PREDATOR-PREY INTERACTIONS BETWEEN ISOCHRYSIS-GALBANA AND OXYRRHIS-MARINA .1. CHANGES IN PARTICULATE DELTA-C-13

Delta C-13 values in particulate organic C were measured during batch growth of cultures of the phototroph Isochrysis galbana and the heterotroph Oxyrrhis marina. There was an increase in delta C-13 during growth of the phototroph which was reversed as the number of predators increased and/or the rate of C-fixation decreased. There was a diurnal change in Isochrysis delta C-13 with lower values at the end of the dark phase. Evidence of C-12 concentration was apparent in mixed cultures, especially with stable predator and prey numbers where delta C-13 values decreased while particulate organic carbon remained constant. In cultures where the phototroph was effectively eliminated, delta C-13 values remained stable; changes in delta C-13 required the presence of the phototroph. Because delta C-13 values in mixed microbial populations can vary in response to various factors. the use of such values in trophic web analyses must be undertaken with caution. Also, care must be taken not to use high concentrations of N or P in batch cultures for either stock maintenance or experimental work because of the implications for C-stress with inadequate dissolved inorganic C (DIC).