Vitamin B <sub>12</sub> -dependent biosynthesis ties amplified 2-methylhopanoid production during oceanic anoxic events to nitrification
Felix J. Elling, Jordon Hemingway, T. Evans, Jenan J. Kharbush, Eva Spieck, Roger E. Summons, Ann Pearson
Abstract
spp. cannot synthesize cobalamin, we postulate that they acquire it from organisms inhabiting a shared ecological niche-for example, ammonia-oxidizing archaea. We propose that during nutrient-rich conditions, cobalamin-based mutualism intensifies upper water column nitrification, thus promoting 2-methylhopanoid deposition. In contrast, anoxia underlying oligotrophic surface ocean conditions in restricted basins would prompt shoaling of anaerobic ammonium oxidation, leading to low observed 2-methylhopanoid abundances. The first scenario is consistent with hypotheses of enhanced nutrient loading during OAEs, while the second is consistent with the sedimentary record of Pliocene-Pleistocene Mediterranean sapropel events. We thus hypothesize that nitrogen cycling in the Pliocene-Pleistocene Mediterranean resembled modern, highly stratified basins, whereas no modern analog exists for OAEs.