Biological considerations in the measurement of dissolved free amino acids in seawater and implications for chemical and microbiological studies

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Abstract
Dissolved free amino acids (DFAAs) are a significant source of C and N for marine bacteria. The various organisms present in coastal surface seawater contain amounts of free amino acids far greater than those dissolved in the water they occupy and certain common ways of treating seawater samples can lead to release from cells, yielding abnormal high amounts of DFAAs. Filtration of 5, 10 and 25 ml volumes through 0.2-0.45 .mu.m pore size membrane filters yielded similar DFAA concentrations as measured by high pressure liquid chromatography, but filtrations of 50 ml or more apparently led to DFAA release. Filter pore size and type had an effect; 0.2 .mu.m polycarbonate Nuclepore and 0.22 or 0.45 .mu.m mixed cellulose ester Millipore filters gave similar results, but 1.0 .mu.m Nuclepore or Whatman GF/F glass fiber filters apparently induced DFAA release from organisms, even with volumes as small as 10 ml. Some previous studies of DFAA concentrations or microbiological rate measurements may have been significantly affected by filtration artifacts. Such artifacts may also involve other dissolved components.