Impact of elevated pCO2 on the ecophysiology of Mytilus edulis

Increasing atmospheric CO2 concentrations equilibrate with the surface water of the oceans and thereby increase seawater pCO2 and decrease [CO32-] and pH. This process of ocean acidification is expected to cause a drastic change of marine ecosystem composition and a decrease in calcification ability of various benthic invertebrates. The studied area, Kiel Fjord, is characterized by high pCO2 variability due to upwelling of O2 depleted and CO2 enriched bottom water. Within less than 50 years, eutrophication of the Baltic Sea has drastically increased the mean pCO2 in the fjord. The observed increase and also the rate of this acidification process is much higher than it is expected for the global ocean as a consequence of increasing atmospheric CO2 concentrations. In contrast to other areas subjected to elevated pCO2, calcifying invertebrates inhabit Kiel fjord and the benthic community is dominated by the blue mussel Mytilus edulis. Mussel larvae settle in the period of the year when highest pCO2 (800-2300 µatm) are encountered, which is, at the same time, the main growth period due to highest phytoplankton densities. In laboratory experiments, calcification rates of M. edulis are maintained at elevated pCO2 levels which are expected to occur by the year 2300. Only at high pCO2 above 3000 µatm, calcification is significantly reduced. One possible reason for this tolerance is the fact that even under control conditions, the extracellular body fluids (haemolymph and extrapallial fluid, EPF) of M. edulis are characterized by low pH and [CO32-] and high pCO2. Therefore, the EPF which is in direct contact with the shell is undersaturated with calcium carbonate also at current, low seawater pCO2. Under elevated pCO2, mussels do not buffer the extracellular acidosis by means of bicarbonate accumulation. Thus haemolymph pH and [CO32-] are reduced even further. Calcification might not be affected by the extracellular acidosis, as an amorphous calcium carbonate (ACC) precursor is most probably formed in intracellular vesicles. Since mussels are able to efficiently regulate the intracellular pH, reduced extracellular pH might therefore have only little impact on the initial calcification process. On the other hand, the production of the organic shell components, e.g. the periostracum, consumes high amounts of energy. Especially in young thin shelled life stages with a higher organic shell content most of the energy allocated to growth is required for shell production. Under elevated pCO2, mussels initially (two months acclimation) up - regulate their metabolic rates which may indicate higher energy demand for ion regulatory processes. Long-term acclimated animals (12 months acclimation) probably switch to an energetically less expensive compensation and do not exhibit elevated aerobic metabolism. However, long-term acclimated mussels are characterized by lower filtration rates. As consequence, after both intermediate and long-term exposure, the scope for growth is reduced in high pCO2 acclimated animals. Additionally, after intermediate and also long-term acclimation to elevated pCO2, protein metabolism is increased, as indicated by an elevation of ammonia excretion rates. This mode of energy generation is less efficient than oxidation of lipid or carbohydrate and may contribute to lower energy availability for growth and calcification. Similar to other aquatic animals, ammonia excretion in mussels seems to be facilitated by NH3 diffusion through Rhesus (Rh) and ammonium transporter (Amt) protein channels and subsequent acid-trapping by separate proton excretion. In order to test the importance of energy supply and elevated pCO2 on mussel calcification, juvenile M. edulis were exposed to a crossed experimental design for seven weeks. Higher food supply enables mussels to calcify also under highly elevated pCO2. In general food supply is the most important factor which determines the growth rates of mussels whereas pCO2 has only a minor effect. In a simultaneous field study, mussels were transplanted to the energy rich high pCO2 inner fjord and to the outer parts of the fjord at lower pCO2 and particulated organic carbon concentrations. Similar to the laboratory experiment, mussels exhibit much higher growth rates in the high pCO2 inner fjord with its higher particulate organic carbon concentrations. This reveals the importance of energy availability impacting CO2 tolerance of M. edulis. Mussels seem to be relatively tolerant to elevated pCO2 both in laboratory experiments and under current high pCO2 conditions in Kiel Fjord. The high energy availability present in the eutrophicated habitat may support the tolerance to elevated pCO2. In the future, increasing atmospheric CO2 concentrations will drastically elevate pCO2 level in this habitat. The benthic life stages seem to be able to cope with the expected levels but plantonic larvae might be vulnerable. However, M. edulis exhibit a high adaptation potential to the rate of acidification in the recent past and might be able to adapt also to higher levels in future. In order to predict the success of M. edulis in future, also effects of elevated temperature and the response of their main predators to these conditions needs to be considered.