Revisiting "You are what you eat, +1‰": Bacterial Trophic Structure and the Sedimentary Record

Abstract:

“You are what you eat, +1‰” is a central principle of carbon stable isotope (δ¹³C) distributions and is widely applied to understand the structure and ordering of macrobiotic ecosystems. Although based on observations from multicellular organisms that are able to ingest “food”, this idea also has been applied to Precambrian ecosystems dominated by unicellular, microbial life, with the suggestion that such systems could sustain ordered trophic structures observable in their isotopes. However, using a new approach to community profiling known as protein stable isotope fingerprinting (P-SIF), we find that the carbon isotope ratios of whole proteins separated from environmental samples show differences only between metabolically-distinct autotrophs; heterotrophs are not ¹³C-enriched. In parallel, a survey of the relative distribution of ¹³C between biochemical classes – specifically acetogenic lipids, isoprenoid lipids, amino acids, and nucleic acids/sugars – across a variety of bacterial species appears to be a function of the main carbon metabolite, not an indicator of heterotrophy vs. autotrophy. Indeed, autotrophy, heterotrophy, and mixotrophy all are indistinguishable when the primary food source is fresh photosynthate, i.e., sugar. Significant assimilation of acetate is diagnosed by acetogenic lipids that are relatively ¹³C-enriched vs. isoprenoid lipids. Mixed-substrate heterotrophy, in contrast, satisfies the classic “…+1‰” rule for bulk biomass, yet simultaneously it collapses the biochemical patterns of ¹³C almost completely. Together these observations point to a paradigm shift for understanding the preservation of bulk organic and lipid δ¹³C signatures in the rock record, suggesting that patterns of δ¹³C_org must primarily reflect changing carbon inputs, not the extent or intensity of heterotrophy.