[CobaltBlue]

the plankton absorb excess to keep the active amount in the atmosphere more nearly steady

Do you mean "the plankton absorb excess which keeps the active amount in the atmosphere more nearly steady"?

The sentence as originally constructed suggests that absorbing the CO2 is somehow a function of the plankton, rather than a marvellous coincidence.

Reason I am asking is that some people believe in the Gaia Hypothesis, that various interworkings of things like plankton, seawater and CO2 operate on some kind of feedback system.

[Doctor Stochastic]

Yes, "which keeps" would have been better. The (local to the plankton) absorbing of CO2 does have the side effect of keeping atmospheric concentrations more nearly constant. There is a feedback system, but the feedback can be positive or negative.