Marine bivalve shell geochemistry and ultrastructure from modern low pH environments: environmental effect versus experimental bias

Abstract

Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels <i>Mytilus galloprovincialis</i> combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pH<sub>T</sub> 7.3 and mean pH<sub>T</sub> 8.1 near CO<sub>2</sub> vents on the east coast of the island. Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive

Ocean acidification impacts mussel control on biomineralisation

“…Crystallographic orientation pole figures confirmed changes in the spread of crystallographic orientations, suggesting further disorganisation of crystal structures, the degrees of spread were observed to be 20° for 380 µatm p CO 2 ambient (Figure 2 (a)), 40° for 550, 30° for 750 and 20° for 1000 µatm p CO 2 . Disturbed ultrastructure was also observed in Mytilus galloprovincialis 15 after transference to acidified seawater at pH 7.3 compared to ambient at pH 8.1. Increasing culture temperature by ambient plus 2°C resulted in calcite crystals with more structural disorientation only at 750 µatm and 1000 µatm where the average calcite growth angle was 60.9° and 78.8° respectively, compared to new growth calcite crystal angle of 28.7° at 380 µatm p CO 2 and ambient plus 2°C (Figure 3 (b), (d) and (f)) (Supplementary Table 2).…”

Historical baselines and the future of shell calcification for a foundation species in a changing ocean

“…Shell composition can also change with pH. The mussel M. galloprovincialis showed a decrease in the amount of aragonitic nacreous material in the shell as pH decreased in proximity to CO 2 -emitting vents [50]. Further, material properties of shells can be affected, with increased rspb.royalsocietypublishing.org Proc.…”

The effects of environment on <i>Arctica islandica</i> shell formation and architecture

“…So far, variations of the crystallographic properties of bivalve biominerals have been exclusively investigated as a response to hypercapnic (acidified) conditions. Mytilus galloprovincialis and Mytilus edulis showed a significant change in the orientation of the prisms forming shell calcitic layer when subjected to hypercapnia (Hahn et al, 2012;Fitzer et al, 2014a). Altered crystallographic organization may derive from the animal exposure to suboptimal conditions.…”

“…For instance, samples do not require any pre-treatment. Unlike EBSD, there is no need for preparing thin sections (∼ 150 µm thick) or etching the shell surface (Griesshaber et al, 2010;Hahn et al, 2012). Therefore, further structural and geochemical analyses can be easily performed on the same sections (Nehrke et al, 2012).…”