Seasonal variations of pH, pCO2, pO2, HCO3− and Ca2+ in the haemolymph: implications on the calcification physiology in Anodonta cygnea

A study about the relationship between the physical–chemical parameters and the calcium carbonate balance between the haemolymph fluid and mantle calcareous structures was carried out in Anodonta cygnea. An intense peak of HCO3− and a highest pH in December–January months may be understood as a preparation period for creating alkaline conditions. An intense pH decrease from January to February in parallel with the HCO3− reduction seems to indicate the beginning process of carbonate precipitation. On the other hand, the following calcium and HCO3− increases in February–May associated with a continuous and gradual pH fall profile may infer two combined aspects: calcium and HCO3− absorption from external environment and a simultaneous intense calcium carbonate deposition in the haemolymph. So, the pCO2 peak in this period reflects a subsequent result on equilibrium balance between HCO3− absorption and deposition. The only significant pO2 increase in the next period, from February to June, is related with an energetic increase to support the metabolic activity favouring the posterior intense pCO2 peaks. The extended time of CO2 production in the haemolymph from May to November should induce an increased metabolic acidosis with subsequent intense formation of both HCO3− and Ca2+ ions in the same period. This seems to result from CaCO3 deposits dissolution in the haemolymph, the most direct calcareous source. Additionally, the later increase of metabolic succinic acid during autumn may greatly potentiate this acidosis increasing the dissolution process. Consequently, the pH profile present two simultaneous alkaline peaks in July and October, probably due to a strong HCO3− release from the CaCO3 dissolution. So, the present seasonal results indicate that in the freshwater bivalve A. cygnea, the low metabolism with higher pH from the early winter is the main cause which may favour a calcareous precipitation, while the high metabolism with lower pH from the early summer may function as an inductor of calcareous dissolution in the haemolymph.