( Macroalgae ( ( play ( ( a ( major role ) ) ( in ( ( the ( carbon cycle ) ) ( of ( coastal ecosystems ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Macroalgae)) (VP (VBP play) (NP (NP (DT a) (JJ major) (NN role)) (PP (IN in) (NP (NP (DT the) (NN carbon) (NN cycle)) (PP (IN of) (NP (JJ coastal) (NNS ecosystems))))))) (. .)))
Macroalgae play a major role in the carbon cycle of coastal ecosystems.
(ROOT (S (NP (NP (DT A) (JJ primary) (NN role)) (PP (IN of) (NP (NP (NNS decomposers)) (PP (IN in) (NP (DT an) (NN ecosystem)))))) (VP (VBZ is) (SBAR (IN that) (S (NP (PRP they)) (VP (VBZ cycle) (NP (NN matter)) (PP (IN by) (S (VP (VBG returning) (NP (NN carbon) (CC and) (JJ other) (NN matter)) (PP (TO to) (NP (DT the) (NN ecosystem)))))))))) (. .)))
A primary role of decomposers in an ecosystem is that they cycle matter by returning carbon and other matter to the ecosystem.