Maintenance of coelomic fluid pH in sea urchins exposed to elevated CO2: the role of body cavity epithelia and stereom dissolution

Experimental ocean acidification leads to a shift in resource allocation and to an increased [HCO3-] within the perivisceral coelomic fluid (PCF) in the Baltic green sea urchin Strongylocentrotus droebachiensis. We investigated putative mechanisms of this pH compensation reaction by evaluating epithelial barrier function and the magnitude of skeleton (stereom) dissolution. In addition, we measured ossicle growth and skeletal stability. Ussing chamber measurements revealed that the intestine formed a barrier for HCO3- and was selective for cation diffusion. In contrast, the peritoneal epithelium was leaky and only formed a barrier for macromolecules. The ossicles of 6 week high CO2-acclimatised sea urchins revealed minor carbonate dissolution, reduced growth but unchanged stability. On the other hand, spines dissolved more severely and were more fragile following acclimatisation to high CO2. Our results indicate that epithelia lining the PCF space contribute to its acid-base regulation. The intestine prevents HCO3- diffusion and thus buffer leakage. In contrast, the leaky peritoneal epithelium allows buffer generation via carbonate dissolution from the surrounding skeletal ossicles. Long-term extracellular acid-base balance must be mediated by active processes, as sea urchins can maintain relatively high extracellular [HCO3-]. The intestinal epithelia are good candidate tissues for this active net import of HCO3- into the PCF. Spines appear to be more vulnerable to ocean acidification which might significantly impact resistance to predation pressure and thus influence fitness of this keystone species.

Oxygen microprofiles and porewater chemistry in sediments of the Skagerrak

Sediment porewater oxygen profiles were measured with micro and needle electrodes in sediment cores of 27 stations in the Skagerrak (northeastern North Sea). Oxygen penetration depth ranged from 3 to 20 mm depth. Fluxes estimated from the oxygen gradients varied from 3 to 18 mmol m**-2 d**-1. Oxygen penetration and flux depend on water depth, but possibly more on the hydrological conditions, related to the import of fresh organic matter by primary production in the water column. Oxygen fluxes were not related to the total organic carbon (TOC) content of the sediments. Stations in the eastern part of the Skagerrak showed high burial rates of TOC. At 6 stations porewater chemistry of Fe, Mn and NO3- was strongly associated with the oxygen distribution. The average relative contribution of terminal electron acceptors to carbon mineralisation was estimated at 85% for O2, 0.5% for Mn, 4.5% for [NO3]3-, 1% for Fe and 9% for [SO4]2-. At one station the occurrence of exceptionally high solid manganese oxyhydroxides was probably related to an active internal manganese cycle.

Biogeochemical parameters of sea water and bottom sediments near the Curieuse Island (Seychelles Islands)

Biogeochemical reef studies carried out in 1981 and 1984 found low concentration of total natural and anthropogenic hydrocarbons in inshore waters. Detection of lignin in marine and bottom sediments indicates that the land has major effect on makeup of organic matter there. Comparison of compositions of organic matter in sea water, suspended matter and bottom sediments indicated that it was altered rapidly by the reef community. Thus, in the inshore zone of the island, runoff from the land is important in supplying nutrients to the reef ecosystem alongside with transport of nutrients by deep waters. Concentrations of nutri¬ents (N, P) in the inshore zone are higher than in waters of the tropical part of the ocean. Nitrogen is the limiting element in development of phytoplankton in the inshore zone.