Buffer capacity of the coelomic fluid in echinoderms.

Area of Science:

• Marine Biology
• Echinoderm Physiology
• Ocean Acidification Research

Background:

• Anthropogenic carbon dioxide increases atmospheric CO2, leading to ocean acidification.
• Ocean acidification affects marine organisms differently based on their internal pH regulation.
• Echinoderms exhibit varied abilities to control internal fluid pH.

Purpose of the Study:

• To investigate the coelomic fluid buffer capacity across different echinoderm taxa.
• To identify factors influencing the buffer capacity of echinoderm coelomic fluid.
• To explore the relationship between gas exchange efficiency and buffer capacity in echinoderms.

Main Methods:

• Comparative analysis of coelomic fluid buffer capacity in Euechinoidea, Cidaroidea, starfish, and holothurians.
• Detailed investigation of buffer constituents and influencing factors in the sea urchin Paracentrotus lividus and starfish Asterias rubens.
• Experimental manipulation of seawater pH and feeding regimes to assess impacts on buffer capacity.

Main Results:

• Euechinoidea exhibit significantly higher coelomic fluid buffer capacity (0.8–1.8 mmol kg−1 SW above seawater) than Cidaroidea, starfish, and holothurians (-0.1–0.4 mmol kg−1 SW).
• The bicarbonate buffer system is the primary contributor (63–92%), followed by coelomocytes (approx. 8%) and a >3 kDa compound in P. lividus (approx. 15%).
• Feeding increased buffer capacity in P. lividus but not A. rubens; decreased seawater pH significantly enhanced buffer capacity in P. lividus, allowing partial pH compensation.

Conclusions:

• Echinoderm taxa possess distinct coelomic fluid buffer capacities, potentially linked to adaptations in gas exchange structures.
• The bicarbonate system, coelomocytes, and specific organic compounds contribute to echinoderm buffer capacity.
• Echinoderms, particularly P. lividus, demonstrate physiological plasticity in buffer capacity in response to feeding and ocean acidification.