Short-term responses to ocean acidification: effects on relative abundance of eukaryotic plankton from the tropical Timor Sea

ABSTRACT

Anthropogenic carbon dioxide (CO₂) emissions drive climate change and pose one of the major challenges of our century. The effects of increased CO₂ in the form of ocean acidification (OA) on the communities of marine planktonic eukaryotes in tropical regions such as the Timor Sea are barely understood. Here, we show the effects of high CO₂ (pCO₂=1823±161 μatm, pH_T=7.46±0.05) versus in situ CO₂ (pCO₂=504±42 μatm, pH_T=7.95±0.04) seawater on the community composition of marine planktonic eukaryotes immediately and after 48 hours of treatment exposure in a shipboard microcosm experiment. Illumina sequencing of the V9 hypervariable region of 18S rRNA (gene) was used to study the eukaryotic community composition. Down-regulation of extracellular carbonic anhydrase occurred faster in the high CO₂ treatment. Increased CO₂ significantly suppressed the relative abundances of different eukaryotic operational taxonomic units (OTUs), including important primary producers. These effects were consistent between abundant (DNA-based) and active (cDNA-based) taxa after 48 hours, e.g., for the diatoms Trieres chinensis and Stephanopyxis turris. Effects were also very species-specific among different diatoms. Planktonic eukaryotes showed adaptation to the CO₂ treatment over time, but many OTUs were adversely affected by decreasing pH. OA effects might fundamentally impact the base of marine biodiversity, suggesting profound outcomes for food web functioning in the future ocean.