Microzooplankton grazing, growth and gross growth efficiency are affected by pCO2 induced changes in phytoplankton biology

• Main Author: Still, Kelly
• Format: Text
• Language: unknown
• Published: Western Washington University 2016
• Subjects: Ocean acidification
• Online Access: https://dx.doi.org/10.25710/kmdq-bj83; https://cedar.wwu.edu/wwuet/478

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 Rhodomonas sp. would be directly affected by elevated pCO 2 and 2) that pCO 2 -induced changes in Rhodomonas sp. would affect grazing, growth rates, and GGE in microzooplankton consumers. To test my first hypothesis, I cultured the ecologically important phytoplankton, Rhodomonas sp., semi-continuously for 17 days under three pCO 2 treatments (400ppmv, 750ppmv and 1000ppmv). During this time I characterized Rhodomonas sp. cell size, C:N, cellular total lipids, growth rate, cellular chlorophyll a concentrations and carbohydrates. Rhodomonas sp. cell bio-volume and total cellular lipids were the only aspects of Rhodomonas sp. found to be significantly affected by pCO 2 . On average, Rhodomonas 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 2 treatments, respectively, compared to the ambient treatment. To test my second hypothesis, the pCO 2 -acclimated Rhodomonas sp. were fed to four microzooplankton species, two tintinnid ciliates ( Favella ehrenbergii (recent name change to Schmidingerella sp.) and Coxliella sp.) and two heterotrophic dinoflagellates ( Gyrodinium dominans and Oxyrrhis marina ). 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 2 acclimated Rhodomonas 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 2 treatments, prey cell bio-volume explained 43, 82 and 88% of the variability in short term grazing rates for O. marina, G. dominans and F. ehrenbergii, respectively. In contrast to the short term grazing results, I found that during long term grazing experiments, G. dominans and Coxliella sp. grazed ambient pCO 2 acclimated Rhodomonas sp. significantly faster than moderate and high cultured cells. O. marina demonstrated a non-linear feeding response in both short and long term grazing experiments, where O. marina ingested moderate pCO 2 acclimated Rhodomonas sp. faster than ambient and high pCO 2 acclimated prey. Microzooplankton growth rates were higher for all microzooplankton species when feeding on Rhodomonas sp. cultured under moderate and high pCO 2 compared to ambient pCO 2 diets. G. dominans and Coxliella sp. were the only grazers that demonstrated a difference in GGE across treatments, showing increased GGE when feeding on prey cultured under elevated pCO 2 . These findings validate my hypothesis that OA-induced changes in Rhodomonas 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.