Our oceans are suffering because of overfishing, pollution, warming and CO2 gas absorption (acidification). The effects are devestating if we don’t act on them. For instance, the loss of oxygen will mean fish species dissapear, iron is lost and dangerous hydrogen sulfide buildups can occur. The temperature rise means the amount of gas the oceans can contain is reduced, and methane, CO2, N2, and other gasses enter the athmosphere. The concentration of CO2 in ocean water is a multiple of that in the air, so it would accelerate global warming, leading to an almost unstoppeable venting of gas.
On idea to counter these trends is to use the oceans to absorb more CO2, by using nutrients found at debts of 200 meter in combination with sunlight at the surface. Algae and seaweed can grow at amazing speeds when fed well, and pyrolizing them (burning without oxygen) means you can dump carbon to the deep where it can stay in an anoxic environment for millions of years, like our burned coal reserves. On a large scale this would certainly reverse the rise in CO2 in our atmosphere. To do this we thougth of Bambooya, a giant island created out of bamboo that would create a maritime base even in the middle of the Pacific.
Another thing that could certainly help would be to generate oxygen in the deep ocean. A growing area of our oceans has become anoxic, due to fertilizers from rivers causing algae blooms. Another factor reducing oxygen is the slowing ‘ocean conveyer belt’, the current from equator to the poles. More oxygen not only sustains oxygen breathing life, but also helps organisms that use photosynthesis by keeping iron dissolved (as rust). Iron can be a limiting factor to growth as was demonstrated by several iron fertilization experiments. But how would you generate oxygen so deep in under sea?
One way to do it would be to have solar panels floating at the surface. Then run a cable to the deep and split water there. This would allow for several other interesting processes, like generating liquid fuel near the surface using the H2 while leaving the O2 in the deep.
Another way would be to place a tidal or current driven turbine to do the same, generate oxygen, perhaps pump nutrient rich water to the surface (something that happens near the coast and sustains fish habitats). Such turbines mid ocean could become sources of liquid fuels for shipping.
The last source of energy at the bottom of the ocean is geothermal. Vulcanic vents can be used to generate the power to drive fertilization or capture carbon. According to the below video the potential is huge. There is thousands of miles of ridges where geothermal energy can be found at shallow depths. Instead of using tubes to bring the gasses and chemicals to the surface perhaps some type of submersible power plant can be developed that can be sunk to the vent, and do it’s usefull work there.