It looks like Isaac Asimov's Nemesis is it.

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.

It looks like Isaac Asimov's Nemesis is it.

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.

It looks like Isaac Asimov's Nemesis is it.

Here's the scene with the water:

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.

It looks like Isaac Asimov's Nemesis is it.

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.