81-8 Geochemical Controls on Euendolithic Microboring by Green Alga, Phaeophila dendroides

Abstract:

Euendolithic microorganisms actively dissolve carbonate substrates, producing microborings that they inhabit. These organisms are commonly found in shallow, marine, CaCO₃-supersaturated seawater, yet they dissolve CaCO₃, working against the thermodynamic tendency toward precipitation. The persistence of euendolithic boring under these conditions implies there must be ecological or physiological benefits to this lifestyle, but these benefits remain poorly constrained. We tested whether euendoliths benefit from this behavior through protection from excess light, availability of carbonate-derived inorganic carbon, and access to substrate-hosted micronutrients, namely Fe, Mn, and Zn. Additionally, we predicted windows during the diel cycle in which carbonate chemistry is more favorable for dissolution. We use the green alga Phaeophila dendroides, enriched from ooid sand collected from the Turks and Caicos Islands. By measuring pH, alkalinity, dissolved inorganic carbon (DIC) concentration, δ¹³C_DIC values, and major ion concentrations, we constrained the carbonate system across a light-availability gradient, a gradient of CaCO₃ saturation state (Ω), a suite of aragonite and calcite substrates containing variable concentrations of trace micronutrients, and sampled at time points across the diel cycle. All full-light treatments consumed DIC and alkalinity to low concentrations, consistent with net photosynthetic carbon uptake coupled with carbonate mineral formation. This pattern occurred across both the Ω_arag gradient and the suite of CaCO₃ substrates, whereas treatments under opaque conditions showed evidence of net dissolution, evident in increases in alkalinity, DIC concentration, and δ¹³C_DIC values. The diel time series provided context for this observation: pH, Ca²⁺ concentration, δ¹³C_DIC values, DIC concentration, alkalinity, and Ω_arag values trace a hysteresis across the light–dark cycle. Nighttime increases in alkalinity and δ¹³C_DIC values were consistent with carbonate dissolution, but this signal was partly overprinted by the much larger effects of daytime photosynthesis and carbonate mineral precipitation. Daytime increases in Ca²⁺ concentrations may reflect delayed release of intracellular Ca²⁺ pools resulting from nighttime dissolution. These results provide preliminary evidence of temporal separation of CaCO₃ dissolution from photosynthetic activity in an euendolithic alga.

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