Complex sentence: All phycobiliproteins are water-soluble, so cannot exist within the membrane as do carotenoids, but aggregate to form clusters that adhere to the membrane called phycobilisomes.
Split into simpler sentences: All phycobiliproteins are water-soluble, so they cannot exist within the membrane like carotenoids can. Instead, phycobiliproteins aggregate to form clusters that adhere to the membrane called phycobilisomes.