Simulated ocean acidification reveals winners and losers in coastal phytoplankton

Abstract

The oceans absorb ~25% of the annual anthropogenic CO2 emissions. This causes a shift in the marine carbonate chemistry termed ocean acidification (OA). OA is expected to influence metabolic processes in phytoplankton species but it is unclear how the combination of individual physiological changes alters the structure of entire phytoplankton communities. To investigate this, we deployed ten pelagic mesocosms (volume ~50 m3) for 113 days at the west coast of Sweden and simulated OA (pCO2 = 760 μatm) in five of them while the other five served as controls (380 μatm). We found: (1) Bulk chlorophyll a concentration and 10 out of 16 investigated phytoplankton groups were significantly and mostly positively affected by elevated CO2 concentrations. However, CO2 effects on abundance or biomass were generally subtle and present only during certain succession stages. (2) Some of the CO2-affected phytoplankton groups seemed to respond directly to altered carbonate chemistry (e.g. diatoms) while others (e.g. Synechococcus) were more likely to be indirectly affected through CO2 sensitive competitors or grazers. (3) Picoeukaryotic phytoplankton (0.2-2 μm) showed the clearest and relatively strong positive CO2 responses during several succession stages. We attribute this not only to a CO2 fertilization of their photosynthetic apparatus but also to an increased nutrient competitiveness under acidified (i.e. low pH) conditions. The stimulating influence of high CO2/low pH on picoeukaryote abundance observed in this experiment is strikingly consistent with results from previous studies, suggesting that picoeukaryotes are among the winners in a future ocean.

Fig 1. Four potential scenarios how phytoplankton bloom development could be altered by ocean acidification explained with the example of chla concentration.

Blue and red lines illustrate control and “treatment”, respectively. (A) Change in bloom amplitude. (B) Change in bloom timing. (C) Change in bloom amplitude and timing. (D) Change in bloom pattern.

Fig 2. Chla development over time.

Red and blue lines display the average of five high and five ambient CO₂ mesocosms, respectively. Shaded areas represent standard deviations from means. Vertical grey lines (Roman numbers I to IV) separate the four experimental phases.

Fig 3. Development of phytoplankton groups quantified by flow cytometry and filter counts.

Red and blue lines display the average of five high and five ambient CO₂ mesocosms, respectively. Shaded areas represent standard deviations from means. Data are displayed on linear (A-H) and logarithmic y-axis (I-P). Note: the exponent in A-H after a group name needs to be multiplied with the y-axis numbering (e.g. 5 Syn x 10⁴ → 50000 Synechoccocus cells mL^-1). Vertical grey lines (Roman numbers I to IV) separate the four experimental phases.

Fig 4. Development of phytoplankton classes based on CHEMTAX pigment taxonomy.

Red and blue lines show the average of five high and five ambient CO₂ mesocosms, respectively. Shaded areas represent standard deviations from means. The y-axis shows the amount of chla contributed by each class. Vertical grey lines (Roman numbers I to IV) separate the four experimental phases.

Fig 5. Relative chla contribution of the 8 phytoplankton classes determined with CHEMTAX to bulk chla.

(A) Average of the control mesocosms. (B) Average of the high CO₂ mesocosms. Vertical grey lines (Roman numbers I to IV) separate the four experimental phases. Chryso = Chrysophyceae; Cyano = Cyanophyceae; Dino = Dinophyceae; Pras = Prasinophyceae; Chloro = Chlorophyceae; Crypto = Cryptophyceae; Prym = Prymnesiophyceae; Dia = Diatoms.

Fig 6. SEM pictures of important pico- and nanophytoplankton species during the two major phytoplankton blooms.

A) Representative overview picture from M1 on day 35 including Arcocellulus sp., Minidiscus sp., and Tetraparma sp. (all three are silicifying species). B) Three Minidiscus sp. cells without organic membrane cover in M6 on day 35. C) Minidiscus sp. without organic membrane cover in M4 on day 27. D) Arcocellulus sp. in M1 on day 27 E) Two Tetraparma sp. cells in M1 on day 35. F) Three spherical cells (probably picophytoplankton) in M1 on day 35. Yellow scale bars are 3 μm long.