Bacteria 



Two basic principles relate to the 

 consumption of the net energy produced 

 annually in the marsh-estuarine ecosystem. 

 The first principle is that most energy 

 produced by the dominant primary producer 

 in the system ( Spartina alterniflora ) is 

 not consumed directly by grazers. Instead, 

 at least 90% (perhaps 95%) either leaches 

 into the water column from living and dead 

 plants as dissolved organic matter, or 

 through various processes enters the sys- 

 tem as detritus. Both forms of this or- 

 ganic matter are then attacked by micro- 

 scopic decomposers or ingested directly by 

 macroconsumers. 



The second major principle of energy 

 consumption in the salt marsh is that the 

 decomposer community (aerobic and anaer- 

 obic) is large, diverse, and extremely 

 active, consuming about 50% of the total 

 energy flowing through the ecosystem, as 

 shown in Figure 4 (Teal 1962). The decom- 

 poser community of the estuarine ecosystem 

 can be divided conveniently into two 

 groups: (1) aerobic heterotrophs (bacteria 

 and fungi) which utilize inorganic matter 

 in standing dead grass stalks, the water 

 column, and aerobic sediments; and (2) an- 

 aerobic bacteria in anoxic (oxygen-poor) 

 sediments. The activity of the aerobic 

 group enhances the nutritive quality of 

 both particulate and dissolved organic 

 matter for the larger consumers. Particu- 

 late organic matter is colonized by the 

 aerobic heterotrophs as it is gradually 

 fragmented into detritus. Its nutritional 

 value is enhanced by increasing the rela- 

 tive nitrogen composition of the particu- 

 late organic carbon (POC), as shown by 

 Odum and de la Cruz (1967). This can be 

 symbolized as follows: POC + O2 + NH4+ -^ 

 bacterial POC-N + CO2. Dissolved organic 

 carbon (plant leachates, etc.) can be as- 

 similated by micro-heterotrophs and also 

 converted into POC-N. Some aerobic bac- 

 teria are also critical elements of the 

 nitrogen cycle, as discussed below. 



Anaerobic decomposers function in a 

 variety of roles in the salt marsh ecosys- 

 tem. They are essential to the geochemical 

 cycles that release plant nutrients in a 

 continuous stream to the primary produc- 

 ers. The nitrogen cycle is especially im- 

 portant because evidence to date indicates 



that nitrogen is the limiting nutrient in 

 the salt marsh (Valiela and Teal 1979). 

 Since the decomposition of cellulose is 

 nitrogen-limited (Pomeroy and Wiegert 

 1980), the decomposition of the large 

 standing stock of organic matter in the 

 system results in a competition for nitro- 

 gen between decomposers and primary pro- 

 ducers. 



Four groups of bacteria are involved 

 in the nitrogen cycle. One group in the 

 sediments (nitrogen fixers) converts atmo- 

 spheric nitrogen to nitrate and nitrite 

 (N2 -* NC3 -+ NO2-); another group (the de- 

 nitrifiers) reduces nitrates and nitrites 

 to atmospheric nitrogen. A third group 

 (ammonifiers) converts dead tissue into 

 ammonia. A fourth group (nitrifiers) oc- 

 cupies the thin, oxidized layer around 

 Spartina roots and converts ammonia from 

 anaerobic sediments into nitrates directly 

 usable by the plant. 



The anaerobic zone in salt marsh- 

 estuarine sediments extends upward almost 

 to the sediment surface because of the 

 enormous oxygen-depleting capacity (chemi- 

 cal oxygen demand) of these sediments. The 

 metabolic activity of anaerobic bacteria 

 is responsible for this oxygen demand. 



Benthic Infauna 



Other organisms in estuarine sedi- 

 ments include metazoan animals larger than 

 bacteria but so small that studying them 

 and documenting their functions are diffi- 

 cult. This group is called the meiofauna, 

 and although it contains various phyloge- 

 netic groups and trophic positions, its 

 overall role apparently is that of a tro- 

 phic intermediary between bacteria and 

 macroconsumers. Nematodes and other meio- 

 fauna appear to be major processors of 

 bacterial tissue, and they are an impor- 

 tant component of the food of many so- 

 called deposit feeders (Sikora 1977; Bell 

 and Coull 1979). Intertidal biomass of 

 nematodes in creek banks in the study area 

 has been measured at 6.4 g ash free dry 

 weight (afdw)/ m^ (Sikora et al. 1977). 



Larger benthic organisms (macroben- 

 thos) in salt marsh estuaries are usually 

 divided into epibenthos and macro-infauna. 

 Because oysters are epibenthos, we will 

 omit further discussion of epibenthos 



11 



