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Calculating 18O enrichment
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Archaea dominate oxic subseafloor communities over multimillion-year time scales

Procedure

Excess atom % 18O enrichment was calculated for each taxon according to the equations for quantifying per OTU atomic enrichment from Tag-SIP data provided by Coskun et al. (60). The atom fraction excess of 18O for each OTU (AOXYGENi) accounts for the background fractional abundance of 18O (0.002000429) using the following formula$AOXYGENi=MLABi−MLIGHTiMHEAVYMAXi−MLIGHTi⋅(1−0.002000429)$where MLABi is the molecular weight of the 16S gene for taxon i in the labeled treatment, MLIGHTi is the molecular weight of the 16S gene for taxon i in the unlabeled treatment, and MHEAVYMAXi is the theoretical maximum molecular weight of a fully labeled 16S gene with 18O. The lowest detection limit in the density shifts were proposed as 0.0034 to 0.0042 g/ml (65), and we selected a conservative lowest limit of 0.005 g/ml for a significant shift. Therefore, throughout this study, we referred to OTUs meeting this criterion as those having incorporated the given substrate.

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