Organic nitrogen and caloric content of detritus: II. Microbial biomass and activity

Abstract

The effects of organic nitrogen, total caloric and/or ‘available’ caloric content on the regulation of microbial dynamics were studied using 23 types of detrital rations. Because most natural benthic systems rarely receive rations above 50 mg N m−2 day−1, the results of this work predicts that the organic nitrogen content of the detrital ration is the primary factor regulating microbial metabolism. Estimates of microbial biomass (total adenylates, A) and activity (rates of O2 consumption, CO2 production and nitrogen fixation) were measured at low (>3 to 100 mg N m−2 day−1) and high rations (>100 mg N m−2 day−1 to 216 mg N m−2 day−1). At low rations microbial biomass, activity and the mineralization (CO2) of ‘available’ detrital carbon per unit microbial biomass (CO2: A ratio) correlated best with the nitrogen content of the detritus while respiration per unit microbial biomass (O2:A) correlated best with ‘available’ caloric content. At high rations, microbial biomass and activity correlated best with ‘available’ caloric content. In all cases the metabolic quotient (CO2:O2) correlated best with ‘available’ calories and in no case did total caloric content become an important factor in microbial dynamics. This work suggests the rate of organic enrichment to benthic systems is an important variable to microbial dynamics as is the organic nitrogen and/or ‘available’ caloric content of the ration. It also suggests the importance of relying on more than one type of measurement and in determining the ‘availability’ of the detritus to detritivores in nutritional studies.