abundant on the surfaces of sediment particles in 

 intertidal flats. Smaller particles, because of their 

 greater surface area, contain larger populations of 

 microbes than coarser sediments (Zobell 1938, 

 Newell 1970). As a result, bacteria and probably 

 also fungi tend to be more abundant in the sedi- 

 ments of mud flats than in sand flats (Levinton 

 1972). Both bacteria and fungi play an importimt 

 ecological role in converting dead organic matter 

 to inorganic nutrients (a process termed minerali- 

 zation), which are then available to support an- 

 other cycle of primary (plant) and secondary (ani- 

 mal) production. Because this mineralization pro- 

 cess occurs in the sediments, nutrients arc made 

 rapidly available to benthic plants, whereas plank- 

 tonic algae, especially in deeper waters, are often 

 severely limited by the slow return of nutrients 

 to the water column. Fungi are commonly found 

 on decomposing plant material, whereas bacteria 

 are abundant decomposers of both plant and ani- 

 mal matter. Fungi tend to act inside the detrital 

 particle by extending long hyphae, while bacteria 

 are most commonly associated with the outer sur- 

 faces. Both fungal and bacterial decomposition are 

 aided by the activities of various animals which 

 serve to break up the detritus into progressively 

 finer particles with larger and larger surface area. 



In addition to their role as mineralizers of de- 

 tritus, bacteria (and probably also fungi) play 

 smother extremely important part in the dynamics 

 of estuarine systems. They also serve as a trophic 

 intermediate between relatively indigestible plants 

 (and plant fractions such as cellulose and lignin) 

 and potential consumers of plant detritus (de la 

 Cruz 1973, Tenore 1977). Many studies of marsh 

 plants, including Spartina (cord grass), /uncus 

 (needle rush), and Salicornia (saltwort), and of 

 seagrasscs, including Zostcra (eelgrass), Halodulc 

 (shoalgrass), and Thalassia (turtle grass), in various 

 estuarine systems around the wcirld have demon- 

 strated that very little of the primary production 

 is grazed directly by herbivores (Teal 1962, 

 Tenore 1977). Most of the plant matter dies and 

 is shed into the water. \ large fraction (probably 

 exceeding 50% in most systems) of the dead plant 

 material that is shod each year from marsh plants 

 and seagrasscs is carried away from the immediate 

 area where the plants grew and is ultimately pro- 

 cessed by dec(jmposcrs and detritivores elsewhere 

 within the estuarine or coastal marine ecosystem 

 (Oduni and dc la Cru/ 1967, dc la Cruz 1973). 



For the same reasons that render the living 

 plant largely indigestible to licrhixorcs, the dead 



plant matter is not directly usable by most detriti- 

 vores (Zobell and Feltham 1942, Newell 1965). 

 The plants (especially Spartina and /uncus) con- 

 tain high levels of carbon but few nutritious pro- 

 teins. By growing on this relatively intractable de- 

 tritus, protein-rich microorganisms provide a 

 means of making some of that energy bound up 

 in plant material available to consumers and 

 higher trophic levels (de la Cruz 1973, Tenore 

 1977). Numerous studies have demonstrated 

 that the consumption of detrital particles by vari- 

 ous detritivores results in the assimilation of the 

 bacteria but in little or no digestion of the detritus 

 itself (Fenchel 1970, Newell 1970, but see also 

 conflicting evidence in Adams and Angelovic 

 1970, Cammen et al. 1978). Passage through the 

 gut often serves to aid the process of mechanical 

 break-down of the detritus despite this lack of di- 

 rect digestion of the detritus (Fenchel 1970). This 

 explains why the reingestion of feces (termed 

 coprophagy) is a commonly observed practice 

 among many marine deposit feeders (detritivores) 

 (Johannes and Satomi 1966, Frankenberg and 

 Smitii 1967). Fecal pellets are enriched in detritus 

 but stripped of much of their bacteria and pre- 

 sumably fungi during passage through the animal's 

 gut. After it is defecated, the fecal pellet is rapidly 

 recolonized by microbes, which because of high 

 bacterial growth rates quickly increase in abun- 

 dance, such that after a sufficient lag time the fe- 

 cal pellet is again suitable for consumption by 

 detritivores (Newell 1965, Fenchel 1970, Har- 

 grave 1970). Many researchers who study marine 

 benthic deposit feeders believe that this processs 

 of microbial renewal on detritus is an important 

 rate-limiting step which determines the abun- 

 dances of various deposit-feeding species in ma- 

 rine benthic communities (Levinton 1972, Levin- 

 ton and Lopez 1977). The best studied of these 

 deposit feeders are snails of the genus Hydrobia, 

 which live on intertidal mud flats. 



In addition to their role as mineralizers and 

 their role as trophic intermediates between detri- 

 tus and its consumers, bacteria in marine systems 

 have a third major function. Some bacteria are 

 capable of growing on dissolved substances from 

 the water ctjlumn. Such activity essentially pro- 

 duces edible particulate matter (the bacteria) 

 from substances that would otherwise be unavail- 

 able to higher-level consiuners. Dissolved sub- 

 stances which some bacteria can utilize in this 

 fashion are of two major types: (1) dissolved or- 

 ganic compoimds, ])re\iousIy excreted or othcr- 



12 



