# What types of fusion reactions happened in population III stars?

I have read that, in smaller stars, such as our Sun, the fusion reaction that takes place is a proton-proton chain, or PP chain for short.

From what I have learned, in larger stars, a different process takes place, known as the CNO cycle, in which carbon, nitrogen and oxygen are utilized as a catalyst.

The first stars were many times more massive than the Sun, but pretty much the only elements they had to make use of were hydrogen and a little bit of helium. So, what types of fusion reactions went on in the first stars?

EDIT: Further to this: Current thinking is that CNO burning will take place on the main sequence for Pop. III stars more massive than $$20 M_{\odot}$$. This is because they cannot be supported by the pp chain alone; they contract and heat up sufficiently to start the triple alpha He burning phase that produce carbon; once the carbon concentration builds up to about $$10^{-10}$$ of hydrogen then the CNO cycle takes over (Ekstrom et al. 2008; Yoon et al. 2012).