Predator-induced changes of bacterial size-structure and productivity studied on an experimental microbial community

Abstract

The grazlng unpact of 3 different protozoan specles on a muted bacter~al community was studled by means of a s ~m p h i ~e d and funct~onally reproducible expenmental m~crobial food web In a 2stage flow-through system In the first stage the algae Rhodomonas sp was grown on an inorganic medlurn wlth its accompanying bacterial community (BC) growing on algal exudates T h ~s m t u r e of algae and bacterla was transferled Into 4 second stage vessels (1) a control and 3 vessels inoculated wlth (2) a heterotrophic nanoflagellate Bodo saltans (3) a scuticoc~l~ate, Cyclldlum glaucoma, and (4) a mxotroph~c flagellate, Ochromonas sp Us~ng Image analysis techn~ques we followed the changes In bactenal s u e &stnbut~ons and bactenal to protozoan total biovolume rat~os over an expermental period of 15 d In add~tion productivity of the grazed and ungrazed BC was measured using [ 3 ~] t h y m ~dine and ['"C]leuc~ne As a consequence of total grazing rates and size-select~ve feedlng we observed 3 d~fferent responses of the initially ident~cal BC to grazing of the 3 protists (1) LOW grazlng by B saltans caused a slow decrease of bactenal cell numbers from 14 to 5 9 X 10h cells m -' , but no s~gnlflcant shlft of the mean cell volume (MCV, average 0 107 pm3) and bacter~al product~on (2) H ~g h e r grazing rates of C glaucoma resulted In the dechne o! bactenal abundance to 3 3 X 106 cells ml-' In parallel with a doubllng of the MCV to 0 207 pm3 and high DNA and protein synthes~s rates Due to the c h a t e S ab~lity to graze also on small prey ( < l 5 pm) an increase in MCV seemed to provide h l g h e ~ grazing resistance or at least decreased vulnerab~lity. (3) Ochromonas sp. showed the hlghest grazlng rates and reduced bacterial numbers by 20 times wlthin 2 d. Only the s m a l l e s t o b v i o u s l y more grazing-protected-bacteria (<0.9 pm) survived, which increased thereafter to almost initial bacterial densities Although the bacterial MCV dropped to 0.038 pm3, [3H]thymidine uptake rates per cell were greatly enhanced and highly variable. Our results reflect the potential of BC responses to different predation regimes and the advantages of phenotypic traits in order to coexlst with various grazers This should be seen in the context of influencing bottom-up effects and the varying potential of individual bacter~al specles to change morphology, growth strategies, and activ~ty patterns.