Abstract
Negative impacts of global climate change are predicted for a range of taxa. Projections predict marked increases in sea surface temperatures and ocean acidification (OA), arguably placing calcifying organisms at most risk. While detrimental impacts of environmental change on the growth and ultrastructure of bivalve mollusc shells have been shown, rapid and diel fluctuations in pH typical of coastal systems are often not considered. Mytilus edulis, an economically important marine calcifier vulnerable to climate change, were exposed to current and future ocean acidification (380 ppm and 1000 ppm pCO2), warming (17°C; 20°C), and ocean acidification and warming (OAW) scenarios in a seawater system incorporating natural fluctuations in pH. Both macroscopic morphometrics (length, width, height, volume) and microscopic changes in the crystalline structure of shells (ultrastructure) using electron backscatter diffraction (EBSD) were measured over time. Increases in seawater temperature and OAW scenarios led to increased and decreased shell growth respectively and on marginal changes in cavity volumes. Shell crystal matrices became disordered shifting toward preferred alignment under elevated temperatures indicating restricted growth, whereas Mytilus grown under OAW scenarios maintained single crystal fabrics suggesting OA may ameliorate some of the negative consequences of temperature increases. However, both elevated temperature and OAW led to significant increases in crystal size (grain area and diameter) and misorientation frequencies, suggesting a propensity toward increased shell brittleness. Results suggest adult Mytilus may become more susceptible to biological determinants of survival in the future, altering ecosystem structure and functioning.
Highlights
Over the last century, atmospheric concentrations of CO2 have increased at an unprecedented rate resulting in changing environmental conditions on both land and sea
Grain areas and diameters were smallest in the control treatment (17◦C × 380 ppm), largest in the elevated temperature treatment (20◦C × 380 ppm), and of intermediate size in the ocean acidification and warming (OAW) (20◦C × 1000 ppm) treatment
While median grain area was lower in the OAW scenario than the temperature only treatment, the maximum grain area observed in mussels from the OAW treatment was up to 7.8 × larger (12,887 μm2 vs. 1680 μm2) and 16.5 × larger than the temperature only (20◦C × 380 ppm) and control treatments, respectively (Figure 4)
Summary
Atmospheric concentrations of CO2 have increased at an unprecedented rate resulting in changing environmental conditions on both land and sea. The shell often consists of two polymorphs of calcium carbonate: the outer prismatic layer comprising calcite crystals, and the inner nacre layer comprising aragonite crystal tablets (Gazeau et al, 2013) Both polymorphs are formed within the extrapallial space following the catalytic conversion of CO2 to bicarbonate by the enzyme carbonic anhydrase (Marin and Luquet, 2004). This catalysis is a crucial process for calcium carbonate crystal nucleation, growth and orientation (Nakahara, 1991; Choi and Kim, 2000; Olson et al, 2013). It is argued that these changes will have negative consequences for shell integrity and material properties (e.g., Beniash et al, 2010; Fitzer et al, 2015b), impacting the potential for mollusks to withstand physical and biological stress (Sadler et al, 2018)
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