The effects of experimental increases in underground carbon dioxide on edaphic protozoan communities

• No patterns in the abundance of amoebae, ciliates, or flagellates.
• Ciliate diversity decreased in response to higher levels of CO2.
• The C/P ratio increased as CO2 flows increased.
• CO2 influenced the composition of the ciliate community.
• Supports the use of ciliates as promising biological indicators of CO2 effects. Further research is needed to species level.

This paper evaluates the effect of CO2 on protozoan communities, a neglected group in CO2 related studies, despite their potential role in influencing bacterial communities, and vegetation growth. For this purpose, the effect of increased CO2 in the soil upon the abundance of protozoa (amoebae, flagellates, and ciliates) and diversity of ciliates was experimentally studied in two soils. Twelve mesocosms were created with 8 m2 cells filled with the upper layer of one of the two soils. The cells were equipped with systems that controlled injection of CO2 at different depths. Two different flows of CO2 (20 and 40 l/h) were studied simulating a potential CO2 leakage from early CSS sites. Water content and pH levels were measured by continuous monitoring systems. There were no clear patterns in the abundance of amoebae, ciliates, or flagellates in response to the injection of CO2. The analysis used to evaluate the community structure of ciliates, such as Equitability, Margalef index, or the Colpodea to Polyhymenophorea ratio showed significant differences with increases in CO2. The results of this study indicate that controlled increments of CO2 in the soil influence the composition of the ciliate community, but have no effect on their total abundance. This paper supports the use of ciliates as promising biological indicators of CO2 effects in Carbon Capture and Storage operations.