THE LIFE ACIDIC: TUBEWORMS' SURVIVAL IN FUTURE OCEANS

Abstract

In tropical coastal cities and ports around the world, the tiny tubeworm has garnered quite a bad rep. As miniscule larvae, these tubeworms make it undetected into seawater-based cooling systems, and once inside, they settle down and metamorphose into their adult forms, with their characteristic calcium carbonate shells causing costly blockages. But their way of life is under threat as a result of our oceans absorbing excess man-made carbon dioxide, as Vengatesen Thiyagarajan, a researcher from the Swire Institute of Marine Science and School of Biological Sciences, Hong Kong, explains: ‘Human carbon dioxide is affecting calcifying animals in two ways. Firstly, the acidification is decreasing the carbonate ion concentration in the seawater, so building the calcium carbonate structure is difficult; and secondly, the decreased pH in the [sea] environment means the animal has to spend more energy maintaining their internal pH.’ As they are troublemakers, many would welcome the demise of tubeworms, but Thiyagarajan points out that these marine animals are also ecologically important, forming coral-reef-like structures that promote biodiversity, as well as cleaning up the water in their role as filter feeders. As their loss would be significant, Thiyagarajan decided to find out just how susceptible these tubeworms are to ocean acidification (p. 4580).Thiyagarajan and his team focused on what effect ocean acidification would have on the ability of larval tubeworms to transition into the adult form, as metamorphosis at normal ambient pH already uses up 50% of the body's energy and is associated with a high mortality rate of 80–90%. With acidification depleting carbonate ions and larvae having to expend more energy on maintaining internal pH, could some larvae still metamorphosis in acidic conditions? To test this, Thiyagarajan's team placed larvae into seawater with a pH of 7.6 and then measured how many made it through metamorphosis. Sure enough, compared with their larval friends kept in seawater with a normal and milder pH of 8.1, significantly fewer larvae made it through to adulthood.However, Thiyagarajan points out: ‘In the future oceans it's not just acidification [that's going to be happening], it's going to be combined with hypoxia, global warming and pollution; there are going be multiple stressors on these animals.’ To test the effects of acidification in a more realistic setting, the team measured metamorphosis success in larvae also subjected to low-oxygen (hypoxic) conditions. However, to their surprise, instead of faring even worse, these doubly stressed larvae seemed to metamorphosis at normal levels and the hypoxia seemed to rescue the larvae from the negative effects of ocean acidification.But how exactly was acidification hindering metamorphosis and hypoxia rescuing these larvae from the negative effects? Unfortunately, without the tubeworm genome sequence, working this is out is a little hard. However, undeterred, the team decided to measure changes in levels of proteins using a technique called two-dimensional gel electrophoresis. Overall, they found larvae that had been subjected to acidic conditions reduced the number of proteins they expressed. They then found that many of these proteins were very similar to proteins in other marine species that were responsible for stress responses and for controlling the calcification process. In larvae treated with both acidification and hypoxia, they didn't see this decrease in protein expression. Although hypoxia rescued the larvae from the negative effects of ocean acidification, the team still don't know what the long-term effects will be for these doubly stressed worms, but the study highlights the importance of studying ocean acidification in combination with other potential future stressors.