Direct Effects of Decreased pH on the Ingestion and Survival of Marine Microzooplankton
Katharine Plemons1 and M. Brady Olson2
1Eckerd College, 2Western Washington University, Shannon Point Marine Center
plemonkm(at)eckerd.edu
Abstract - Introduction - Materials & Methods - Results - Discussion - References - Acknowledgements
Climate models predict that by the year 2100, atmospheric pCO2 may increase two to three fold, resulting in shifts in dissolved inorganic carbon speciation, and H+ increases of nearly 150%. Studies with CO2 levels elevated to these predicted levels show effects on a wide range of marine flora and fauna. However, it is unclear how reductions in ocean pH will affect microzooplankton ecology. We conducted a series of experiments to test the sensitivity of temperate, coastal microzooplankton to reductions in ocean pH.
In one set of experiments, we exposed the tintinnid ciliate Favella ehrenbergii and the heterotrophic dinoflagellate Oxyrrhis marina to a seawater pH concentration gradient and measured survival over a 24-h period. Both microzooplankton species showed acute sensitivity to reductions in pH, with increased mortality at pH levels of 5 and 6 for O. marina and F. ehrenbergii, respectively. In a second set of experiments, we measured short-term ingestion rates of Favella ehrenbergii to test for subacute effects to low pH. We found that F. ehrenbergii ingestion rates were reduced at non-lethal pH concentrations. Our results showed that reductions in pH directly affect survival and feeding of these microzooplankton, but these effects occurred only at pH levels below those predicted by climate models, indicating microzooplankton may not be directly affected by ocean acidification. However, microzooplankton may be affected by CO2-dependent alterations in the physiology and biochemistry of their prey. Future studies should include these indirect linkages in order to fully comprehend how ocean acidification will impact microzooplankton ecology.
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Over the past 200 years, humans have influenced climate dynamics by loading CO2 into the atmosphere (Riebesell, 2004). Climate models predict that, by the year 2100, atmospheric pCO2 may increase two to three fold, precipitating shifts in dissolved inorganic carbon speciation and a 150% increase in H+. Studies with CO2 levels elevated to these predicted levels show effects on a wide range of marine flora and fauna. However, little is known about how reduced ocean pH will affect marine microbial communities (Doney et. al. 2009).
A series of experiments were completed to test the sensitivity of temperate, coastal microzooplankton to reductions in ocean pH. We hypothesized that reductions in pH would have direct negative effects on zooplankton survival and feeding.
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In the first set of experiments, we exposed the tintinnid ciliate Favella ehrenbergii and the heterotrophic dinoflagellate Oxyrrhis marina to a seawater pH concentration gradient (pH 8, 7,6,5,4,3) and measured survival over a 24-h period using live swimming tests ( F. ehrenbergii ) and epifluorescent microscopy ( O. marina ).
In a second experiment, we measured short-term feeding rates of F. ehrenbergii to test for sub acute effects of low. Previously starved F. ehrenbergii were allowed to feed for 6 min. on a saturating concentration of the prey dinoflagellate Heterocapsa triquetra. Using epifluorescent microscopy we counted the number of F. ehrenbergii with food-containing vacuoles across pH treatments.
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Both microzooplankton species showed acute sensitivity to reductions in pH, with significant mortality beginning at pH levels of 5 and 6 for O. marina and F. ehrenbergii , respectively (Figures 1 & 2).
During the experiments, we saw that a reduction in pH level affected the ability of F. ehrenbergii to digest prey cells. Even though F. ehrenbergii survived at pH level 6, they had significantly more food within their vacuoles compared to cells in pH treatments 7 and 8 (Figures 3 & 4).
Results from our ingestion experiment suggest that F. ehrenbergii ingestion rates are reduced at non-lethal pH concentrations (Fig. 5).
However, there was considerable variability in the data and feeding rates were not statistically different.
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Reductions in pH directly affect survival and feeding of the tested microzooplankton, but these effects occurred only at pH levels below those predicted by climate models, indicating microzooplankton may not be directly affected by ocean acidification. However, microzooplankton may be affected by CO2-dependent alterations in the physiology and biochemistry of their prey. Future studies should include these indirect linkages in order to fully comprehend how ocean acidification will impact microzooplankton ecology.
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I thank Brady Olson for outstanding guidance and instruction. I thank Brian Bingham for accepting me into the
MIMSUP program and giving me such an honorable opportunity. Dr Strom allowed me to use her equipment; and, her lab technicians, Kerri Fredrickson and Kelley Bright taught me how to use it. Horng-Yuh Lee taught me how to use the pH meter. I also thank Dr. Sulkin for allowing me access to the Shannon Point Marine Center. Finally, I thank the National Science Foundation (grant # OCE0741372) for granting me an excellent opportunity to conduct hands on research and present it at scientific conferences. This experience was very gratifying and will help me with my future career goals.