Metabolic responses of subtropical microplankton after a simulated deep-water upwelling event suggest a possible dominance of mixotrophy under increasing CO 2 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: Biological Sciences; Other Biological Sciences; Global Change Biology; North Atlantic
• Online Access: https://doi.org/10.3389/fmars.2020.00307; http://ecite.utas.edu.au/139161

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 CO 2 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 (0.750.0 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 O 2 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 O 2 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 O 2 h −1 (μg chl- a ) −1 , it indicated a community where microzooplankton proliferated (>10.0 μL O 2 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 pCO 2 during Phase II [α = 23010 −5 μL O 2 h −1 L −1 (μatm CO 2 ) −1 (phase-day) −1 , R 2 = 0.30] and between chl- a and pCO 2 during Phase III [α = 10010 −5 μg chl- a L −1 (μ atmCO 2 ) −1 (phase-day) −1 , R 2 = 0.84]. At the end of Phase II, a harmful algal species (HAS), Vicicitus globosus , bloomed in the high pCO 2 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 O 2 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.