Are there any known microbes capable of living in almost pure carbon dioxide?
These aren't technically extremophiles, but one paper identified twenty species of bacteria that grow under a low pressure, mostly CO₂ atmosphere1. 

One relatively well studied species is Serratia liquefaciens (a Proteobacterium) that has been shown to grow under 


  low temperature (0 °C), low pressure (0.7 kPa), and anoxic, CO2-dominated atmosphere-conditions intended to simulate the near-subsurface environment of Mars2.


References:

1: Schuerger, A. C., &amp; Nicholson, W. L. (2016). Twenty species of hypobarophilic bacteria recovered from diverse soils exhibit growth under simulated martian conditions at 0.7 kPa. Astrobiology, 16(12), 964-976.

2: Fajardo-Cavazos, P., Morrison, M. D., Miller, K. M., Schuerger, A. C., &amp; Nicholson, W. L. (2018). Transcriptomic responses of Serratia liquefaciens cells grown under simulated Martian conditions of low temperature, low pressure, and CO 2-enriched anoxic atmosphere. Scientific reports, 8(1), 1-10.