Biochemistry of heterotrophic bacterial growth
Bacterial cells are 80% water, 18% organic dry matter and 2% inorganic dry matter
The organic dry matter component is C60H87O23N12P1 . The CNP molar ratio of bacteria is thus 60:12:1.
Bacterial synthesis proceeds as follows when oxygen is present:
6(C6H12O6) + 4NH3 +16O2 ----------------> 4 (C5H7NO2) + 16 CO2 + 28H2O
This shows that, when oxygen is present:
when bacteria are actively growing they release CO2, which causes pH to go down.
It also shows that bacteria are about 55% efficient at incorporating organic C into their biomass: the rest is lost as CO2.
It also shows that the molar ratio of C:N of the bacterial food source should be at about 36:4 = 9:1 ; this equates to a C:N weight ratio of about 8:1.
Each gram of carbon used requires about 1.2 grams of oxygen
In our pond example, we need to remove about 0.48 mgN/L. From these calculations we need to add about 8 x 0.48 = 3.84mg C/L and we also need the capacity to apply at least 4.6 mg 0 2 /L over and above the normal requirement. In our 5 million litre pond, these requirements equate theoretically to about 19 kg carbon and 23 kg oxygen per day. Molasses is about 18% C by weight, so about 100 kg of molasses is required per day. With regard to oxygen, extreme caution is required as there are a great many variables that can effect the oxygen demand in a situation where the pond is being loaded with an organic carbon source such as molasses. The safest and most practical way to proceed is to be aware of the theoretical aspects as outlined here but initiate trials at around 10 – 20 kg molasses per ha per day and to monitor ammonia regularly. As more molasses is added, the oxygen demand will increase as the pond becomes more heterotrophic. On our farm, we use molasses to help with controlling ammonia so we monitor ammonia and add molasses as required, keeping the total daily additions to less than 30 kg per ha per day. We have found that by doing this and maintaining the bloom at 35 cm Secchi and PO4 at less than 0.1mg/L we can get long term stable blooms even with 50% protein feed inputs as high as 200 kg per ha per day