It looks like Isaac Asimov's [Nemesis][1] is it.

![enter image description here][2]


Here's the scene with the face in the water (ch. 33 - Mind):

> She was staring at the creek, and she suddenly realized that while she
> had been communicating with the voice in her mind, the creek had been
> the only thing she had been sensing. She had not been aware of
> anything else around her. It was as though her mind had enclosed
> itself, in order to make it more sensitive to the one thing that had
> filled it.
> 
> And now the veil lifted. The water was moving along the rocks,
> bubbling over them, swirling in a small eddy in a space marked off by
> several of those bubbles. The small bubbles turned and broke, even as
> new ones formed, setting up a pattern that, in essence, didn’t change,
> and in fine detail was never repeated.
> 
> Then, one by one, the bubbles broke noiselessly and the water was flat
> and featureless, but still turned. How could she see it turn if it
> were featureless? Because it glistened very slightly in the pink light
> of Nemesis. It turned and she could see it turn because the shimmers
> formed arcs that spiraled as they turned and coalesced. Her eyes were
> caught in it, slowly following the turns as they collected into the
> caricature of a face, two dark holes for eyes, a slash for a mouth.
> 
> It grew sharper, as she watched, fascinated.
> 
> And it took on definition and became a face, staring up at her with
> empty eyes, yet real enough to recognize.
> 
> It was the face of Aurinel Pampas.

The rest of your description checks out as well - main character named Marlene, with a highly adaptable brain. Otherwise-barren planet covered with "bacterial" life. Colonists going insane due to the hive-mind bacteria trying to communicate - all there.


  [1]: http://www.goodreads.com/book/show/302625.Nemesis
  [2]: https://i.sstatic.net/xCI9k.jpg