Microbial tropicalization driven by a strengthening western ocean boundary current

• Published in: Global change biology 2020-10, Vol.26 (10), p.5613-5629
• Main Authors: Messer, Lauren F., Ostrowski, Martin, Doblin, Martina A., Petrou, Katherina, Baird, Mark E., Ingleton, Timothy, Bissett, Andrew, Van de Kamp, Jodie, Nelson, Tiffanie, Paulsen, Ian, Bodrossy, Levente, Fuhrman, Jed A., Seymour, Justin R., Brown, Mark V.
• Format: Article
• Language: English
• Subjects: Aquatic crustaceans; Australia; Boundary currents; Carnivores; Chlorophyll; Chlorophyll A; Climate change; Cyanobacteria; East Australian Current; Fatty acids; Genes; Genomes; Lipids; Marine animals; Metabolism; Microbial activity; microbial community; microbial indicators; Microorganisms; ocean boundary currents; Ocean currents; Pacific Ocean; Photosynthesis; Phytoplankton; Predators; Prochlorococcus; Range extension; Seawater; Stocks; Strengthening; Structure-function relationships; Trophic levels; Trophic status; Trophic structure; Tropical climate; Tropical environments; tropicalization; Western boundary currents
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.15257

Western boundary currents (WBCs) redistribute heat and oligotrophic seawater from the tropics to temperate latitudes, with several displaying substantial climate change‐driven intensification over the last century. Strengthening WBCs have been implicated in the poleward range expansion of marine macroflora and fauna, however, the impacts on the structure and function of temperate microbial communities are largely unknown. Here we show that the major subtropical WBC of the South Pacific Ocean, the East Australian Current (EAC), transports microbial assemblages that maintain tropical and oligotrophic (k‐strategist) signatures, to seasonally displace more copiotrophic (r‐strategist) temperate microbial populations within temperate latitudes of the Tasman Sea. We identified specific characteristics of EAC microbial assemblages compared with non‐EAC assemblages, including strain transitions within the SAR11 clade, enrichment of Prochlorococcus, predicted smaller genome sizes and shifts in the importance of several functional genes, including those associated with cyanobacterial photosynthesis, secondary metabolism and fatty acid and lipid transport. At a temperate time‐series site in the Tasman Sea, we observed significant reductions in standing stocks of total carbon and chlorophyll a, and a shift towards smaller phytoplankton and carnivorous copepods, associated with the seasonal impact of the EAC microbial assemblage. In light of the substantial shifts in microbial assemblage structure and function associated with the EAC, we conclude that climate‐driven expansions of WBCs will expand the range of tropical oligotrophic microbes, and potentially profoundly impact the trophic status of temperate waters. Waters along south‐eastern Australia are undergoing rapid warming driven in large part by the expanding influence of the East Australian Current (EAC). We reveal that the EAC efficiently transports tropical microbes into temperate waters leading to 'tropicalization' of local microbial assemblages. This alteration to community structure at the base of the food chain is associated with significant reductions in standing stocks of total carbon and chlorophyll a and a shift towards smaller phytoplankton and carnivorous copepods. We suggest that climate‐driven expansions of western boundary currents more generally will potentially have profound impacts on the trophic status of temperate waters.