J. Non-Equilib. Thermodyn. 37 (2012), 291–313 DOI 10.1515 / jnetdy-2011-0235 © de Gruyter 2012 Heat output by marine microbial and viral communities Essmaiil Djamali, James D. Nulton, Peter J. Turner, Forest Rohwer and Peter Salamon Communicated by Thomas Maskow Keywords. Calorimetry, marine microbes, microbes, viral communities. Abstract The Marine Microbial Food Web (MMFW) includes heterotrophic microbes and their protist and viral predators. These microbes consume dissolved organic matter thereby making the MMFW a major component of global biogeochemical and energy cycles. However, quantification of the MMFW contribution to these cycles is dependent on a handful of tech- niques, all of which require laboratory-derived conversion factors. Here we describe a differential calorimeter capable of measuring the small amounts of heat produced by marine microbes and viruses at natural populations. Using this ultra-sensitive calorimeter, we show that heat production in the presence of viruses is significantly larger than in their absence. This in- creased heat output occurs despite a net decrease in the number of mi- crobes. This provides direct evidence for top-down control of microbial populations by viruses and shows that there is increased re-mineralization. A comparative statics model was developed to interpret the calorimeter measurements. The spirit of the model is thermodynamic – it restricts its view to net changes in the populations and net heat produced. The model predicts that approximately 25% of the total heat production during the growth phase of a pelagic microbial community is due directly to viral ac- tivities. This result has implications for the energy budget of our planet and for climate prediction.