RADIOTRACER DETERMINATION OF INGESTION AND ASSIMILATION OF PERIPHYTICALGAE, BACTERIA, AND ADSORBED AMINO-ACIDS BY SNAILS

We examined the use of radiotracers to determine if H-3-amino acids (A A) and C-14-bicarbonate could differentiate grazing by Physella virgat a, a pulmonate gastropod, on the bacterial and algal components in per iphyton. Large amounts of H-3-AA became associated with periphyton as a result of adsorption to periphyton, rather than microbial uptake. As a consequence of this abiotic adsorption of H-3-AA by periphyton, upt ake of H-3 by grazers could be due to ingestion of either H-3-labeled bacteria or detritivory on material to which H-3-AA were adsorbed. Phy sella was able to ingest and assimilate adsorbed H-3-AA directly in th e absence of any bacterial uptake of amino acids. In contrast to H-3-A A, relatively little C-14-bicarbonate adsorbed to periphyton, and esse ntially all of the uptake could be attributed to photosynthetic activi ty. C-14-bicarbonate is, therefore, a reliable tracer for determining grazing activity on the algal component of periphyton assemblages. Cal culated assimilation efficiencies for snails grazing on periphyton to which H-3-AA was adsorbed (78%) were greater than those of snails graz ing on periphyton in which bacteria had incorporated H-3-AA (50%) or p eriphyton algae labeled with C-14-bicarbonate (32%) suggesting that ad sorbed organic carbon and detritivory may be important to this snail's nutrition.