Effects of heat waves on virioplankton and virus-driven processes in the planktonic food web - the viral component of the HeatPlank3R- mesocosm experiment Summer in Spring

Abstract

Viruses are the most abundant biologicalentities in the ocean and play a pivotal role in the biogeochemical cycles, incontrolling the biomass of the microbial community and in maintaining geneticdiversity amongst it. While recycling organic carbon und nutrients, viral lysisshifts the food web towards a more regenerative pathway, weakening the carbonflux to higher trophic levels, increasing system heterotrophy and finallyreducing the efficiency of the biological carbon pump. Since viral replicationrates and viral life style (lytic versus lysogenic) are closely linked to hostabundance and metabolism, it is reasonable to assume that viral infection ratesrespond directly to temperature effects on host cells but existing data arescarce. Moreover, no conclusive data on viral lysis rates of heterotrophicbacteria and the induction of lysogens in combination with short-termtemperature increases have been reported. During the 20-days-lasting in situ mesocosm experiment.Summer inSpring Heat wave effects on Mediterranean Plankton communities: Resistance,Resilience and Recovery (Heat Plank 3R) at MEDIMEER we followed daily the dynamics of total viral abundance and viralsubpopulations in three mesocosms, where the ambient water temperature wasincreased by three degrees during 10 days simulating a heat wave and in threecontrol mesocosms. Moreover, phage mediated mortality of bacteria and lysogenywere determined in the presence and absence of nanoflagellate grazers on day 1,two times during the heating and ones afterwards in order to study thepotential effects of virus driven processes on biogeochemical cycles in marinesystems facing strong heat waves during spring. Totalviral abundance and the cytometric populations of low and medium fluorescenceviruses, comprising essentially bacteriophages, increased during theexperimental course while the high fluorescence viruses, mostly cyanophages andviruses infecting picoeukaryotes, decreased. No discernable effect of the heatwave treatment on viral populations could be detected. Phage production rates and phage mediated bacterialmortality increased with time and were significantly higher in the absence ofgrazers in the heat wave treatment suggesting preferential grazing on infectedbacteria. Moreover, the heat wave stimulated phage production in the presenceof grazers. No discernable effect of heat waves no lysogeny could be detected. The consequences forthe plankton carbon fluxes in this global change scenario are discussed.