When you cast your seed into the open, you'd better be sure that the conditions will be ideal to ensure the success of the next generation. And that approach seems to have served sea urchins perfectly well: that was until now. As global sea temperatures rise and the oceans begin to acidify, the tiny gametes that sea urchins entrust to the waves are under increasing risk. Monique Binet from CSISO, Australia, and an international team of collaborators say, ‘Under near-future scenarios of ocean acidification, the swimming behaviour of marine invertebrate sperm is altered’. Sperm use a pH gradient (from low to high pH) to activate the energy-supplying mitochondria that power the gametes when released into the ocean, so the team decided to test how future climate scenarios might affect the membrane potential of mitochondria that produce ATP, and how individual swimming performance is affected by acidification.
After collecting Centrostephanus rodgersii sea urchins from the rocky shoreline of Sydney's North Harbour and transporting them back to the lab at Macquarie, the team encouraged the sea urchins to release sperm ready to test the effects of environmental acidification. Next, they exposed the fresh sperm to seawater from one of three scenarios –current ocean pH, –0.3 pH units below current ocean pH (the projected coastal water conditions in 2100) and –0.5 pH units below current ocean pH (the projected coastal water conditions in 2300) – before adding JC-1 stain to the sperm, which fluoresces green when the mitochondrial membrane potential is low, but produces orange/green fluorescence when the membrane potential is high. Then, after allowing 15 min for the stain to penetrate the sperm, the team set the gametes a swimming test – they filmed the sperm in a drop of seawater under a microscope – to find out how well they performed and recorded the ratio of green to orange/green fluorescence in order to measure the membrane potential of the sperm mitochondria in all three environmental scenarios.
Not surprisingly, the sperm mitochondria became increasingly unhappy as the pH dropped, with the membrane potential falling up to 42% at –0.3 pH units, and plunging by as much as 55% in the 2300 pH scenario. However, the –0.3 pH unit reduction did not appear to impact the sperm swimming speed adversely – sperm swam even faster than under present day conditions – but the performance of the –0.5 pH unit sperm suffered as their swim speed plummeted. Although the sperm were not equally affected by the adverse conditions; there were large variations in swimming speed in both of the future climate conditions.
So, even though the sperms’ mitochondria were impaired, and Binet and her colleagues point out that the sperm are currently near their pH tolerance tipping point, the team is optimistic about the future of C. rodgersii sea urchins. Explaining that lower mitochondrial activity will allow the sperm to eke out their meagre energy reserves for longer and extend their longevity, the team says, ‘Increased sperm longevity increases fertilisation success when sperm–egg encounter rates remain high over prolonged periods’. And they also suggest that the broad range of sperm responses may provide the sea urchins with an opportunity to side-step their apparently bleak future. The team says, ‘Substantial inter-individual variation in responses of sperm swimming to ocean acidification may increase the scope for selection of resilient phenotypes, which, if heritable, could provide a basis for adaptation to future ocean acidification’.