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[BPT03-P04] Variations of the growth and Mg/Ca ratios in sea urchins (Mesocentrotus nudus) cultured under the long-term acidification experiment
Keywords:sea urchin, Mg/Ca, ocean acidification, long-term culture experiment
It has been suggested that the ocean acidification (OA) is expected to adversely affect marine ecosystems, especially carbonate organisms. In this study, we cultured sea urchins (Mesocentrotus nudus) more than 4 years since they were juvenile under the different pCO2 conditions. The growth and Mg/Ca in spines of individual sea urchin, which is mainly composed of calcite, have been periodically measured during the culture experiment that is still ongoing. Then we investigate the effects of OA on the growth and incorporation of Mg/Ca in spines of sea urchins. In addition, variation of Mg/Ca with respect to the life cycle of M. nudus is also examined.
In this study, sea urchins were raised under the following five pCO2 conditions (µatm): (i) 2000, (ii) 1000, (iii) 700, (iv) 350 (controlled setting), (v) 250. The conditions from (i) to (iii) were adjusted by addition of CO2 and that of (v) was adjusted chemically. For the culture experiment, 7 sea urchins were placed in individual bottles at each setting. The test diameter and the body weight were measured monthly and bimonthly in the first and latter half of culture period, respectively, and one spine from individual sea urchin was also collected at this time. The Mg/Ca ratios contained in the spines of sea urchins were measured with an inductivity coupled plasma optical emission spectrometer (Agilent 720).
As the result of measurements of the growth, the growth rate of test diameter and the body weight decreased with increasing pCO2 with two large reductions; one is between 250 and 350 and the other is between 700 and >1000 µatm, suggesting OA impacts may already affect the growth of M. nudus at present and further suppress it in the future. Interestingly, the spine Mg/Ca ratios showed a clear positive relation against pCO2. Since it has been reported that calcites with higher Mg contents have higher solubility, spines of M. nudus would become more vulnerable under the projected OA conditions. In other words, M. nudus would be projected to suffer from both the reduction of Ω for calcite and increasing in solubility during the growth of spines due to increasing of pCO2 itself and higher Mg/Ca of spines under higher pCO2 conditions, respectively, although the mechanism of Mg/Ca increase against increasing in pCO2 is uncleared. For the life cycle of cultured sea urchins, spawning was identified around August 2018 and after that daily growth rates of the test diameter become relatively lower and stable compared to those before the spawning in all pCO2 conditions. Since temporal variations of Mg/Ca in spines also show large differences between Mg/Ca ratios before and after spawning; higher and lower Mg/Ca before and after spawning, respectively. Therefore Mg/Ca in M. nudus could be an indicator for distinguish juvenile and/or adult in their life cycle.
In this study, sea urchins were raised under the following five pCO2 conditions (µatm): (i) 2000, (ii) 1000, (iii) 700, (iv) 350 (controlled setting), (v) 250. The conditions from (i) to (iii) were adjusted by addition of CO2 and that of (v) was adjusted chemically. For the culture experiment, 7 sea urchins were placed in individual bottles at each setting. The test diameter and the body weight were measured monthly and bimonthly in the first and latter half of culture period, respectively, and one spine from individual sea urchin was also collected at this time. The Mg/Ca ratios contained in the spines of sea urchins were measured with an inductivity coupled plasma optical emission spectrometer (Agilent 720).
As the result of measurements of the growth, the growth rate of test diameter and the body weight decreased with increasing pCO2 with two large reductions; one is between 250 and 350 and the other is between 700 and >1000 µatm, suggesting OA impacts may already affect the growth of M. nudus at present and further suppress it in the future. Interestingly, the spine Mg/Ca ratios showed a clear positive relation against pCO2. Since it has been reported that calcites with higher Mg contents have higher solubility, spines of M. nudus would become more vulnerable under the projected OA conditions. In other words, M. nudus would be projected to suffer from both the reduction of Ω for calcite and increasing in solubility during the growth of spines due to increasing of pCO2 itself and higher Mg/Ca of spines under higher pCO2 conditions, respectively, although the mechanism of Mg/Ca increase against increasing in pCO2 is uncleared. For the life cycle of cultured sea urchins, spawning was identified around August 2018 and after that daily growth rates of the test diameter become relatively lower and stable compared to those before the spawning in all pCO2 conditions. Since temporal variations of Mg/Ca in spines also show large differences between Mg/Ca ratios before and after spawning; higher and lower Mg/Ca before and after spawning, respectively. Therefore Mg/Ca in M. nudus could be an indicator for distinguish juvenile and/or adult in their life cycle.