Can a conclusion be based on a flawed premise, especially when the premise is a sub-conclusion.

For example, in the following stimulus (taken from PowerScore Critical Reasoning Bible), the main conclusion ("However, this characteristic is actually quite common.") is supported by several premises including a sub-conclusion ("Thus plankton cause the surface of the earth to be cooler and this benefits the plankton."). I believe that the sub-conclusion is flawed because there's the speaker didn't proved that the cooler surface of the earth benefits the plankton.

Here's the stimulus:

It is well known that many species adapt to their environment, but it is usually assumed that only the most highly evolved species alter their environmentt in ways that aid their own survival. However, this characteristic is actually quite common. Certain species of plankton, for example generate a gas that is converted in the atmosphere into particles of sulphate. These particles cause water vapor to condense,thus forming clouds. Indeed, the formation of clouds over the ocean largely depends on the presense of these particles.More cloud cover means more sunlight is reflected, and so the earth absorbs less heat.Thus plankton cause the surface of the earth to be cooler and this benefits the plankton.

Thank you.