title: Microzooplankton grazing, growth and gross growth efficiency are affected by pCO2 induced changes in phytoplankton biology
Creators: name: Still, Kelly
Publisher: name: Western Washington University
Year: 2016

description: <p>Accumulating evidence shows that ocean acidification (OA) alters surface ocean chemistry and, in turn, affects aspects of phytoplankton biology. However, very little research has been done to determine if OA-induced changes to phytoplankton morphology, physiology and biochemistry may indirectly affect microzooplankton, the primary consumers of phytoplankton. This is one of the first studies to explore how OA may indirectly affect microzooplankton ingestion, population growth and gross growth efficiency (GGE). I hypothesized 1) that the physiology, biochemistry and morphology of the phytoplankton <em>Rhodomonas </em>sp. would be directly affected by elevated pCO<sub>2</sub> and 2) that pCO<sub>2</sub>-induced changes in <em>Rhodomonas </em>sp. would affect grazing, growth rates, and GGE in microzooplankton consumers. To test my first hypothesis, I cultured the ecologically important phytoplankton, <em>Rhodomonas </em>sp., semi-continuously for 17 days under three pCO<sub>2 </sub>treatments (400ppmv, 750ppmv and 1000ppmv). During this time I characterized <em>Rhodomonas </em>sp. cell size, C:N, cellular total lipids, growth rate, cellular chlorophyll <em>a </em>concentrations and carbohydrates. <em>Rhodomonas </em>sp. cell bio-volume and total cellular lipids were the only aspects of <em>Rhodomonas </em>sp. found to be significantly affected by pCO<sub>2</sub>. On average, <em>Rhodomonas </em>sp. cell bio-volume increased by ~60% and ~100% and total cellular lipids increased by 36% and 50% when cultured under moderate and high pCO<sub>2 </sub>treatments, respectively, compared to the ambient treatment. To test my second hypothesis, the pCO<sub>2</sub>-acclimated <em>Rhodomonas </em>sp. were fed to four microzooplankton species, two tintinnid ciliates (<em>Favella ehrenbergii</em> (recent name change to <em>Schmidingerella </em>sp.) and <em>Coxliella </em>sp.) and two heterotrophic dinoflagellates (<em>Gyrodinium dominans </em>and <em>Oxyrrhis marina</em>). Two experimental designs were used to test whether microzooplankton grazing and growth are affected by OA through changes in prey state. My data confirm my hypothesis that microzooplankton grazing is affected by OA-induced changes to their prey. In three out of the four grazers tested, short term ingestion rates were either higher or non-linear when grazers fed on moderate and high pCO<sub>2 </sub>acclimated <em>Rhodomonas</em> sp., compared to the ambient treatment cells. Using multiple linear regression models to test for the factors that explain the observed variation in microzooplankton short term ingestion rates across pCO<sub>2</sub> treatments, prey cell bio-volume explained 43, 82 and 88% of the variability in short term grazing rates for <em>O. marina, G. dominans </em>and <em>F. ehrenbergii, </em>respectively. In contrast to the short term grazing results, I found that during long term grazing experiments, <em>G. dominans </em>and <em>Coxliella </em>sp. grazed ambient pCO<sub>2</sub> acclimated <em>Rhodomonas</em> sp. significantly faster than moderate and high cultured cells. <em>O. marina </em>demonstrated a non-linear feeding response in both short and long term grazing experiments, where <em>O. marina </em>ingested moderate pCO<sub>2</sub> acclimated <em>Rhodomonas </em>sp. faster than ambient and high pCO<sub>2</sub> acclimated prey. Microzooplankton growth rates were higher for all microzooplankton species when feeding on <em>Rhodomonas </em>sp. cultured under moderate and high pCO<sub>2</sub> compared to ambient pCO<sub>2</sub> diets. <em>G. dominans </em>and <em>Coxliella </em>sp. were the only grazers that demonstrated a difference in GGE across treatments, showing increased GGE when feeding on prey cultured under elevated pCO<sub>2</sub>. These findings validate my hypothesis that OA-induced changes in <em>Rhodomonas </em>sp. morphology and biochemistry affects microzooplankton grazing and growth. If the alteration of phytoplankton morphology and nutritional quality observed in this study is wide spread across phytoplankton taxa under OA, and this, in turn, affects microzooplankton grazing and growth dynamics as seen here, it will serve as a mechanism to alter future biogeochemical processes in pelagic marine food webs.</p>