DEPARTURE FROM THE BIOSPHERE 369 



A. The Loss of Carbon present in Carbohydrate Macromolecules 



(a) Starch 



Bacteria and fungi are able to secrete a amylases into the surrounding 

 medium and in particular this can be done by Bac. subtilis, mesentericus, 

 macerans and polymyxa. 



(h) Pectins 



As we have already seen these are methylated polymers of D-galacturonic 

 acid (polyuronides and not polysaccharides). 



The chief organisms attacking pectin are found among the Entero- 

 bacteriaceae and such spore bearers as Bac. macerans and most of the 

 Clostridia. 



The breakdown of pectin is a complex process and takes place in several 

 stages. 



1. Conversion of protopectin to pectin (this occurs during the retting 

 of flax). The enzyme responsible is protopectinase, which is present in 

 Aspergillus, CI. felsineum, etc. 



2. Demethylation of pectin to pectin acid. The enzyme is pectase which 

 is found in many bacteria. 



3. Hydrolysis of the 1-4 linkage, spHtting the macromolecule. Here the 

 corresponding enzyme is pectinase which is present in Asp. oryzae, 

 Rhizopiis tritici, etc. 



(c) Cellulose 



Since cellulose constitutes the major part of the insoluble material in 

 plants, the speed at which it is broken down is one of the important factors 

 in the carbon cycle. 



A number of micro-organisms capable of decomposing cellulose exist : 

 myxobacteria, Clostridia, Actinomycetes and many fungi. The presence 

 of a soluble cellulose has only been shown in a few cases. In other cases, 

 the way in which the cellulose is broken down has not been decided. 



B. The Loss of Carbon present in Amino Acids 



The ketonic acids which result from the action of microbial proteases 

 and the oxidative deamination of the resulting amino acids, can be directly 

 reduced to hydroxy-acids 



2H 



R— CO— COOH > R— CHOH— COOH 



The ketonic acids can also be decarboxylated with the loss of one carbon 

 atom to form an aldehyde. 



R— CO— COOH -> COo + R— CHO 



