A journal paper co-authored by E&E Professor Cathy Pfister was recently published in Scientific Reports. Entitled “Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification”, the paper investigated microbial processes in association with the widespread Mediterranean seagrass Posidonia oceanica and how they contributed to water column productivity. The authors point out that as the ocean continues to acidify, the results of the study “suggest that microbial ammonification rates in association with seagrass will decrease. However, the demonstrated role that P. oceanica plays in enhancing carbon fixation by surrounding [particulate organic matter], particularly in low pH seawater, suggests an underappreciated role for resilient foundational species in a changing ocean.”
Researchers in France and Italy also contributed to the study, which was made possible with funding from the University of Chicago's France and Chicago Collaborating in the Sciences (FACCTS) and the National Research Agency Investments for the Future (“4Oceans-Make Our Planet Great Again” grant, ANR-17-MOPGA-001).
Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6–17.4 with the greatest effect at CO2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.