Tommy Gold looks increasingly prescient. Refer to his book, “The Deep Hot Biosphere”, written nearly twenty years ago. (And check him out on Wikipedia - he also first stated how the ear hears different frequencies and that pulsars are spinning neutron stars.)
These organisms are close to the root of the tree. Photosynthesis came much later.
Tom Gold wrote that paper after the discovery of life forms in underwater thermal vents, during that period he was trying to build a theory of non-biological formation of fossils fuels the discovery of life forms living in extreme conditions at extreme depths made him reconsider his stance somewhat but not to abandon it.
His paper was about microbial life feeding on petroleum and so he postulated that it would have to reach several KM in depth at least.
These organisms are close to the root of the tree. Photosynthesis came much later.