CONSEQUENCES OF PROTIST-STIMULATED BACTERIAL PRODUCTION FOR ESTIMATING PROTIST GROWTH EFFICIENCIES

The trophic link between bacteria and bacterivorous protists is a comp lex interaction that involves feedback of inorganic nutrients and grow th substrates that are immeadiately available for prey growth. These i nteractions were examined in the laboratory and in incubations of conc entrated natural assemblages of bacterioplankton. Growth dynamics of e stuarine and marine bacterivorous protists were determined in laborato ry culture using Vibrio natriegens as prey and were compared to growth of protists on bacterioplankton assemblages concentrated by tangentia l flow filtration from four northwest Florida Estuaries. Biomass trans fers from bacteria to protists were monitored by tracing elemental car bon and nitrogen in particulate fractions of protist added and grazer free controls. Gross growth efficiencies of the protists on naturally occurring bacteria were within the range determined in lab estimates o f growth efficiency on cultured bacteria (similar to 50%). However, ba cterial response to protist excretion products was different in the la b and field incubations, and bacterial growth contributed to the bioma ss available to protists in the field incubations. As determined by ra dioisotope-labeled substrate incorporation, a time lag in bacterial re ponse to protist excretion products was observed for laboratory batch cultures, allowing accurate estimation of growth efficiency. In incuba tions with concentrated natural bacterial assemblages, bacterial growt h response coincided with protist growth and excretion. The additional bacterial production on protist excretion products reached a maximum of 2-3-fold higher than protist-free controls. In addition, ammonium c oncentrations increased with protist grazing and growth in lab culture s, but ammonium excreted by protists in concentrates did not accumulat e. The C:N values for the bacterial concentrates suggests that these b acteria were nitrogen limited. It is speculated that dissolved organic carbon, concentrated by tangential flow filtration (> 100,000 MW memb rane) with the bacterioplankton, was utilized by bacteria when nitroge n was supplied as ammonium and amino acids from protist excretion. Thu s, estimates of protist growth efficiency on naturally occurring bacte rioplankton, corrected for protist-stimulated bacterial production, we re in the range of 13-21%.