Threat to the nutrition base in the oceans – phytoplankton and acidificationThe the entire food chain in the ocean is represented by the microscopic organisms of the marine phytoplankton. These include diatoms (siliceous algae), coccolithophores (calcareous algae), and the cyanobacteria (formerly called blue algae), which, because of their photosynthetic activity, are responsible for around half of the global primary productivity. Because phytoplankton requires light for these processes, it lives exclusively in surface ocean waters. It is therefore directly affected by ocean acidification. In the future, however, due to global warming, other influencing variables such as temperature, light or nutrient availability will also change due to global warming. These changes will also determine the productivity of autotrophic organisms, primarily bacteria or algae, which produce biomass purely by photosynthesis or the incorporation of chemical compounds. It is therefore very difficult to predict which groups of organisms will profit from the changing environmental conditions and which will turn out to be the losers. Ocean acidification is of course not the only consequence of increased CO2. This gas is, above all, the elixir of life for plants, which take up CO2 from the air or seawater and produce biomass. Except for the acidification problem, increasing CO2 levels in seawater should therefore favour the growth of those species whose photosynthetic processes were formerly limited by carbon dioxide. For example, a strong increase in photosynthesis rates was reported for cyanobacteria under higher CO2 concentrations. This is also true for certain coccolithophores such as Emiliania huxleyi. But even for Emiliania the initially beneficial rising CO2 levels could become fatal. Emiliania species possess a calcareous shell comprised of numerous individual plates. There is now evidence that the formation of these plates is impaired by lower pH values. In contrast, shell formation by diatoms, as well as their photosynthetic activity, seems to be hardly affected by carbon dioxide. For diatoms also, however, shifts in species composition have been reported under conditions of increased CO2 concentration.
Challenge for the future: Understanding acidificationIn order to develop a comprehensive understanding of the impacts of ocean acidification on life in the sea, we have to learn how and why CO2 affects various physiological processes in marine organisms. The ultimate critical challenge is how the combination of individual processes determines the overall CO2 tolerance of the organisms. So far, investigations have mostly been limited to short-term studies. To find out how and whether an organism can grow, remain active and reproduce successfully in a more acidified ocean, long term (months) and multiple-generation studies are neccessary.
- The final, and most difficult step, thus is to integrate the knowledge gained from species or groups at the ecosystem level. Because of the diverse interactions among species within ecosystems, it is infinitely more difficult to predict the behaviour of such a complex system under ocean acidification.
- In addition, investigations are increasingly being focused on marine habitats that are naturally characterized by higher CO2 concentrations in the seawater. Close to the Italian coast around the island of Ischia, for instance, CO2 is released from the sea floor due to volcanic activity, leading to acidification of the water. This means that there are coastal areas directly adjacent to one another with normal (8.1 to 8.2) and significantly lowered pH values (minimum 7.4). If we compare the animal and plant communities of these respective areas, clear differences can be observed: In the acidic areas rock corals are completely absent, the number of specimens of various sea urchin and snail species is low, as is the number of calcareous red algae. These acidic areas of the sea are mainly dominated by seagrass meadows and various non-calcareous algal species.
- 2.12 > Low pH values in the waters around Ischia cause corrosion of the shells of calcareous animals such as the snail Osilinus turbinata. The left picture shows an intact spotted shell at normal pH values of 8.2. The shell on the right, exposed to pH values of 7.3, shows clear signs of corrosion. The scale bars are equal to one centimetre.
- The further development of such ecosystem-based studies is a great challenge for the future. Such investigations are prerequisite to a broader understanding of future trends in the ocean. In addition, deep-sea ecosystems, which could be directly affected by the possible impacts of future CO2 disposal under the sea floor, also have to be considered. In addition, answers have to be found to the question of how climate change affects reproduction in various organisms in the marine environment. Up to now there have been only a few exemplary studies carried out and current science is still far from a complete understanding. Whether and how different species react to chemical changes in the ocean, whether they suffer from stress or not is, for the most part, still unknown. There is an enormous need for further research in this area.