the oldest photosynthesizing organism

Jacinta: It's probable that photosynthesis evolved slowly, bitsily, growing gradually more sophisticated, more complete - by which I mean growing to be what we recognize it as today. For example, modern organisms use photosynthesis to make oxygen, and starches and sugars, from water, but in earlier environments it's likely that something other than water was used.
Canto: Right, if water can be utilized by some sort of redox reaction, then so can other molecules. But how long has water existed on Earth?

Jacinta: Don't sidetrack me Canto. One of the world's foremost authorities on photosynthesis and its origins is Carl Bauer of Indiana University. In the graphic above, taken from the website of Bauer's lab, he presents an outline of the origin of 'oxygen evolving organisms', by which he presumably means photosynthesizing organisms that generate oxygen.
Canto: Right, so he's taking biological carbon fixation back to 3.8 billion years or so.
Jacinta: At that time, our atmosphere was rich in carbon dioxide. In any case, no matter what molecules were used to produce fuel for metabolism, Bauer has found, through detailed phylogenetic analysis, that Rhodobacter, aka purple bacteria are the oldest lineage of photosynthetic organisms on the planet.
Canto: But they don't 'evolve oxygen' as he puts it?
Jacinta: Right, they're anoxygenic. In fact, there are five known branches of microbes that engage in chlorophyll-based photosynthesis, and only one of them, cyanobacteria, evolve oxygen.
Canto: Can you explain what phylogenetic analysis might be - not for me of course, but for our vast readership out there.
Jacinta: Of course. To examine the phylogeny of an organism, or indeed an organ, is to trace its evolutionary development. The phylogenetic analysis we're talking about here involves molecular phylogenetics. Closely related organisms have similar molecular structures, re their genetic material and their proteins. There's a pattern of similarity to related organisms traceable back in time, revealing a pattern of evolution. There's more to it than that, but it'll do for now.
Canto: So tell me more about these anoxygenic purple bacteria.
Jacinta: Well, Rhodobacter have long been of interest to molecular biologists because of their interesting metabolic processes, and they're the most studied of micro-organisms. They live in water - freshwater or marine environments, and their diverse methods of survival - not just through photosynthesis but through a variety of processes - has made them a favourite for study, due to our own concerns about survival.
Canto: Okay, I'll keep them in mind - but you've pointed out that they're aquatic, which raises the question again - when did water first appear on Earth, and what about non-photosynthesizing organisms that might be older than Rhodobacter?
Jacinta: Yes, all that is interesting, along with the actual origins of these Rhodobacter, and the origins of viruses, which we haven't gotten into as yet. Everybody is obsessed with water as an essential source of life, so that when we look for life elsewhere, we tend to look for signs of water - but it ain't necessarily so.
Canto: Wow, lots to explore there - can't wait for our next little chat!