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Compound-specific isotope analysis of amino acids reveals dependency on grazing rather than detritivory in mangrove food webs

Yota Harada*, Shing Yip Lee, Rod M. Connolly, Brian Fry

*Corresponding author:

ABSTRACT: Colonisation of decaying leaves fallen from mangrove trees by bacteria and fungi is thought to play an important role at the base of food webs in most tropical estuaries. Compound-specific isotope analysis of amino acids (CSIA-AA) has enabled the previously difficult methodological task of measuring plant, bacterial, and fungal energy flows to food webs. Here we assessed the biosynthetic origins of amino acids at the base of a mangrove food web using the CSIA-AA approach. Trophic positions of the two most common mangrove fauna - fiddler crabs and sesarmid crabs - estimated from nitrogen isotopes in phenylalanine and glutamic acid approached 2, suggesting that they are herbivores rather than microbivores. Consistent with this, carbon isotope fingerprints in AAs did not support the importance of essential AAs derived from fungi and bacteria but rather suggested the importance of those originated from plants, especially microalgae. These results suggest that (a) microbial mineralization of decaying leaves supports the production of more easily assimilated microalgae; and (b) bacteria and fungi as intermediates also routinely incorporate plant-derived AAs into their biomass.