Effects of ocean acidification on radionuclide bioaccumulation in marine invertebrates

Atmospheric CO2 has been increased dramatically as a result of human activities over last two centuries. CO2 is absorbed by surface ocean and causes elevated pCO2 (a decrease in seawater pH level) and changes carbonate chemistry, which refer to "Ocean Acidification". Changing acidity and carbonate chemistry affect marine organisms, especially those with calcium carbonate structures and skeletons. Calcium carbonate shell of the marine calcifiers suffers from high pCO2 in seawater due to the lowered carbonate ion in seawater. The seawater pH has already dropped (8,05-8,10) since industrial revolution and is expected to reduce 7,8 at the end of this century and 7,5 by 2300 according to pessimistic atmospheric CO2 release scenarios. In this study the biokinetics of metal / radionuclide (uptake and depuration) in bivalves and shrimp was studied by using high sensitive radiotracer method under three different pH values (8,1: current sea water pH, 7,8: the predicted pH value for the year 2100, 7,5: the predicted pH value for the year 2300). Aquarium pH levels were controlled by a pH-stat system (IKS Aquastar). The aquariums and sea water parameters used in the experiments were established in the form of model systems arranged in accordance with the environmental living conditions of investigated organisms. The radionuclide activities were measured by gamma and alpha spectrometers. The effects of high pCO2 (lowered pH) driven by anthropogenic CO2 on metal/radionuclide bioaccumulation in bivalves and shrimp studied in a series of laboratory controlled radiotracer experiments. High pCO2 caused increased 109Cd, but not 110mAg, uptake in juvenile mussel Mytilus galloprovincialis. On the other hand, high pCO2 did not affect Cd and Ag bioaccumulation (uptake, depuration and tissue distribution) in adult mussel. Decreased pH did not affect uptake, depuration, tissue and intracellular distribution of 109Cd, 134Cs and 57Co while it caused coloration in the shell of the oyster (Ostrea edulis). Shrimp (Palaemon adspersus) moulted more frequently and accumulated Ag more efficiently in the lowest pH condition (pH:7,5) compared to modest pH (7,8) and control pH (8,1). Sediment distribution coefficients (Kd: the transfer of metal/radionuclide from deep sediment to water) of 110mAg increased by lowered pH, while 109Cd and 134Cs remains unaffected. Transfer factor value was independent of pH, while the 110mAg concentration factor value increased with decreasing pH in manila clam (Ruditapes philippinarum). Po-210 bioaccumulation in Ruditapes philippinarum was not affected by pH. The physiology and metabolic rate affects metal bioaccumulation in marine organisms. Therefore, filtration rate, haemocyte viability, immunomodulation (neutral red uptake) and circulating cell-free nucleic acids were determined in Mytilus galloprovincialis exposed to lowered pH over 82 days period. Filtration rate and haemocyte viability significantly lessen at lowered pH (7,5) compared to control condition while circulating cell-free nucleic acids and immune system remain unchanged. To sum up, effect of ocean acidification on metal and radionuclide bioaccumulation in bivalves and shrimp dependent on the exposure pathway and duration, development stage, select species and radionuclides/metals. We also inferred that lowered pH can modulate some metabolic rate and physiology parameters, but not immune defence and circulating cell free nucleic acid in mussel (Mytilus galloprovincialis). The result of this study also suggests that the effects of ocean acidification on bioaccumulation of metal and radionuclides in a marine organism should be supported by the information provided by metabolic and physiological status of organism as well as biochemical potential biomarker.