Excretion of organic matter by marine phytoplankton: Do healthy cells do it?1

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Abstract
It is widely accepted that marine phytoplankton lose appreciable photoassimilated carbon into seawater as organic compounds; the phenomenon is usually referred to as excretion. Two lines of evidence for excretion are found in the literature: measurements in field studies of “organic 14C” in productivity experiments and the appearance of organic matter in laboratory culture filtrates. Laboratory experiments and a literature review lead to the suggestion that the evidence for excretion is marred by two procedural artifacts, inadequate assessment of control blanks in the 14C field measurements and the results of cultural shock in laboratory studies. Evidence of extensive excretion by phytoplankton is not good.