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Theme leader: Ulf Riebesell uriebesell (at) ifm-geomar.de IFM-GEOMAR
WP 4 - Sensitivity of calcification
WP 5 - Sensitivity of autotrophic processes
WP 6 - Performance: reproduction and growth
WP 7 - Microbial diversity and activity, in particular nitrogen cycling
WP 8 - Impact on trophic interactions
WP 9 - From process studies to ecosystem models
If CO2 emissions continue to rise at current rates, the resulting changes in seawater chemistry will expose, before the end of this century, marine organisms to conditions which they probably have not experienced during much of their evolutionary history, which could pose a threat to the ecological fitness of pH/CO2-sensitive species and groups. Thus, as oceans continue to acidify, there is an increasing risk of loss of biodiversity and of profound ecological shifts.
Our present knowledge on the effects of ocean acidification (OA) on the marine biota is largely based on experimental work with single species and strains maintained in short-term incubations often exposed to abrupt and extreme changes in carbonate chemistry. Based on the presently available data, little is known about the responses of genetically diverse populations, synergistic effects from other stressors, and the ability of organisms to undergo physiological and genetic adaptations. A large gap in our understanding also concerns the transfer of responses from the organism to the community and ecosystem levels or the replacement of OA-sensitive by OA-tolerant species. In view of these uncertainties, it is presently not yet possible to define critical thresholds (tipping points) for tolerable pH decline or to predict the pathways of ecosystem changes where threshold levels have been surpassed.
To close this gap in our understanding and to allow a systems-based assessment of the risks and uncertainties associated with ocean acidification, Theme 2 (Fig. 1.) takes an integrated approach combining the expertise of molecular and cell biologists, biochemists, plant and animal physiologists, marine ecologists and ocean biogeochemists. The interaction between Theme 2 scientists across disciplines and work packages is further strengthened by joint community level manipulative experiments employing benthic and pelagic mesocosms and allowing all partners to address specific aspects of their work in complex biological system. This enables the collection of comprehensive multidisciplinary datasets on organism responses covering a wide range of relevant biological processes and strophic levels. These form the basis for an integrated assessment of ecosystem sensitivities and Earth system feedbacks. Following this approach, the overarching questions of Theme 2 are:
What are the effects of ocean acidification and related changes in seawater chemistry on marine organisms, what are the underlying mechanisms of the observed responses and the potential for adaptation, how are they modulated by other environmental stressors, and what are the consequences for marine ecosystems and ocean biogeochemical cycles?
To address these questions for a wide range of potentially sensitive biological processes, Theme 2 activities are structured according to key ecosystem components and functional groups.
The sensitivity of calcifying organisms to ocean acidification are studied in WP4 Sensitivity of calcification on a variety of groups with different carbonate minerals and pathways of carbonate precipitation, using a combination of physiological and molecular experimental approaches at the individual and community level. Particular attention is paid to elucidating (i) the mechanisms of calcification at the molecular, cellular and organism levels, (ii) synergistic effects with global warming, (iii) strain and species–specific differences in calcification responses and (iv) the potential for adaptation. Calcifying groups that are investigated include coccolithophores, foraminifera, pteropods and cold water corals.
The effects of rising CO2 and related changes in seawater chemistry, including changes in nutrient and trace metal chemistry, are studied in WP5 Sensitivities of autotrophic processes. Off-shore pelagic mesocosm experiments are conducted to quantify effects of acidification on species composition and succession, primary production, nutrient and carbon utilization, diazotrophic nitrogen fixation, organic matter exudation, and biogas production, such as DMS, DMSP and volatile organics, in natural open sea plankton communities.
The consequences of these responses for elemental stoichiometry, partitioning of organic matter into dissolved and particulate phases, and vertical transport are assessed. Carbon acquisition and energy fluxes are measured at the cellular level in key phytoplankton taxa. These results are fed into cell models of the carbon concentrating mechanism and photochemistry in eukaryotic algae to simulate the CO2-related responses of marine phytoplankton on the basis of the involved biochemical reactions (Fig. 2).
