Is acidification of samples for isotopic analysis of carbon and nitrogen necessary for shoreline marine species?

The acidification of samples for the simultaneous measurement of stable carbon (delta C-13) and nitrogen (delta N-15) isotopes represents an important methodological question still not clarified because the removal of calcium carbonate (CaCO3) from samples may affect the outcome of the stable-isotope analysis. We investigated the effect of acidification on samples of 25 different taxa of benthic marine organisms from a rocky shore, to determine whether acidification affects the isotopic values of delta C-13 and delta N-15. After washing with distilled water and drying, each sample was divided into two parts; one part was acidified by adding 1 M HCl with a pipette, until no more CO(2)was released, whereas the other part was retained as a non-acidified sample. Organisms with little CaCO(3)showed no difference in carbon isotopic signatures after acidification, from those in the non-acidified part. Some organisms with a high CaCO(3)content did show differences in carbon isotopic signatures after acidification. The nitrogen isotopic signature presented a significant difference; however, to what extent this difference is biologically important is discussed. Avoiding sampling parts containing a high concentration of carbonate is an efficient way to circumvent this problem. Acidification of samples as a pre-treatment for isotopic analysis is not useful for marine organisms without calcareous structures and is necessary only when the sampled parts contain significant amounts of carbonate.

Automated summary

The effects of acidification on marine organism samples in terms of their carbon and nitrogen stable isotopes were studied, showing that organisms with low CaCO3 content had no difference in carbon isotopic signatures after acidification, while those with high CaCO3 content did show differences.