213 Cushman Symposium: Microfossils of Extremophiles: Living in the Danger Zone
This session will assemble studies documenting microfossil presence and/or microfossil-generating taxa in extreme environments, widely defined present and past, and those elucidating adaptations employed by extremophiles, to inform interpretations of the geologic record.
Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 303C
Benthic foraminiferal community structure in submerged cave systems of the Yucatan Peninsula and Cozumel Island, Mexico
Paper number: 213-9
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM
Authors
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Joan BernhardWoods Hole Oceanographic Institution, Woods Hole, MA
Abstract
Karst subterranean estuaries (KSEs) are zones within coastal aquifers where meteoric water and seawater interact, resulting in steep physicochemical gradients in salinity, oxygen and organic matter availability. These systems can be considered extreme environments due to these sharp gradients, with reduced organic matter input and lack of light past the cave entrance, and oxygen concentrations that can range from fully oxygenated to hypoxic or anoxic. Foraminifera are a diverse group of microbial eukaryotes with a well-documented fossil record, playing important roles in benthic ecosystems. They have long been used as paleoenvironmental indicators in marine environments, including coastal and anchialine systems. While the utility of fossil and subfossil foraminiferal assemblages from karst subterranean estuaries (KSEs) and submarine caves is established, fewer studies have examined the composition of living foraminifera in these systems, which is a critical step toward establishing their ecological roles in these systems and, thus, their micropaleontologic usefulness.
Here, we examined the species composition and environmental drivers of benthic foraminiferal assemblages, integrating traditional morphological approaches with molecular methods. Metabarcoding (i.e., sequencing of environmental DNA) results from four KSEs in the Yucatan Peninsula and Cozumel Island, Mexico will be presented, with further comparison using traditional picking methods in two contrasting caves: the hypoxic to anoxic Cenote Crustacea, and the oxygenated Cenote El Aerolito. In the picked samples, CellHuntTM Orange CMTMR (Setareh Biotech) and Rose Bengal were used to identify metabolically active and recently living individuals, respectively, allowing for a comparison of the living, recently living, and total assemblages across environmental gradients. Living (fluorescently-labelled) foraminifera were identified in every sample collected from the hypoxic to anoxic cave (Cenote Crustacea) and the well-oxygenated cave (Cenote El Aerolito). Differences in taxonomic composition and densities were observed between the two systems, with the assemblages recovered from the low-oxygen to anoxic KSE composed primarily of agglutinated taxa. Metabarcoding data from four KSEs (including Cenotes Crustacea and El Aerolito) revealed limited taxonomic resolution, highlighting gaps in existing molecular-reference databases for foraminifera. These findings contribute to a broader understanding of benthic ecosystem function in KSEs and provide further insights into the environmental tolerances and distribution patterns of foraminifera inhabiting physically and chemically challenging, or extreme, habitats. Funded by NSF OCE 2136377 and NSF OCE 2136322.
Geological Society of America Abstracts with Programs. Vol. 57, No. 6
doi: 10.1130/abs/2025AM-413331
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