Metabolic responses of subtropical microplankton after a simulated deep-water upwelling event suggest a possible dominance of mixotrophy under increasing CO2 levels

• Published in: Frontiers in Marine Science
• Main Authors: Tames-Espinosa, M, Martinez, I, Romero-Kutzner, V, Coca, J, Alguero-Muniz, M, Horn, HG, Ludwig, A, Taucher, J, Bach, L, Riebesell, U, Packard, TT, Gomez, M
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Research Foundation 2020
• Subjects: mixotrophy; phytoplankton; ocean acidification; mesocosms; nutrient fertilization; subtropical North-Atlantic; potential respiration; plankton metabolism; North Atlantic; Ocean acidification
• Online Access: https://eprints.utas.edu.au/40134/; https://eprints.utas.edu.au/40134/1/139161%20-%20Metabolic%20responses%20of%20subtropical%20microplankton%20after%20a%20simulated%20deep-water.pdf

In the autumn of 2014, nine large mesocosms were deployed in the oligotrophic subtropical North-Atlantic coastal waters off Gran Canaria (Spain). Their deployment was designed to address the acidification effects of CO2 levels from 400 to 1,400 μatm, on a plankton community experiencing upwelling of nutrient-rich deep water. Among other parameters, chlorophyll a (chl-a), potential respiration (Φ), and biomass in terms of particulate protein (B) were measured in the microplankton community (0.7–50.0 μm) during an oligotrophic phase (Phase I), a phytoplankton-bloom phase (Phase II), and a post-bloom phase (Phase III). Here, we explore the use of the Φ/chl-a ratio in monitoring shifts in the microplankton community composition and its metabolism. Φ/chl-a values below 2.5 μL O2 h−1 (μg chl-a)−1 indicated a community dominated by photoautotrophs. When Φ/chl-a ranged higher, between 2.5 and 7.0 μL O2 h−1 (μg chl-a)−1, it indicated a mixed community of phytoplankton, microzooplankton and heterotrophic prokaryotes. When Φ/chl-a rose above 7.0 μL O2 h−1 (μg chl-a)−1, it indicated a community where microzooplankton proliferated (>10.0 μL O2 h−1 (μg chl-a)−1), because heterotrophic dinoflagellates bloomed. The first derivative of B, as a function of time (dB/dt), indicates the rate of protein build-up when positive and the rate of protein loss, when negative. It revealed that the maximum increase in particulate protein (biomass) occurred between 1 and 2 days before the chl-a peak. A day after this peak, the trough revealed the maximum net biomass loss. This analysis did not detect significant changes in particulate protein, neither in Phase I nor in Phase III. Integral analysis of Φ, chl-a and B, over the duration of each phase, for each mesocosm, reflected a positive relationship between Φ and pCO2 during Phase II [α = 230·10−5 μL O2 h−1 L−1 (μatm CO2)−1 (phase-day)−1, R2 = 0.30] and between chl-a and pCO2 during Phase III [α = 100·10−5 μg chl-a L−1 (μ atmCO2)−1 (phase-day)−1, R2 = 0.84]. At the end of Phase II, a harmful algal species (HAS), Vicicitus globosus, bloomed in the high pCO2 mesocosms. In these mesocosms, microzooplankton did not proliferate, and chl-a retention time in the water column increased. In these V. globosus-disrupted communities, the Φ/chl-a ratio [4.1 ± 1.5 μL O2 h−1 (μg chl-a)−1] was more similar to the Φ/chl-a ratio in a mixed plankton community than to a photoautotroph-dominated one.