This may be implied by the statements in the argument, but it is not the main conclusion because plankton are only used as an example to support the more general claim that many species beneficially alter their environments.

The author never makes a claim that the plankton’s effects on the environment benefit the Earth as a whole. This is easy to assume based on knowledge that global warming is harmful, but it is not something the author says.

This accurately captures the author’s main conclusion. The rest of the argument, i.e. the plankton example, provides support for the author’s claim that beneficial environment alteration is common and not restricted to highly-evolved species.

This is not something the author claims. Based on the argument, plankton might be sufficient to emit particles that cause cloud cover, but we don’t know if they’re necessary or if there may be other sources of these particles.

The author never says anything about how other species are affected by the changes plankton make. The focus is on one species at a time, not the wider effects.