Excretion of dissolved organic carbon by phytoplankton cells

Phytoplankton cells release a wide range of soluble organic compounds into the surrounding water medium. These make an important contribution to the overall dissolved material present in the ecosystem and are referred to as dissolved organic carbon (DOC) or dissolved organic matter (DOM). Many of these compounds are derived directly from the process of photosynthesis (see later), hence their consideration in this part of the book.

DOC released by phytoplankton cells includes carbohydrates (Mil nster, 1984), polypeptides (Walsby 1974), free amino-acids, organic acids (particularly glycollic acid - Pant and Fogg, 1976) and cyclic AMP (Francko and Wetzel, 1982). Considerable molecular diversity occurs within each of these major groups. Analysis of soluble carbohydrates in the Plussee (Germany), for example, involved the determination of 26 monosaccharides and six disaccharides (MUnster, 1984). The major derivation of lake water concentrations of these organic compounds from phytoplankton cells is indicated by environmental observations of seasonal correlations of DOC with algal biomass, diurnal correlations with phytoplankton activities (Chapter 3.7), and laboratory studies on photosynthesis and DOC release (see below).

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