Deciphering the complexity of element incorporation into bivalve shells
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Abstract
Bivalve shells can function as extraordinary archives of environmental change. Unlike stable light isotopes, however, unravelling environmental signals from element impurities of bivalve shells remains an extremely challenging task. With the aim of disentangling the controls on the incorporation of trace and minor elements into bivalve shells, this study conducted a series of laboratory experiments and field studies.
Possible pathways by which trace and minor elements are transported from the ambient water to the calcifying front are delineated. Chapter 2 demonstrated that increasing aqueous Ca2+ levels from 3 to 6 mM did not facilitate shell growth of the freshwater bivalve, Corbicula fluminea. However, the amounts of Mn, Cu and Pb incorporated into the shells significantly decreased, indicating the potential competition with Ca2+ in the same transport pathways. Blocking the Ca2+-channels by lanthanum and Verapamil significantly reduced Mn, Cu, Zn and Pb incorporation into the shells, and Mn/Cashell and Cu/Cashell decreased when inhibiting the Ca2+-ATPase by ruthenium red. However, the amounts of Mg, Sr and Ba incorporated into the shells were virtually unaffected.
Building on the findings of chapter 2, specimens of C. fluminea were grown under different combinations of water chemistry, temperature and food availability. As shown in Chapter 3, the Sr/Cashell ratio of C. fluminea exhibited a negative correlation with temperature and a positive correlation to Sr/Cawater, but was not affected by changing food or shell growth rate. Ba/Cashell was influenced by an intertwined set of environmental factors, Ba/Cawater and shell growth rate. Sr/Cashell from freshwater can serve as a proxy for water temperature if the variability of strontium-to-calcium in the water is small. Interpreting Ba/Cashell values, however, is much more challenging because they are controlled by a large number of environmental and physiological variables. Sr/Cashell and Ba/Cashell of C. fluminea specimens from estuarine settings in which element-to-calcium ratios are more conservative can potentially serve as paleoenvironmental proxies.
Chapter 4 investigated the combined effects of seawater pH and temperature on the growth and sodium composition of the shells of Mytilus edulis and Patinopecten yessoensis. Exposure of M. edulis to low pH (7.7 and 7.4) caused a decrease of shell formation, whereas a 6 °C warming depressed the rate of shell growth in P. yessoensis. While the amount of Na incorporated into the shells of P. yessoensis did not increase in acidified seawater, an increase of Na/Cashell with decreasing pH was observed in M. edulis. Moreover, a combined analysis of the shell growth and sodium content provides a more detailed understanding of shell formation processes. Under acidified conditions, mussels may maintain more alkaline conditions favorable for calcification, but a significant decrease of shell formation indicates that the mineralization processes are impaired. The opposite occurs in scallops; unaffected shell growth implies that shell mineralization functions well.
Chapter 5 investigated the transgenerational epigenetic effect on the element composition of bivalve shells. Findings showed that reduced seawater pH projected for the end of this century led to a significant decrease of shell production of newly settled juvenile Manila clams, Ruditapes philippinarum. However, juveniles from parents exposed to low pH grew significantly faster than those from parents grown at ambient pH, exhibiting a rapid transgenerational acclimation to an acidic environment. Irrespective of parental exposure, the amount of Na incorporated into shells increased with decreasing pH, implying active removal of excessive protons through the Na+/H+ exchanger. However, the shells with a prior history of transgenerational exposure to low pH recorded significantly lower amounts of Na than those with no history of acidic exposure.
Chapter 6 presented the Mn/Cashell time series of the freshwater mussel, Hyriopsis cumingii (Lea, 1852) from a shallow eutrophic lake. Mn/Cashell time-series of studied shells exhibited a remarkable degree of synchrony. There is an inter-annual variability of Mn/Cashell records during 2011–2015. Mn/Cashell also displayed a pronounced intra-annual variability with maxima consistently occurring during the summer. The high reproducibility of Mn/Cashell time-series among contemporaneous specimens highlighted the existence of strong environmental rather than biological control on the incorporation of Mn into the shells. The striking feature of Mn/Cashell maxima occurred synchronously when reducing conditions beneath the sediment-water interface (SWI). The latter resulted in increases of Mn2+ in the sediment pore water and organic particles which were rapidly taken up by the mussels and incorporated into their shells. Therefore, Mn/Cashell can serve as a proxy of the mobility of Mn at the SWI.