'A litre of sea water may contain a billion organisms of these single-celled plants, which are the first links in the marine food chain.
(A litre of sea water; may contain; a billion organisms of these single-celled plants) (these single-celled plants; are; the first links in the marine food chain)
A litre of sea water these single-celled plants
may contain are
a billion organisms of these single-celled plants
the first links in the marine food chain