Responses of Free-Living Planktonic Bacterial Communities to Experimental Acidification and Warming

Abstract

Climate change driven by human activities encompasses the increase in atmospheric CO₂ concentration and sea-surface temperature. Little is known regarding the synergistic effects of these phenomena on bacterial communities in oligotrophic marine ecosystems that are expected to be particularly vulnerable. Here, we studied bacterial community composition changes based on 16S rRNA sequencing at two fractions (0.1-0.2 and >0.2 μm) during a 10- day fully factorial mesocosm experiment in the eastern Mediterranean where the pH decreased by ~0.3 units and temperature increased by ~3 °C to project possible future changes in surface waters. The bacterial community experienced significant taxonomic differences driven by the combined effect of time and treatment; a community shift one day after the manipulations was noticed, followed by a similar state between all mesocosms at the third day, and mild shifts later on, which were remarkable mainly under sole acidification. The abundance of Synechococcus increased in response to warming, while the SAR11 clade immediately benefited from the combined acidification and warming. The effect of the acidification itself had a more persistent impact on community composition. This study highlights the importance of studying climate change consequences on ecosystem functioning both separately and simultaneously, considering the ambient environmental parameters.

Keywords: acidification; climate change; mesocosms; small bacteria; warming.

Figure 4

Percentage contribution of bacterial phyla (0.1–0.2 μm fraction) in the mesocosms at T3, T6 and T10. “Other” phyla include Acidobacteria, Chlamydiae, Chloroflexi, Firmicutes, TM6, TM7 and ZB3. For the treatment abbreviations, please refer to Figure 1.

Figure 1

Percentage contribution of bacterial phyla (>0.2 μm fraction) in the mesocosms at T–1, T0, T3, T6 and T10. “Other” phyla include Acidobacteria, Chlorobi, Chloroflexi, Firmicutes, Gemmatimonadetes, NC10, Nitrospirae, Spirochaetes and ZB3. C: control mesocosms, W: mesocosms where the temperature was increased by 3 °C (labeled after warming), OA: mesocosms where the pH was decreased by 0.3 units (labeled after ocean acidification), GH: mesocosms where the temperature was increased by 3 °C and the pH was decreased by 0.3 units (labeled after greenhouse).

Figure 2

The abundances of (a) heterotrophic bacteria (cells mL⁻¹) and (b) Synechococcus (Synechococcus, cells mL⁻¹) in the mesocosms at T–1, T0, T3, T6 and T10. Colored asterisks above symbols denote significant differences from the controls. For the treatment abbreviations, please refer to Figure 1.

Figure 3

Distance-based redundancy analysis (dbRDA) plot of the distance-based linear model of the pH and temperature measurements and the concentrations of nitrate (NO₃^-) and ammonium (NH₄⁺) fitted to the bacterial community composition (genus level). The length and direction of the vectors indicate their relative strength and direction of relationship in the ordination plot, respectively. Community similarity was calculated via the Bray–Curtis index after a square–root transformation of the bacterial genera read counts. The samples are presented based on the mesocosm treatment (symbols) and experimental day (numbers above symbols). For the treatment abbreviations, please refer to Figure 1.