Extracellular pH changes in echinoderms as a consequence of ocean acidification

Atmospheric carbon dioxide levels have recently surpassed 400 ppm, a substantial increase since the 1750’s value of 280 ppm. The ocean, an enormous sink of this CO2, is projected to experience a change in seawater pH of up to 0.77 pH units by the year 2300. Regionally polar latitudes are expected to show the effects of ocean acidification earliest as CaCO3 saturation levels are already low due to cold waters.This research aimed to develop and test a technique that allowed the measurement of extracellular pH (pHe) in developing echinoderm larvae when exposed to pH values predicted for the future. Morphological changes were also recorded in parallel with pHe. To investigate if responses varied with latitude, I looked at the effect of lowered seawater pH on the pHe of five echinoderm species: the tropical sand dollar Arachnoides placenta, the temperate summer spawning urchin Evechinus chloroticus, and temperate winter spawning Pseudechinus huttoni, the polar sea urchin Sterechinus neumayeri and the polar sea star Odontaster validus. Different developmental stages were used (blastula, gastrula, pluteus and bipinnaria) to investigate how pHe regulation changed during development. Body regions of the developing larvae were also distinguished (gut, oesophagus and arm) to identify if pHe regulation differed throughout the pluteus and bipinnaria larvae. To determine the affects of ocean acidification, pH levels of ambient pH, pH 7.8, pH 7.6, and pH 7.4 were used for the tropical and temperate experiments and ambient pH, pH 7.7, pH 7.5 and pH 7.2 for the polar experiments.

The technique to measure extracellular pH was developed using the nontoxic fluorescent probe 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS). After incubation of larvae in 30 μM HPTS, dual photographs were taken using the probes excitation optimums of 405 nm and 460 with a single emission optimum at 520 nm to produce a false colour pH map of larval pHe in MATLAB.

The temperate sea urchin E. chloroticus showed the least tolerance to lowered seawater pH as pHe was significantly decreased in the gut (pHe 7.91 to pHe 6.5), oesophagus (pHe 6.59 to pHe 5.97) and arms (pHe 6.65 to pHe 6.17) when raised under pH 7.6. Sterechinus neumayeri blastula did not show an effect to lowered pH but the pluteus larvae had significantly decreased pHe of up to 0.44 units (gut pHe) in all regions in treatment pH 7.2. The temperate P. huttoni was the most tolerant to a lowering seawater pH with only the gut region responding with a significantly lowered pHe (pHe 6.67 to pHe 6.64) in treatment pH 7.4. Overall polar species had the lowest pHe in all regions which decreased as seawater pH decreased. A 0.4 pH unit drop in pH (pH 7.6-7.8) resulted in significant abnormality of the early cell stage and size reductions of the pluteus in all species and sea star bipinnaria.

The effects of ocean acidification on the ability of echinoderms to regulate their extracellular pH are species-specific. Several parameters, such as taxonomic differences, physiology, genetic makeup and the population’s evolutionary history may contribute to this variability. This study highlights that although many species may be negatively affected by ocean acidification, the ability for polar species to regulate pHe is surprisingly effective; they are only affected when pH decrease is extreme. Further research is needed, especially looking at multiple stresses, as ocean acidification will not happen on its own but in conjunction with ocean warming, increases in UV-B irradiance, overfishing and pollution. The technique developed in this study will allow insight into how internal systems of many species cope with climates associated changes.

Stuck E. F., 2014. Extracellular pH changes in echinoderms as a consequence of ocean acidification. MSc thesis, University of Otago, 158 pp. Thesis.

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