Ocean acidification impacts bacteria – phytoplankton coupling at low-nutrient conditions

• Published in: Biogeosciences
• Main Authors: Hornick, T., Bach, L.T., Crawfurd, K.J., Spilling, K., Achterberg, E.P., Woodhouse, J.N., Schulz, K.G., Brussaard, C.P.D., Riebesell, U., Grossart, H.-P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Ocean acidification
• Online Access: https://www.vliz.be/imisdocs/publications/34/299534.pdf

The oceans absorb about a quarter of the annuallyproduced anthropogenic atmospheric carbon dioxide(CO2/, resulting in a decrease in surface water pH, aprocess termed ocean acidification (OA). Surprisingly littleis known about how OA affects the physiology of heterotrophicbacteria or the coupling of heterotrophic bacteriato phytoplankton when nutrients are limited. Previous experimentswere, for the most part, undertaken during productivephases or following nutrient additions designed tostimulate algal blooms. Therefore, we performed an in sitularge-volume mesocosm (?55m3/ experiment in the BalticSea by simulating different fugacities of CO2 (fCO2/ extendingfrom present to future conditions. The study wasconducted in July–August after the nominal spring bloom, inorder to maintain low-nutrient conditions throughout the experiment.This resulted in phytoplankton communities dominatedby small-sized functional groups (picophytoplankton).There was no consistent fCO2-induced effect on bacterialprotein production (BPP), cell-specific BPP (csBPP)or biovolumes (BVs) of either free-living (FL) or particleassociated(PA) heterotrophic bacteria, when considered asindividual components (univariate analyses). PermutationalMultivariate Analysis of Variance (PERMANOVA) revealeda significant effect of the fCO2 treatment on entire assemblagesof dissolved and particulate nutrients, metabolic parametersand the bacteria–phytoplankton community. However,distance-based linear modelling only identified fCO2as a factor explaining the variability observed amongst themicrobial community composition, but not for explainingvariability within the metabolic parameters. This suggeststhat fCO2 impacts on microbial metabolic parameters occurredindirectly through varying physicochemical parametersand microbial species composition. Cluster analyses examiningthe co-occurrence of different functional groups ofbacteria and phytoplankton further revealed a separation ofthe four fCO2-treated mesocosms from both control mesocosms,indicating that complex trophic interactions might bealtered in a future acidified ocean. Possible consequences fornutrient cycling and carbon export are still largely unknown,in particular in a nutrient-limited ocean.