Variability in Protist Grazing and Growth on Different Marine Synechococcus Isolates

Abstract

ABSTRACTGrazing mortality of the marine phytoplanktonSynechococcusis dominated by planktonic protists, yet rates of consumption and factors regulating grazer-Synechococcusinteractions are poorly understood. One aspect of predator-prey interactions for which little is known are the mechanisms by whichSynechococcusavoids or resists predation and, in turn, how this relates to the ability ofSynechococcusto support growth of protist grazer populations. Grazing experiments conducted with the raptorial dinoflagellateOxyrrhis marinaand phylogenetically diverseSynechococcusisolates (strains WH8102, CC9605, CC9311, and CC9902) revealed marked differences in grazing rates—specifically that WH8102 was grazed at significantly lower rates than all other isolates. Additional experiments using the heterotrophic nanoflagellateGoniomonas pacificaand the filter-feeding tintinnid ciliateEutintinnissp. revealed that this pattern in grazing susceptibility among the isolates transcended feeding guilds and grazer taxon.Synechococcuscell size, elemental ratios, and motility were not able to explain differences in grazing rates, indicating that other features play a primary role in grazing resistance. Growth of heterotrophic protists was poorly coupled to prey ingestion and was influenced by the strain ofSynechococcusbeing consumed. AlthoughSynechococcuswas generally a poor-quality food source, it tended to support higher growth and survival ofG. pacificaandO. marinarelative toEutintinnissp., indicating that suitability ofSynechococcusvaries among grazer taxa and may be a more suitable food source for the smaller protist grazers. This work has developed tractable model systems for further studies of grazer-Synechococcusinteractions in marine microbial food webs.