178 PROVASOLI [CHAP. 8 



B. Excretion of Organic Nitrogen 



Extra-cellular nitrogenous products have been noted frequently in cultures 

 of bacteria or blue-green algae fixing nitrogen, but no data were available on 

 the nature of these substances before the work of Fogg (1952) on Anabaena 

 cylindrica : liberation of extra-cellular organic nitrogen accompanies its growth. 

 The relative amounts of the excreted nitrogen vary during growth : as much as 

 50% of the total nitrogen taken up is excreted during the early logarithmic 

 growth; it decreases to a minimum (10-20%) at half growth; increases again 

 in older but still healthy and growing cultures ; and moribund material liberates 

 large quantities of soluble organic nitrogen (but cultures in these conditions 

 were not used by Fogg in his work). The amount excreted is not affected 

 appreciably by cultural conditions except in the later stages of growth : it may 

 be increased by nutritional deficiencies such as Fe, or slightly decreased by Mo 

 deficiencies (Mo deficiencies affect nitrogen fixation and nitrite assimilation in 

 A. cylindrica; Wolfe, 1954). 



The nitrogenous excretion consists principally of polypeptides with lesser 

 amounts of amide N (amides are high in young cultures and decrease as they 

 become older). As much as 4-8 mg/1. of extra-cellular N are produced in culture. 

 The polypeptides after acid hydrolysis yield amino acids. Chromatographic 

 analysis suggests that the polypeptide fraction varies in composition : the 

 polypeptide of a 28-day culture in a medium without added N contained in 

 decreasing quantities serine, threonine, glutamic acid, glycine and tyrosine, 

 and traces of alanine, valine, leucine; a 12-day culture in a medium with 

 ammonium phosphate gave in decreasing quantities, glutamic acid, alanine, 

 valine, leucine, glutamine, glycine, tyrosine, phenylalanine, aspartic acid. 



Production of polypeptides is not restricted to nitrogen-fixing organisms : 

 algae representing four different classes liberate considerable amounts of extra- 

 cellular N and polypeptides (Fogg and Westlake, 1955). However, since far 

 greater amounts are released in old cultures, autolysis rather than excretion is 

 responsible for the increased amount of soluble organic N (Table IV). The 

 production of polypeptides may be widespread amongst micro-organisms : it is 

 common in non-nitrogen-fixing bacteria (Proom and Woiwood, 1949); several 

 bacilli produce polypeptides with antibiotic properties like polymixin, baci- 

 tracin, valinomycin, tyrocidin and gramicidin. 



The extra-cellular nitrogenous substances may or may not serve as nutrients 

 to other organisms. The polypeptide of Anabaena does not give a perceptible 

 reaction with ninhydrin, suggesting a ring structure which may not be sus- 

 ceptible to attack by the usual proteolytic enzymes. It has no antibiotic 

 properties and is not utilizable as a N-source by A. cylindrica, Chlorella sp. and 

 Oscillatoria sp. ; it is not known whether bacteria can utilize them (Fogg, 1952). 



Peptide nitrogen occurs dissolved in lakes in amounts ranging from 0.057 

 to 0.436 mg N/1. (Domogalla, Juday and Peterson, 1925). Fogg and Westlake 

 (1955) found smaller quantities of peptide N in English lakes : the concentration 



