Decomposers, Bacteria, and Microbenthos 383 



bacterivorous protozoan was present. This was true even though the 

 protozoan reduced the bacterial numbers by half. 



This phenomenon was studied in microcosms at Barrow during 1973 

 using Carex and organisms from the ponds. Details are given in Barsdate 

 et al. (1974). Briefly, the experimental systems were 1 -liter flasks 

 containing 500 ml of autoclaved pond water, sterilized Carex, and an 

 inoculum of organisms. Three inocula were used: pond bacteria obtained 

 by filtering sediment suspensions through a 3-Mm pore size Nuclepore 

 Filter; bacteria plus the bacteria-feeding ciliate Tetrahymena pyriformis 

 isolated from Pond B; or detritus direct from the pond and containing all 

 the living organisms. The organisms in these microcosms attained a 

 plateau after 50 to 100 hours. In grazed systems, the bacterial density was 

 always 2 to 5 times lower than in the ungrazed systems. One theory to 

 explain the increased activity and productivity was that the grazers were 

 recycling nutrients. Accordingly, measurements were made of the 

 movements of phosphorus such as the transfers of ^^P from the water to 

 the bacteria, from bacteria to the water, from bacteria to ciliates, and from 

 ciliates back to the water. Also, measurements were made of the actual 

 mineralization rate of PO4 from the Carex. Finally, a conceptual model of 

 the phosphorus cycle of the microcosm was constructed. 



The turnover rate of the phosphorus was always higher in the grazed 

 than in the ungrazed systems over a wide range of phosphate 

 concentrations in the water. This was mainly caused by a 4-fold increase in 

 the rate of uptake and release of phosphorus by the grazed bacteria; the 

 ciliates only contributed 4 to 5% of the total phosphorus excretion of the 

 system. Therefore, we could not confirm the experiments from which 

 Johannes (1965) concluded that protozoa excreted a significant amount of 

 phosphorus that otherwise is tied up in the bacterial biomass. Rather, 

 these results indicate that bacterial activity is stimulated by some 

 mechanism other than the protozoan excretion of phosphorus. This 

 increased activity then results in the increased rate of phosphorus cycling. 

 Finally, the actual mineralization of phosphorus from the Carex was 

 increased 40 times by the presence of the grazer. 



The mechanism or process responsible for this stimulation of 

 bacterial activity is not known. Some possibilities are the creation of micro- 

 turbulence, the excretion of growth-promoting substances other than 

 phosphorus, the stopping of competition among the bacteria for some 

 factor that limits growth or density, and the selection for rapidly growing 

 bacteria. 



Although these microcosms did mimic the natural system to some 

 degree by using Carex and natural flora and fauna, it is clear that only the 

 qualitative results can be applied to our conception of the natural system. 

 Thus, we can say that phosphorus does not appear to be limiting the 

 bacterial growth and that protozoan grazing somehow acts to increase 

 bacterial decomposition rates. In this way, protozoa and other microfauna 



