Abstract
The exposure to environmental variations in pH and temperature has proven impacts on benthic ectotherms calcifiers, as evidenced by tradeoffs between physiological processes. However, how these stressors affect structure and functionality of mollusk shells has received less attention. Episodic events of upwelling of deep cold and low pH waters are well documented in eastern boundary systems and may be stressful to mollusks, impairing both physiological and biomechanical performance. These events are projected to become more intense, and extensive in time with ongoing global warming. In this study, we evaluate the independent and interactive effects of temperature and pH on the biomineral and biomechanical properties of Argopecten purpuratus scallop shells. Total organic matter in the shell mineral increased under reduced pH (~ 7.7) and control conditions (pH ~ 8.0). The periostracum layer coating the outer shell surface showed increased protein content under low pH conditions but decreasing sulfate and polysaccharides content. Reduced pH negatively impacts shell density and increases the disorder in the orientation of calcite crystals. At elevated temperatures (18 °C), shell microhardness increased. Other biomechanical properties were not affected by pH/temperature treatments. Thus, under a reduction of 0.3 pH units and low temperature, the response of A. purpuratus was a tradeoff among organic compounds (biopolymer plasticity), density, and crystal organization (mineral plasticity) to maintain shell biomechanical performance, while increased temperature ameliorated the impacts on shell hardness. Biopolymer plasticity was associated with ecophysiological performance, indicating that, under the influence of natural fluctuations in pH and temperature, energetic constraints might be critical in modulating the long-term sustainability of this compensatory mechanism.
Highlights
The exposure to environmental variations in pH and temperature has proven impacts on benthic ectotherms calcifiers, as evidenced by tradeoffs between physiological processes
Low values of carbonate saturation state occurred at low pH scenarios, but all treatments showed saturated conditions for calcite (Ω > 1, Table 1), the calcium carbonate mineral polymorph precipitated by Argopecten purpuratus
Shell integrity is critical for the survival of marine calcifiers, and this study demonstrated that the scallop Argopecten purpuratus maintains shell biomechanical properties under low pH and low temperatures
Summary
The exposure to environmental variations in pH and temperature has proven impacts on benthic ectotherms calcifiers, as evidenced by tradeoffs between physiological processes. Still, reduced seawater pH can affect shell properties, inducing mineral dissolution and changing the mineral organization and mechanical p roperties[22,26,27,28,29,30] Both changes in pH and temperature affect a suite of physiological processes (e.g., ingestion and metabolic rates), which drives trade-offs that may alter shell calcification, structure, and functionality in mollusks. Mollusks may cope with low pH-induced dissolution by increasing protection with a thicker shell p eriostracum[24,32,33], adjusting the amount and composition of organic material occluded within the shell mineral[16,34,35], or altering shell mineral properties[22,27,28,29,33] These studies suggest that shell properties may be part of complex compensatory mechanisms to maintain overall performance and homeostasis when mollusks are exposed to low pH conditions. In the context of combined impacts of temperature and pH, it is still poorly understood how biomineral and biomechanical properties of mollusk shells participate in these physiological compensations
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