# How could an Earth-like planet develop huge pinkish-purple forests on ocean surfaces?

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.

• Welcome to Worldbuilding, and bravo on your initial research! I agree with your rationale on the pigmentation choice; I'd expect the bacteriochlorophylls to be dominant in the 700-1000 nm range, right near the peak emission of a K-type star. One question: Can you talk a bit more about the division between the two oceans? If simple tides can be enough to bridge the gap, I feel like eventually the two oceans would merge on short geological timescales. – HDE 226868 Aug 8 '18 at 22:25
• This might seem stupid, but does the surface of the ocean mean ocean floor or sea level? – Totillity Aug 8 '18 at 22:26
• @Totility Sea level. The idea is that these organisms would want to be above water to get as much sunlight as possible, but they would never leave the sea because they need NaCl in order to produce energy. In shallow water some of them would anchor to the floor, and in deep sea they would all just float about. – Eben Cowley Aug 8 '18 at 22:50
• @HDE226868 You make a good point, I'm going to add an edit based on this fact. – Eben Cowley Aug 8 '18 at 22:51

So as I understand: (I'm using bacteria even though they aren't bacteria)

Period 1: Two types of bacteria independently evolve, one which makes energy extremely efficiently from sun but needs salt, and another which is a chemoautotroph.

Period 2: The 2nd type of bacteria starts using oxygen instead of making it, causing global warming, causing decreased salinity.

Period 3: A new, 3rd type of bacteria evolves in which the first type of bacteria is in symbiosis with the 2nd, similar to chloroplasts and mitochondria.

Well, as far as I can tell, this can happen, but Occam's razor can be used on it. Why not remove all the complicated tides and oceans part, and just have them evolve in the same ocean? Here's the idea:

Period 1: Life evolves, the ocean is really salty.

Period 2: Some bacteria evolve to use salt to make energy more efficiently. Maybe the ions are incorporated into the electron transport chain. The others continue with business as usual.

Period 3: Some bacteria evolve to become photo-synthesizers, they start global warming, causing reduced salinity.

Period 4: Salinity not yet at dangerously low levels for the salt users, some incorporated into other bacteria like mitochondria and chloroplasts once were.

Period 5: Salinity is so low that salt users outside start dying, the hosts of salt-users which are now organelles evolve increasing salinity inside their cytoplasm.

Now the bacteria with salt-using organelles make energy really efficiently and dominated the world.

Hope this helps.