Culture studies of the benthic foraminifera Elphidium williamsoni: Evaluating pH, ∆[CO 32−] and inter-individual effects on test Mg/Ca

Abstract

We cultured the benthic foraminifera, Elphidium williamsoni, in a physicochemically stable, large volume (400 l) recirculating seawater system over a range of pH (7.6, 8.1 and 8.3) for 8 weeks. During this period the foraminifera typically deposited 1–3 new chambers which were analysed by electron probe microanalysis. Intertest Mg/Ca variations are large (± 22%, 1 standard deviation, at ambient pH) and in good agreement with variance reports from other benthic foraminifera species. These variations do not reflect changes in ambient water chemistry but must reflect biological variability between individual foraminifera, e.g. dependent on genotype or phenotype. Variations in test chemistry may reflect variability in the composition of the calcifying fluid used for test construction, variability in the amount of Mg incorporated into the test calcite or variations in the relative proportions of high and low Mg calcites incorporated in each test. Within each of the pH treatments we observed no relationship between amount of calcite deposited in the culture system and test Mg/Ca. Foraminifera test Mg/Ca is a commonly used palaeothermometer. Our study shows that variations in the relative proportions of individual foraminifera of different geochemistry, that make up a sample for chemical analysis, may affect bulk determinations of test chemistry and subsequent estimates of past climate. Foraminifera cultured at low pH produced significantly thinner chamber walls but we observed no relationship between culture pH (or seawater ∆[CO 3 2−]) and test Mg/Ca. Pore water gradients are unlikely to develop in the culture substrate and E. williamsoni is an epibenthic species so it is doubtful that this observation reflects changes in the ambient chemistry of pore waters. This suggests that a correlation between ambient water ∆[CO 3 2−] and test Mg/Ca is not ubiquitous in all calcitic foraminifera species. E. williamsoni is an intertidal species and is adapted to living in highly variable physicochemical conditions. Further work is required to ascertain the significance of these results to other benthic foraminifera species.