Redux Observable: How to use action.payload in latter part of chain?

I've ran into this issue quite a few times where I want to access action.payload further down the chain. But by then, the argument passed to mergeMap has already changed to something else.

Given my action looks like this:

{
  type: BUY_GEMS,
  payload: { value: 123, productIdentifier: "ABC123" }
}

And this epic:

function purchaseGems(action$, store) {
  return action$
    .ofType(BUY_GEMS)
    .mergeMap(action => {
      const { productIdentifier } = action.payload; // <-------- works because it's the first mergeMap in this sequence
      return Observable.fromPromise(
        // Some promise call
      ).catch(error => Observable.of(buyGemsRejected(error)));
    })
    .mergeMap(action => {
      const { value } = action.payload; // <----------- doesn't work because "action" is now just the response of the Promise above.
      ...
    });
}

How would I do this?