81-9 Conch in a Bucket: Diurnal Carbon Cycling by an Epilithic and Euendolithic Microbial Ecosystem

Abstract:

Diurnal variations in rates of photosynthesis and respiration of shallow marine benthic organisms like corals or seagrasses can drive substantial shifts in the carbonate chemistry and dissolved inorganic carbon (DIC) δ¹³C values over the course of a day. These diurnal patterns can influence the temporal distribution of calcification (both abiotic and biotic) and potentially bias carbonate mineral δ¹³C values relative to longer term average seawater DIC δ¹³C values. We wanted to know whether these same diurnal changes can be driven by mineral-hosted shallow marine microbial communities: epiliths (microbes that live on mineral surfaces) and euendoliths (microbes that live in cavities within minerals that they create via active dissolution). To answer this question, we designed a field experiment we called “Conch in a Bucket”. We conducted this experiment at the Smithsonian Carrie Bow Cay research station, which is part of the Mesoamerican Barrier Reef system in Belize. The intertidal and shallow subtidal zone adjacent to Carrie Bow Cay is characterized by many skeletal carbonates that are heavily colonized by euendolithic chlorophyte algae and cyanobacteria, in addition to epilithic algae. We selected a conch shell with a visually diverse epilithic and endolithic microbial community and placed the conch in a clear 5 L Cambro container filled with seawater. We tracked temperature, pH, dissolved oxygen, and photosynthetically-active radiation (PAR) continuously in situ and sampled for major ions, DIC (concentration and δ¹³C values), and alkalinity hourly for 24 hours. After the experiment was complete, we extracted the euendolithic microbial community from the shell using a gentle EDTA leach and characterized them by microscopy and 16S rRNA gene sequencing. To provide environmental context, we also collected a similar dataset from the adjacent seagrass meadow. Our data reveal substantial shifts in the carbonate chemistry of the water across a diel cycle driven by the microbial community on and in the conch shell. These dynamics looked significantly different than the dataset from the seagrass meadow, in which the signal of respiration was much stronger than the signal of photosynthesis. Our data demonstrate that shell-hosted epiliths and euendoliths can represent hotspots of productivity and have the potential to drive significant diurnal shifts in ambient seawater carbonate chemistry.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.