the microorganisms incorporate constitu- 

 ents such as nitrogen, phosphorous, and 

 dissolved organic carbon compounds from 

 the surrounding medium into their cells 

 and thus enrich the detrital complex. The 

 microorganisms also secrete large quanti- 

 ties of extracellular materials that 

 change the chemical nature of detritus and 

 may be nutritionally available to detriti- 

 vores. After initial leaching and decay, 

 those processes make microorganisms the 

 primary agents in the chemical changes of 

 detritus. 



The microbial component of macrophyte 

 detritus is highly complex and contains 

 organisms from many phyla. These various 

 components interact and influence each 

 other to such a high degree that they are 

 best thought of as a "decomposer commun- 

 ity" (Lee 1980). The structure and activ- 

 ities of this community are influenced by 

 the feeding activities of detritivorous 

 animals and environmental conditions. 



Microflora in Petri tivore Nutrition 



Microbial carbon constitutes only 10% 

 of the total organic carbon of a typical 

 detrital particle, and microbial nitrogen 

 constitutes no more than 10% of the total 

 nitrogen (Rublee et al. 1978; Lee et al . 

 1980). Thus, most of the organic compo- 

 nents of the detritus are of plant origin 

 and are limited in their availabil ity "to 

 detritivores. 



Carbon uptake from a macroalga, 

 Gracilaria , and the seagrass Zostera 

 marina by the deposit-feeding polychaete, 

 Captella capitata , was measured by Tenore 

 (1977). Uptake of carbon by the worms was 

 directly proportional to the microbial 

 activity of the detritus (measured as 

 oxidation rate). The maximum oxidation 

 rate occurred after 1*^ days for Gracilaria 

 detritus and after ICO days for Zostera 

 detritus. This indicates that the charac- 

 teristics of the original plant matter 

 affect its availability to the microbes, 

 which in turn limits the assimilation of 

 the detritus by consumers. 



Most of the published evidence shows 

 that detritivores do not assimilate 

 significant portions of the non-microbial 

 component of macrophytic detritus. For 

 example, Newell (1965) found that deposit- 

 feeding molluscs removed the nitrogen from 



sediment particles by removal of the 

 microorganisms but did not measurably 

 reduce the total organic carbon content of 

 the sediments which was presumably domi- 

 nated by detrital plant carbon. When the 

 nitrogen-poor, carbon-rich feces were 

 incubated in seawater, their nitrogen con- 

 tent increased because of the growth of 

 attached microorganisms. A new cycle of 

 ingestion by the animals again reduced the 

 nitrogen content as the fresh crop of 

 microorganisms was digested. In a study 

 of detrital leaf material, Morrison and 

 White (1980) found that the detritivorous 

 amphipod Mucrogammarus sp. ingested the 

 microbial component of live oak ( Cliercus 

 virginica ) detritus without altering or 

 consuming the leaf matter. 



While the importance of the microbial 

 components of detritus to detritivores is 

 established, some results have indicated 

 that consumers may be capable of assimi- 

 latina the plant carbon also. Cammen 

 (1980) found that only 26% of the carbon 

 requirements of a population of the 

 deposit-feeding polychaete Nereis succinea 

 would be met by ingested microbial bio- 

 mass. The microbial biomass of the in- 

 gested sediments could supply 90% of the 

 nitrogen requirements of the studied poly- 

 chaete population. The mysid Mysis steno- 

 lepsis , commonly found in Zostera beds, 

 was capable of digesting cell-wall com- 

 pounds of plants (Foulds and Mann 1978). 

 These studies raise the possibility that 

 while microbial biomass is assimilated at 

 high efficiencies of 50% to 100% (Yingst 

 1976; Lopez et al. 1977) and supplies 

 proteins and essential growth factors, 

 the large quantities of plant material 

 that are ingested may be assimilated at 

 low efficiencies (less than 5%) to supply 

 carbon requirements. Assimilation at this 

 low efficiency would not be readily quan- 

 tified in most feedinq studies (Cammen 

 1980). 



The microbial degradation of seagrass 

 organic matter is greatly accelerated by 

 the feedinq activities of detritivores and 

 microfauna, although the exact nature of 

 the effect is not clear. Microbial res- 

 piration rates associated with turtle 

 grass and Zostera detritus were stimulated 

 by the feedinq activities of animals, 

 apparently as a result of physical frag- 

 mentation of the detritus (Fenchel 1970; 

 Harrison and Mann 1975a). 



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