[Do soluble organic nitrogen compounds cause unusual algal blooms in seawater?]

Up to now the nature and importance of the macromolecular forms of organic nitrogen in the marine environment is poorly understood. There is no doubt on the capacity of phytoplankton to utilize low molecule weight dissolved organic nitrogen compounds competitively with bacteria. In marine ecosystems the members of the plankton community show a complex close coupling with biological substances. In spite of trace concentrations the high dynamics in uptake and release processes cause strong fluxes of these compounds. In the case of inorganic nutrient deprivation or reduced photosynthesis under light deficiency algal utilization of organic nitrogen becomes noteworthy. Even under high nitrate supply an algal development occurred which was mainly triggered by amino acid uptake as was demonstrated during field experiments in the German Bight. The consumption of amino acids is controlled by several uptake systems, which are specified for transportation of different groups of amino acids through the cell wall. Since the uptake systems can also differ among species, the amino acid composition of the environment could preferentially enhance the growth of some selected species in comparison with other components of natural phytoplankton assembles. In the scope of increasing water pollution and the resulting shift in nutrient distribution one may expect the development of phytoplankton species, which had not been supported so far. In estuaries and costal zones photosynthesis is effected by light absorbance from particulate material. This phenomenon additionally will sustain algae, which are able to compensate a photosynthesis deficit by heterotrophic uptake. Though interrelationships are not clear up to now, it becomes evident that increasing impact of dissolved organic substances cause strong changes in species composition of marine ecosystems.