I'd really like to write a story taking place on an alien world that involves a totally new kingdom of life, and I'd like it to be at least somewhat realistic. The idea I have right now is a whole bunch of organisms which exist near the surface of oceans, floating or grounded, which use some kind of root system to modulate the concentration of NaCl in their leaves (or whatever they're surface area maximizing structures are above water) which contain what was once haloarchaea. The planet is orbiting a yellow dwarf at roughly the same distance as Earth, so as I understand, the haloarchaea's phototrophy would be more efficient than photosynthesis, and could(?) produce enough energy for the rest of the organism to grow and reproduce.
Alright, here's my explanation for how this kingdom would have evolved (if you are a biologist, you might quickly notice that I am not one):
The conditions that I'll start with are a terrestrial planet with Earth-like mass and one moon. The main difference between the planet and early Earth is the abundance of oxygen and no substantial amount of greenhouse gasses. Thus, a lot of the water is frozen at the poles, except we have 2 big oceans near the equator, both of which contain a bunch of hot springs, so microbial life evolves independently in both. I'm not sure how to explain this part, but ocean A has ridiculously high salinity, a little over 2 M NaCl. Ocean B has salinity similar to Earth's oceans.
Due to the high salinity and the presence of the yellow dwarf, microbes similar to haloarchaea begin to dominate ocean A. You're probably thinking "the haloarchaea of ancient Earth couldn't have survived in that climate," but, as I understand, part of the purple-Earth hypothesis is that some of them developed the ability to live through the the oxygen crisis and survive to present day, so let's just say that the archaea here have that ability naturally.
Due to the single moon of the planet, tides change and every so often the oceans almost touch.
I'm not exactly sure why, but exciting things are happening in ocean B. While the archaea of ocean A are so far happy not evolving and simply soaking up energy from the sun, some organisms in ocean B realized they can capitalize on the high amounts of oxygen to perform cellular respiration. They have their own Cambrian explosion and we begin to see lots of filter feeders anchored to ocean floor as well as primitive mineral-digesting fungi on the coastlines (oh yeah, there are significant temperature gradients so lots of winds means oxygenated water, not sure if that's consistent with the atmosphere I've described).
But this means trouble for ocean A, organisms from ocean B are producing carbon dioxide, causing the atmosphere to very slowly heat up, thus melting the ice caps. This initiates a positive feedback loop because there is also some $CO_2$ trapped in the ice. The increasing amount of liquid water means decreasing salinity for ocean A, it also means that when the tides are right, there is a connection between oceans A and B. This puts a lot of pressure on the archaea to evolve past their dependence on high salinity.
As liquid water becomes more and more plentiful, a mid-salinity region becomes established between the two lakes. Some brave haloarchaea from ocean A and some bold miscellaneous microbes from ocean B pioneer this area, it's inhospitable to both parties. Perhaps what happened next was that one of the ocean B microbes tried to eat an archaea, then happened to find out that it would get way more energy by providing the archaea with a high salt concentration. It got so much energy from doing this it was able to form a multicellular colony of itself and the archaea. Eventually, cells began to differentiate and specialize in modulating salinity, housing the archaea, and protecting the organism from the environment.
This strategy turns out to be extremely viable (would it?) and these types of hybrids diversify and dominate the ocean surfaces. As salinity continues to decrease, some of them increase in size to maximize energy production as well as the amount of salt that can be absorbed, eventually forming massive pinkish purple "ocean forests."
So, what parts of the process that I've described were most unrealistic? How could they be improved? Does the entire process need to be re-written to get the desired end-result?
EDIT: Originally I had the idea that the oceans connecting at certain tides would lead to to some microbes being better adapted to the change in salinity, so that hybridization would become more plausible. However, HDE points out in the comments that this would lead to the oceans merging in a short period of time. My new idea is that there is a wide, high-altitude region between the two oceans where they come closest to each other. Rain storms gradually erode this region away until the oceans are almost touching at high tide right when the polar ice caps begin to melt.