4.
can reproduce by spores.​

Many larger algae reproduce by spores and are also capable of sexual reproduction. ... Spores are most conspicuous in the non-seed-bearing plants, including liverworts, hornworts, mosses, and ferns. In these lower plants, as in fungi, the spores function much like seeds.

Explanation:

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water pollution kills aquatic life. aquatic life includes algae and other plant life that use photosynthesis. photosynthesis takes   c02 out of the air.  

c02is a greenhouse gas.