|Title page of Erwin Chargaff’s 1978 Heraclitean Fire, which gives an ideal of Heraclitus’ notion that the sun was “alive” in some sense, comprised of a “living fire” or fire principle of life, so to say. |
The concept of “living fire” seems to have first arisen in the work of Heraclitus; the following are his main discussions of this point of view:
“This world order, the same for all beings, was created neither by gods nor by humans; rather, it was always and is and will be eternal living fire kindled in measures and quenched in measure.”— Heraclitus (c.470BC), Diels translation 
In 1967, Eugen Fink, in discussion with Martin Heidegger, comments on this:
“At first we interpret only the second half of the fragment. Lightning, we could say, is the sudden fire, the sun is the fire in orderly passage of the course of time, but πυρ αειςωον [eternal living fire] is something we do not find in the phenomenon like the lightning and the sun.”
Another variation of the translation is as follows:
“The universe, that is the all, is made neither of gods nor men, but ever has been and ever will be an eternal living fire, kindling and extinguishing in destined measure.”— Heraclitus (c.470BC) 
On "fire", in general, Heraclitus held the following view:
“All things are an exchange for fire, and fire for all things … the transformations of fire are, first of all, sea; and half of the sea is earth, half whirlwind.”— Heraclitus (c.470BC) 
This is one interpretation of four element theory.
1. Heidegger, Martin and Fink, Eugen. (1967). Heraclitus Seminar (translator: Charles Seibert) (pg. 56). Northwestern University Press, 1979.
2. Stokes, Philip. (2002). Philosophy 100: Essential Thinkers (pg. 15). Enchanted Lion Books.
3. Chargaff, Erwin. (1978). Heraclitean Fire: Sketches from a Life Before Nature. Paul & Co Pub Consortium.
● Scott, George P. (1985). Atoms of the Living Flame: an Odyssey into Ethics and the Physical Chemistry of Free Will. University Press of America.