In the episode "Water," we learn that what appears to be a fresh-water lake is some sort of acid that the Terrians dip their lightening shafts into for recharging. They use the acid in turn to recharge the planet. The episode "Moon Cross" deals, in part, with the yearly ritual the Terrians perform when their two moons are in conjunction. In this ritual they return to the earth -- a kind of dying that they consider "heaven." But it is in the two-part episode "Better Living Through Morganite" that features the symbiotic nature of the planet and the Terrians most prominently.
The colonists discover some glowing rocks that store and release energy. They hope they can be used as an energy source and ask Julia, who also serves as their resident scientist, to analyze the rocks further. As they explore the underground caves where they found the rocks, Danziger and Bess witness the Terrians as they perform a ritual with their lightening staffs that results in an earthquake followed by the appearance of more "sunstones." When they tell the group what they saw, Julia reports that her analysis indicates that the rocks resemble the Terrians and perhaps the planet and the Terrians are one species. She speculates that the veins of rock may be "nerves, or circulation or intelligence -- it could be their communication network."  Morgan and Bess Martin have acquired a device called a Geo-Lock which is used to make mining claims by petrifying user-specified sections land. Morgan has figured out the code that allows him to turn the geo-lock on, but hasn't bothered to decode the release key. In greedy haste to lay claim to the sunstones before anyone else can, he turns it on anyway so that it freezes an area one kilometer in diameter. Morgan's rash action sets off a crisis. The geo-lock has not only killed one of the Terrians, but has frozen what Julia says is a vital part of the living planet. The Terrians take hostages who they intend to kill in response if the geo-lock isn't released by the next day. Morgan is doing his best to crack the code but time is running out as Devon and Bess watch. Julia breaks in on communication gear to say that she thinks the sunstones can help release the geo-lock.
Julia: I've started communicating through the sunstones but not because I've learned their language. They've learned mine.
Devon: How do you know that?
Julia: I was trying to signal some basic syntax. Just, you know, rudimentary English and they sent back a language more complex than I've ever seen before.
Devon: So what are you thinking?
Julia: I'm thinking that the Terrians are so furious, not because we froze some extraneous nerve endings of the planet but because we froze a significant portion of its cerebral cortex.
Devon: Well then how can it help us?
Julia: Well it's impaired but it's still powerful It figured out the binary language of my scanner in minutes. I'm wondering how long it would take to crack the geo-lock.
Devon: Julia, if there's any chance at all then do it. And hurry. 
[W]e have to live with the Earth in partnership, otherwise the rest of creation will, as part of Gaia, unconsciously move Earth itself to a new state, one where we humans may be no longer welcome.