20.6.16 | After an extensive overhaul of the Marine Environmental Ecology Unit (MAREE) by Silvia Hardenberg and her team, scientists at the Leibniz Centre for Tropical Marine Research are now able to undertake more in-depth testing of the effects of global change on marine organisms.

In a new series of laboratory experiments headed by Prof. Dr. Justin Ries of Northeastern University in Boston, a team including his graduate student Louise Cameron and ZMT postdoc Claire Reymond, is currently testing the effects of ocean acidification and warming on corals from both tropical and sub-polar waters. Here, Prof. Ries explains the studies and the scope of the research, which is a joint collaboration between the Leibniz Centre for Tropical Marine Research (ZMT), the Northeastern University in Boston (USA), the Alfred Wegener Institute (AWI) , the Max-Planck-Institute for Marine Microbiology (MPI), and the GEOMAR Helmholtz Centre for Ocean Research Kiel.

Why do you conduct these experiments?
Justin Ries: We want to find out how corals will respond to future acidification and warming of the oceans. As atmospheric CO2 goes up, the pH of the seawater goes down. As it becomes more acidic, it will be harder for some marine organisms to build their calcareous shells and skeletons. In our experiment, we are investigating the responses of different species of corals to acidification and warming that is predicted over the next 400 to 500 years. We create future conditions in the lab and grow corals in these environments for 60 to 90 days whilst measuring how their rates of skeleton-building change in response.

Are effects of ocean acidification always negative for marine organisms?
Justin Ries: Many organisms show a negative reaction, but some exhibit a parabolic response, where a slight increase in acidity actually helps the organism to grow. Some species of crustacea, for example, have been observed to grow faster as a result of acidification. Hence, it is not simply inorganic chemistry controlling the shell-building process. Most organisms produce a shell or skeleton from a fluid within a partially or fully isolated compartment – their so-called calcifying fluid. An important question is how calcifying marine organisms are able to mitigate or buffer changes in the pH of that fluid amidst acidification of their surrounding seawater. Crustacea seem to have very strong control of the fluid, probably because they shed their exoskeleton and have to regrow it within hours otherwise they get eaten. It therefore makes sense that they are better able to offset the effects of acidification than an organism that grows more slowly, such as corals, and has comparably less control over the pH of their calcifying fluid.

How are you measuring the response of marine calcifiers to ocean acidification?
Justin Ries: We are measuring the pH of the coral calcifying fluid with pH microelectrodes inserted into their fluid, and also by studying the boron isotopic composition of their calcareous skeleton. Boron isotopes are a natural component of seawater and sensitive to changes in seawater pH. Since corals are probably letting seawater into their calcifying fluid, an increase in the pH of their calcifying fluid should be seen in the different forms, or isotopes, of boron that get trapped in their skeleton. The isotope work is done in collaboration with the AWI, while the microelectrode work is being done in collaboration with the MPI.

What are the expected outcomes?
Justin Ries: We are testing the hypothesis that organisms with stronger control over the pH of the calcifying fluid are more resilient to acidification. So we expect that corals able to maintain their growth rates under higher-CO2 conditions will have stronger control over the pH at their site of calcification. Once we better understand the exact pathway by which corals are affected by ocean acidification, we can predict which species will be most vulnerable.

In what way is the MAREE ideal for these studies?
Justin Ries: We worked with the MAREE’s incredible staff, Silvia Hardenberg, Nico Steinel and Christian Brandt, to establish the MAREE as a state-of-the-art experimental ocean acidification system. We converted the large multi-species mesocosms to a controlled and highly replicated 48-tank experimental array capable of investigating the independent and combined effects of warming and acidification on marine organisms. We designed and set up a flow-through seawater system for the experimental array, so the system is continuously renewed with natural seawater, which is essential for most isotope studies. We also established a low-temperature acidification system to look at the responses of cold-water coral species. It is a big undertaking to have both a tropical and a cold-water ocean acidification system in the same room. But now, ZMT scientists can conduct experiments at the MAREE investigating the effects of ocean warming and acidification on nearly any benthic marine organism on the planet, from the equator to the poles.

Prof. Dr. Justin B. Ries is from the Department of Marine and Environmental Sciences at Northeastern University’s Marine Science Center in Boston (USA). He is currently undertaking a fellowship at the Hanse-Wissenschaftskolleg in Delmenhorst in collaboration with the ZMT, AWI, and MPI. He is a visiting scientist at the ZMT from December 2015 through August 2016.