TRACKING THE NEOPROTEROZOIC RISE OF EUKARYOTES AND EMERGENCE OF METAZOA USING LIPID BIOMARKERS

Lipid biomarkers yield unique insights for deep time geobiology concerning ancient marine chemistry and biospheric evolution. The earliest record for animals (Metazoa) comes from a 100 million year record of distinctive C₃₀ sterane biomarkers (24-isopropylcholestanes, 24-ipc steranes) produced by demosponges as old as 713-635 Myr, found in thermally immature sedimentary rock bitumens and kerogens from Huqf Supergroup of the South Oman Salt Basin (Love et al., 2009). This constituted the first evidence for animals predating the Marinoan glaciation and is supported by a close match with metazoan divergence age estimates from molecular clocks (e.g. Sperling et al., 2010) and the recent finding of “sponge-grade metazoan” fossils from Cryogenian strata in South Australia (Maloof et al., 2010).

Here we present a status report on i) the phylogenetic distribution of the 24-ipc sterols in modern sponges and, ii) the temporal patterns of extractable and kerogen-bound ancient steranes in Proterozoic rocks. Currently, lipid biomarker evidence suggest that eukaryotes did not become abundant and widespread primary producers in the oceans until the early Neoproterozoic, but were prolific prior to the Sturtian glaciation (pre- 716 Ma).

While 24-isopropylcholesterol (and related structures) can only be biosynthesized by some demosponge genera, we find that 24-ipc compounds are major sterol constituents of certain genera from the order Halichondrida. Algae are not a likely source for Neoproterozoic 24-ipc steranes as explained previously (Love et al., 2009). A single cell genomics study (Siegl et al., 2011) which recently proposed that poribacterial symbionts could possibly synthesize 24-ipc sterols has subsequently been shown to have yielded a false result and that these poribacteria do not have the genetic capacity to make 24-ipc or indeed any sterols (Kamke et al., 2013).

References:

Kamke, J., Sczyrba, A., Ivanova, N., Schwientek, P., Rinke, C., Mavromatis, K., Woyke, T. and Hentschel, U. (2013) The ISME Journal 7, 2287-2300.

Love, G.D., Grosjean, E., Stalvies, et al. (2009) Nature 457, 718-722.

Maloof, AC, Rose, CV, Beach, R, et al. (2010) Nature Geosciences 3, 653-659.

Siegl, A, Kamke, J, Hochmuth, T, et al. (2011) The ISME Journal 5, 61-70.

Sperling, E, Robinson, JM, Pisani, D, Peterson, KJ (2010) Geobiology 8, 23-46.