( ( This ( organic material ) ) ( ( ( represents ( a ( rich ( food source ) ) ) ) ( for ( ( heterotrophic bacteria ) ( in ( the system ) ) ) ) ) . ) )
(ROOT (S (NP (DT This) (JJ organic) (NN material)) (VP (VBZ represents) (NP (DT a) (JJ rich) (NN food) (NN source)) (PP (IN for) (NP (NP (JJ heterotrophic) (NNS bacteria)) (PP (IN in) (NP (DT the) (NN system)))))) (. .)))
This organic material represents a rich food source for heterotrophic bacteria in the system.
(ROOT (S (SBAR (IN Because) (S (NP (DT all) (NNS animals)) (VP (VBP require) (NP (NP (DT an) (JJ external) (NN source)) (PP (IN of) (NP (NN food))))))) (, ,) (NP (PRP they)) (VP (VBP are) (VP (VBN called) (S (ADJP (JJ heterotrophic))))) (. .)))