Influence of bacteria on shell dissolution in dead gastropod larvae and adult Limacina helicina pteropods under ocean acidification conditions

Ocean acidification(OA) increases aragoniteshell dissolutionin calcifying marine organisms. It has been proposed that bacteria associated with molluscan shell surfaces in situ could damage the periostracumand reduce its protective function against shell dissolution. However, the influence of bacteria on shell dissolutionunder OA conditions is unknown. In this study, dissolutionin dead shells from gastropod larvae and adult pteropods ( Limacina helicina) was examined following a 5-day incubation under a range of aragonitesaturation states (Ωarag; values ranging from 0.5 to 1.8) both with and without antibiotics. Gastropod and pteropod specimens were collected from Puget Sound, Washington (48°33′19″N, 122°59′49″W and 47°41′11″N, 122°25′23″W, respectively), preserved, stored, and then treated in August 2015. Environmental scanningelectron microscopy (ESEM) was used to determine the severity and extent of dissolution, which was scored as mild, severe, or summed (mild + severe) dissolution. Shell dissolutionincreased with decreasing Ωarag. In gastropod larvae, there was a significant interaction between the effects of antibiotics and Ωarag on severe dissolution, indicating that microbes could mediate certain types of dissolutionamong shells under low Ωarag. In L. helicina, there were no significant interactions between the effects of antibiotics and Ωarag on dissolution. These findings suggest that bacteria may differentially influence the response of some groups of shelled planktonic gastropods to OA conditions. This is the first assessment of the microbial–chemical coupling of dissolutionin shells of either gastropod larvae or adult L. helicinaunder OA.