Effects in CO2 and light on the carbon acquisition of key diatom species in the Southern Ocean
Final Report Abstract
The ecology and biogeochemistry of the Southern Ocean is strongly dominated by key diatom species and the flagellate Phaeocystis. The mechanisms regulating the dominance of these species in the Southern Ocean ecosystem are poorly understood. According to the IPCC (scenario IS92a), surface water CO2 concentrations are expected to increase almost three-fold for the year 2100 relative to preindustrial values and to lower the pH ('ocean acidification') by 0.4 units. Rising temperatures will impact surface ocean stratification, which in turn will affect light climate and nutrient input from deeper layers. Climate models indicate that especially the Southern Ocean will be affected by these environmental changes. How will these changes shape phytoplankton community structure and possibly alter productivity? To answer this question this project studied the effects of CO2, light and iron on physiological key processes such as photosynthesis, carbon and trace metal acquisition in laboratory as well as in shipboard incubation experiments. The performance of laboratory experiments allowed to gain a mechanistic understanding on physiological key processes in single phytoplankton species (Chaetoceros, Pseudo-nitzschia, Fragilariopsis, and Phaeocystis). Among the tested species, only the growth of Chaetoceros was promoted in response to high pCO2. The investigation of photosynthetic carbon acquisition revealed the operation of very efficient carbon concentrating mechanisms (CCMs) in all species, but there were species-specific differences in CO2-dependent regulation of individual CCM components. Species like Chaetoceros might benefit from reduced energy requirements and/or a better energy use efficiency for higher growth in high CO2 conditions, while in other phytoplankton species such as Pseudo-nitzschia energy requirements might remain unaffected. The role of species competition in structuring phytoplankton communities was tested in species competition experiments with Chaetoceros and Pseudo-nitzschia. These experiments revealed that Pseudo-nitzschia may be a candidate that excretes allelochemicals providing it a competitive advantage over Chaetoceros and potentially other diatom species. To examine the CO2 sensitivity of natural phytoplankton assemblages, shipboard incubation experiments were conducted with populations from different regions of the Southern Ocean. CO2/iron perturbation experiments with a natural phytoplankton population of the Weddell Sea showed a CO2-dependent increase in primary producfivity only in response to iron enrichment. These changes in productivity were accompanied by a pronounced taxonomic shift from weakly to heavily silicified diatoms (i.e. from Pseudo-nitzschia to Fragilariopsis). Under iron-depleted conditions, however, this functional shift was absent and thinly silicified species dominated all PCO2 treatments (Pseudo-nitzschia and Synedropsis). These results highlight that high pCO2 may increase primary productivity in iron-enriched regions of the Weddell Sea, causing a positive feedback on the biological pump and thus atmospheric CO2. To test how the iron source (dust vs. inorganic iron) influences growth and phytoplankton species composition under different CO2 scenarios, shipboard perturbation experiments with a natural phytoplankton population south of the Polar Front were performed. While at ambient PCO2, control and dust treatments were dominated by Pseudo-nitzschia, the addition of FeCl3 led to a dominance of Nitzschia. Next to changes by the iron source, also PCO2 signiflcantly altered the structure of the community. Thus, high PCO2 promoted the growth of Phaeocystis in control and dust treatments while in FeCl3-treatments Chaetoceros prospered. The stimulation in growth of Chaetoceros by high PCO2 confirms previous observations. These findings clearly show that the quality of the iron source influences the phytoplankton species composition differently and that these responses are further modulated by high PCO2. Altogether, these findings provide new insights into the physiology and ecology of Southern Ocean phytoplankton and improve our understanding of changes in the Southern Ocean ecosystem under the impact of environmental perturbations.
Publications
- 2010. Inorganic carbon uptake by Ross Sea phytoplankton across natural and experimental CO2 gradients. Journal of Phycology, 46, 433-443
Tortell, P.D., Trimborn, S., Li, Y., Rost, B., Payne, C.D.
- 2013. Sensitivity of Antarctic phytoplankton species to ocean acidification: growth, carbon acquisition and species interaction. Limnology and Oceanography, 58: 997-1007
Trimborn, S., Brenneis, T., Sweet, E., Rost, B.
(See online at https://doi.org/10.4319/lo.2013.58.3.0997)
