Supporting ourselves in space means developing and demonstrating technologies on Earth.
This is one excellent example: Nemo's Garden, a project to build underwater hydroponics bays. A structure is anchored to the seafloor a few meters underwater; compressed air is used to inflate it. Plants grow as they do in any other hydro environment. The system collects fresh water from evaporation and condensation, so no external water source is required. The temperature and humidity are very stable and pests are eliminated. Gas exchange with the seawater (presumably) allows for O2/CO2 transport. Nutrient solutions would still be required, as well as some electrical power for monitoring, pumps, etc.
This is something that can be deployed without large real-estate expenses, without straining fresh water supplies and without the risk of weather-induced losses. Under the right conditions these systems could produce clean fresh water as a byproduct. Island nations with minimal arable land could meet their food needs without imports even as rising sea levels threaten their growing areas.
Something interesting: plants in the test systems grow faster than they should. One theory is that the increased pressure at depth may allow for more efficient respiration. If true, that means the hydro bays of future colonies could be operated at greater than 1 atmosphere for increased efficiency. It's at least something to consider during engineering trades. It's possible this is simply that the partial pressure of CO2 is closer to ideal at normal concentration and increased total pressure, in which case a reduced-pressure atmosphere with high CO2 concentration might achieve the same result. We won't know until someone does the research.
In the long run, the technologies required for permanence in space will enable human habitation of the worst Earth has to offer: deserts, tundra, mountaintops, deep caves and under water. Those same technologies could make life easier and safer for people who live on the margins today.
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