With nearly no knowledge, I forge ahead in ignorance:
As you suggest, the initial ones would most likely survive on the chemical seeps and, hopefully, some residual heat. Their job would be to eat and survive.
Assuming the rift is narrow enough, then atmospheric and chemical gases may mix and pool to a "very thin" layer along the bottom. We only need a centimeter of VThin to support a nice little spread of microbial action.
Assuming the above micro-atmosphere in the rift, then our second generation of microbes would be less worried about survival, and more interested in living along the fringes of the seeps. In lining the edges of the seeps, they take additional nutrients from the air and sand, and fix certain nutrients back into it. They come in one kind: voraciously eat, reproduce, and quickly die. This changes the outer chemistry of the seep, and gradually changes the micro-atmosphere.
The third generation are adpated to only require nutrients from the sand, and the current mix of chemicals in the air. They are similarly voracious, but they are aided by a fourth generation which reverse some of the fixed chemicals in the soil, probably at the expense of parasitism on the third gen microbes.
A fifth and, probably, sixth generation adapt to the increasingly complex sands prepared by the previous generations, subtly shifting the chemical compositions, and exchanging them with those found in the air, as well. It's at this point that photosynthesis probably starts to come into play -- perhaps the sixth gen is the first slower, stronger grower that fixes itself in place, manages a chemical cycle in the sands and air, and uses the sun as a catalyst.
Two subsequent types see specializations in the photosynthetic root stock, filling in niches where chemical accumulations present opportunties.
Meanwhile, two other types develop extremophile functions -- one perhaps heading up the rift valley, able to live in near-trace atmosphere, while the other may head back into the seep, being better able to process the chemicals than the original, which may go extinct (if it hasn't already).
So where are we now? We have microbes designed to live in the seep, on the edges of the seep, on the sands around it, on the rift walls. I think the most complex interactions will be on the sands, and perhaps along the rift walls, where a symbiotic relationship is probably necessary for survival.
