54. Who is the master? Trade-off between competitive ability and resistance to elevated pCO2

Giannina S.I. Hattich

Background
Ocean acidification negatively impacts calcification and growth rate of calcifying phytoplankton species. Under future conditions species and genotypes
that are more resistant towards elevated pCO should thus have an advantage.

Theory, however, states that “a jack in every trait is a master of none”. Applying this theory, highly resistant species and genotypes towards elevated pCO might be weaker competitors. If selection in nature acts along such a trade-off curve the
selective outcome under competition between all genotypes and elevated CO at the same time should reflect the expectations raised by the trade-off curve.

Methods
We set out to test if a trade-off between competitive ability and the resistance to ocean acidification exists among nine genotypes of the planktonic calcifier Emiliania huxleyi. The resistance to elevated pCO
was assessed by comparing the plastic response in growth rate and carrying capacity among genotypes. The relative competitive ability was assessed in a separate experiment by measuring competitive exclusion in all 36 two-genotype combinations and ranking the competitive ability of each of the genotypes accordingly.

Findings
The resistance to elevated pCO conditions differed significantly between the tested genotypes. Preliminary results also show that genotypes differed in competitive ability and that competitive ability was further altered under high pCO. Awaiting the final analyses, these results point towards the existence of a trade-off between competitive ability and resistance to enhanced seawater CO concentrations.

Conclusions

Knowledge of the selective force of pCO including a potential tradeoff in competitive phytoplankton assemblages will enhance the predictive power for
projecting future changes In phytoplankton
communities in a high-CO world.