213-11 All Benthic Foraminifera Living in Anoxia Are Not Biologically Identical: Use of Different Metabolisms By Microscale Neighbors

Session: Cushman Symposium: Microfossils of Extremophiles: Living in the Danger Zone

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Abstract:

The occurrence of certain foraminifera in benthic settings seemingly lacking dissolved oxygen is an enigma because foraminifera have classically been considered aerobes. We preserved sediments and their fauna on the seafloor of the Santa Barbara Basin (SBB; California, USA) when bottom-water oxygen was undetectable, an extreme marine condition. A well-developed bacterial mat blanketed the sediment-seawater interface of the varved sediments collected at ~571 m water depth; concurrent light measurements indicated complete darkness. Pools of conspecifics were isolated from sediments, well cleaned, and extracted for their RNA to enable assessment of gene expression (i.e., metabolism) while the foraminifers were living in the euxinic (anoxic + sulfidic) porewaters of the SBB seafloor. We will present general physiologic and metabolic (sequence) results on two co-occurring species: the calcareous Nonionella stella and the monothalamid thecate “SBB saccamminid”. Both taxa perform oxygen-independent (anaerobic) energy metabolism and use inorganic pyrophosphate to augment mitochondrial ATP production. N. stella, which sequesters chloroplasts sourced from diatoms and uses peroxisomes and mitochondria, performs dark CO₂ fixation (chemomixotrophy), denitrification (type of anaerobic respiration), and additional unusual metabolic pathways, some new to foraminifera. The SBB saccamminid, which has bacterial endosymbionts instead of sequestered diatom chloroplasts, lacks peroxisomes, does not perform denitrification, and has modified mitochondrial-related organelles (MROs) that do not perform aerobic respiration. Instead, this species tolerates intracellularly produced oxygen (from reactive oxygen species [ROS] detoxification), a trait that allows them to survive even though they are not aerobic per se. From a geological / paleontological perspective, such observations on species-specific foraminiferal physiology and metabolism are crucial in interpreting past environmental conditions and biogeochemistry, processes that significantly impact the geological and paleontological record. Further, it is imperative to realize that all foraminifera do not have the same biological abilities; such variability clearly can impact geochemical proxies entrained in carbonate tests (shells). Funded by NASA 80NSSC21K0478 and the WHOI Investment in Science Program.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10425

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