Inhabitation of bathyal hydrocarbon seeps by early-branching benthic foraminifera: Implications for Neo-Proterozoic ecosystem functioning

Little is known about mechanisms allowing agglutinated and thecate (i.e., organic-walled) foraminifera associated with extreme environments to survive. Here, we present cytological observations of organic-walled and agglutinated monothalamid and milioline tubothalamids collected from sulfur-oxidizing microbial mats of Gulf of Mexico hydrocarbon seeps. This is the first ultrastructural analysis of allogromids, a basal group of organic-walled thecate foraminifera, from seeps. We show most allogromids contained numerous ingested bacteria of various forms; organelle abundance and distribution varied among their morphotypes. Saccaminids (agglutinated monothalamids) had abundant phagocytosed bacteria, including putative methanotrophic and sulfur-oxidizing bacteria, indicating foraminiferal activity in seeps. A porcelaneous tubothalamid morphotype contained phagocytosed bacteria of one morphology, suggesting food selectivity. Increasing our knowledge of the cytology and ecology of these modern representatives of early-evolving foraminifera could help elucidate their evolutionary history. Thus, we augment understanding of extremophile foraminifera, adding to our burgeoning understanding of microeukaryote protists and microfossil adaptations.