• Bradley et al. (2010, Organic Geochemistry) identified adenosylhopane as the first biosynthetic intermediate in hopanoid side chain formation, using [[methylobacterium-extorquens|M. extorquens]] mutants.
• Wakeham et al. (2012, Deep-Sea Research) integrated lipid biomarkers with chemistry and microbiology at the Cariaco Basin chemocline.

Challenges in Interpretation

Our surveys revealed several complications for interpreting the sedimentary hopanoid record:

1. Mixed provenance: coastal marine sediments receive hopanoids from both marine and terrigenous sources, with distinct molecular fingerprints.
2. Depth-dependent production: marine BHPs are not primarily from surface waters — subsurface, suboxic/anoxic production is dominant.
3. Structural cosmopolitanism: most BHP side-chain structures are phylogenetically widespread and not diagnostic of specific organisms.
4. Low marine diversity: marine pelagic BHP structural diversity is much lower than terrigenous, consistent with lower phylogenetic diversity of hopanoid producers in the ocean.