Chemical Changes During Decomposition 



The two general processes that occur 

 during decomposition, loss of plant com- 

 pounds and synthesis of microbial biomass, 

 can be incorporated into a generalized 

 model of chemical changes. Initially, the 

 leaves of turtle grass, manatee grass, and 

 shoal grass contain 9% to 22% protein, 6% 

 to 31% soluble carbohydrates, and 25? to 

 44% ash (dry weight basis), depending on 

 species and season (Dawes and Lawrence 

 1980). Direct assays of crude fiber by 

 Vicente et al . (1978) yielded values of 

 59% for turtle grass leaves; Dawes and 

 Lawrence (1980) classified this material 

 as "insoluble carbohydrates" and calcu- 

 lated values of 34% to 41% for this spe- 

 cies by difference. Initially, losses 

 through translocation and leaching will 

 lead to a decrease in certain components. 

 Thus, the organic carbon and nitrogen con- 

 tent will be decreased, and the remaining 

 material will consist primarily of the 

 highly refractive cell wall compounds 

 (cellulose, hemicellulose, and liqnin) and 

 ash (Harrison and Kann 1975b; Thayer 

 et al. 1977). 



As microbial degradation progresses, 

 the nitrogen content will increase through 

 two processes: oxidation of the remaining 

 nitrogen-poor seagrass compounds and syn- 

 thesis of protein-rich microbial cells 

 (typically 30% to 50% protein) (Thayer 

 et al. 1977; Knauer and Ayers 1977). The 

 accumulation of microbial debris, such as 

 the chitin-containing hyphal walls of fun- 

 gi, may also contribute to the increased 

 nitrogen content (Suberkropp et al . 1976; 

 Thayer et al. 1977). Nitrogen for this 

 process is provided by adsorption of inor- 

 ganic and organic nitrogen from the sur- 

 rounding medium, and fixation of atmos- 

 pheric N . For tropical seagrasses, in 

 particular, there is an increase in ash 

 content during decomposition because of 

 deposition of carbonates during microbial 

 respiration and growth of encrusting algal 

 species, and organic carbon usually con- 

 tinues to decrease (Harrison and t'ann 

 1975a; Knauer and Ayers 1977; Thayer 

 et al. 1977). 



Chemic al Changes as Indicators of Food 

 Value 



Nitrogen content has long been con- 

 sidered a good indicator of the food value 



of detritus and has been assumed to repre- 

 sent protein content (Odum and de la Cruz 

 1967). Subsequent analyses of detritus 

 from many vascular plant species, however, 

 have shown that up to 30% of the nitrogen 

 is not in the protein fraction (Harrison 

 and Mann 1975b; Suberkropp et al . 1976; 

 Odum et al . 1979). As decomposition pro- 

 gresses, the non-protein nitrogen fraction 

 as a proportion of the total nitrogen can 

 increase as the result of several process- 

 es: complexing of proteins in the lignin 

 fraction (Suberkropp et al . 1976); produc- 

 tion of chitin, a major cell wall compound 

 of fungi (Odum et al . 1979b); and decompo- 

 sition of bacterial exudates (Lee et al . 

 1980). As a result, actual protein con- 

 tent may be a better indicator of food 

 value. Thayer et al . (1977) found that 

 the protein content of Zostera leaves 

 increased from standing dead to detrital 

 fractions, presumably due to microbial 

 enrichment. The role of the non-protein 

 and protein nitrogen compounds in detriti- 

 vore nutrition is not presently well 

 understood. 



Like many higher plants, tropical 

 seagrasses contain phenolic acids known as 

 allelochemicals. These compounds are known 

 to deter herbivory in many plant groups 

 (Feeny 1976). Six phenolic acids have 

 been detected in the leaves, roots, and 

 rhizomes of turtle grass, manatee grass, 

 and shoal grass (Zapata and McN'illan 

 1979). In laboratory studies two of these 

 compounds, ferulic acid and p-coumaric 

 acid, when present at concentrations found 

 in fresh leaves, inhibited the feeding 

 activities of detritivorous amphipods and 

 snails grazing on S^. al terniflora detri- 

 tus. During decompositon the concentra- 

 tions of these compounds decreased to 

 levels that did not significantly inhibit 

 the feedinq activities of the animals 

 (Valiela et al . 1979). 



Seagrass leaves may also contain com- 

 pounds that inhibit the growth of microor- 

 ganisms; this in turn would decrease the 

 usable nutritional value of the detritus. 

 Water soluble extracts of fresh or re- 

 cently detached Z^. marina leaves inhibited 

 the growth of diatoms, phytoflagellates, 

 and bacteria (Harrison and Chan 19B0). 

 The inhibitory compounds are not found in 

 older detrital leaves or ones that have 

 been partially desiccated. 



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