The influences of hypercapnia (a condition where body CO2 concentration exceeds tolerable levels) and decreasing seawater pH and their interaction with changing temperatures are studied in selected metazoan species from benthic and pelagic realms in WP6 Performance: reproduction and growth. Special emphasis aree given to species considered as early indicators, including fish and invertebrates with likely different levels of CO2 and pH sensitivity. Another focus is on early life stages, such as eggs and larvae, as these are known to be most sensitive to environmental stresses. The main objectives are to quantify (i) changes in various measures of performance and the physiological mechanisms setting and modifying those performance levels and (ii) the capacity to shift tolerance thresholds through acclimation or evolutionary adaptation. These objectives are addressed by invasive (optodes and chromatography techniques) and non-invasive (NMR, optical techniques) methods determining cellular acid-base regulation and associated changes in metabolic and calcification rates. This includes analyses of energetics, protein synthesis, gene expression capacity and the integrity of calcified structures from fish otoliths to invertebrate exoskeletons. CO2 effects on physiological performance with regard to oxygen consumption, circulatory and ventilatory activity, fecundity or egg production, hatching, success and somatic growth, in relation to temperature dependent acid-base and ion regulation are studied in model species of the various groups.
Prokaryotic organisms, the primary drivers of carbon and nutrient remineralisation in the sea, may be affected by ocean acidification both directly through changes in bacterial diversity and activity and indirectly via changes in phytoplankton and the allocation of photosynthetic products. Sedimentary microbial processes, including nitrogen cycling, may also be affected by OA-related changes in benthic macrofauna communities. These effects are examined in WP7 Impact of ocean acidification on microbial diversity and activity, in particular nitrogen cycling. Analyses of gene expression using the newly developed 454 pyrosequencing method coupled with 16S rRNA gene based fingerprints and real-time quantitative PCR allows us to generate large sequence datasets to assess responses of the microbial community to ocean acidification.
Links between direct effects of ocean acidification at the organism level (WP4 and WP6) and indirect effects on food web structure and function are studied in WP8 Impact on trophic interactions. This includes effects of reduced calcification on the relative importance of coccolithophore mortality via zooplankton grazing versus viral lysis (Fig. 3). A further focus is to elucidate how changes in organism size, predator susceptibility and food quality translate into changes at the food web level, such as slope of size-abundance spectra, autotrophic:heterotrophic balance, shifts between carbon and nutrient limited bacterial growth and biodiversity.
In the research on each of these ecosystem components a strong emphasis is given to understanding the mechanisms underlying the observed responses and to assess the potential for acclimation and adaptation. An efficient transfer of knowledge gained in experimental, process-oriented research in WPs 4-8 for global system analyses performed in Theme 3 are achieved through a meta-analysis of process studies and process parameterisations, and by combining models and data in a data-assimilative framework using the mesocosm datasets in WP9 From process studies to ecosystem models. In return, feedback from the modelling work is informed the experimental work in EPOCA about uncertainties in models and the relevant process parameterisations.
Joint mesocosm experiments: An up-scaling of research at the cellular and organism to the community and ecosystem level is achieved in joint benthic and pelagic mesocosm CO2 enrichment experiments. Most of the research groups contributing to WPs 4-8 participate in these collaborative activities and provide their expertise to obtain an integrated assessment of whole community responses to seawater acidification. Joint mesocosm activities are organized by PML (benthic mesocosms) and IFM-GEOMAR (pelagic mesocosms). An off-shore free-floating mesocosm system, consisting of 6 mesocosm units each containing 65 m3 of enclosed seawater (see Fig.4.), is presently being developed at IFM-GEOMAR and were tested during a field campaign in the Baltic Sea in July of 2007. In June/July of 2008, EPOCA’s first pelagic mesocosm experiment was conducted in the Gotland Sea (Proper Baltic Sea) jointly with the German project SOPRAN (Surface Ocean Processes in the Anthropocene) to study the CO2 sensitivity of a plankton community dominated by diazotrophic cyanobacteria. For this expedition two mid-sized research vessels (R/V Alkor and Heincke) have been reserved, a third vessel needed to board additional EPOCA scientists may be contributed by Dutch or UK partner institutes. A joint pelagic-benthic mesocosm study to assess the response of high Arctic communities to ocean acidification is proposed for the summer of 2010 in the Ny Ålesund on Svalbard.
Langer G. et al., 2006. Species-specific responses of calcifying algae to changing seawater carbonate chemistry. Geochemistry, Geophysics, Geosystems 7, Q09006. doi:10.1029/2005GC001227.
The Royal Society, 2005. Ocean acidification due to increasing atmospheric carbon dioxide. 60 p. London: The Royal Society.
Thierstein H. R. & Young J. R., 2004. Coccolithophores. 565 p. Berlin: Springer-Verlag.