Viral production in the Gulf of Trieste (Northern Adriatic Sea): preliminary results using different methodological approaches.

Although the temporal and spatial variability of virioplankton in the northernmost part of the Adriatic Sea has been repeatedly explored suggesting that viruses constitute an extremely dynamic component of the plankton community and hypothesizing their importance in marine food webs and mucilage events, there is still no information about viral replication rates. Hence, the contribution of viruses to bacterial mortality and the cycling of organic matter in this part of the Adriatic basin are still not fully comprehended. Assessment of the role of viral lysis requires a robust means of estimating viral production. Since, up to now, none of the available methods evolved to a state of a standard yet, in this preliminary study 3 different experimental approaches were simultaneously assayed (viral production estimated by radiotracer incorporation method [TdR], dilution technique for the estimate of viral production in already infected bacteria [DIL] and serial dilution method in manipulated phage-host assemblage [SER]). The present study provided the first evidence of viral production rates in this study area, that resulted in comprising between ∼3.5–15×108 viruses L−1 h−1 and critically faced up the results obtained by different techniques with the consideration that they suffer from different biases. Based on TdR and DIL viral proliferation estimates, viral lysis was responsible for the loss of 54 to 95% of the bacterial standing stock, while the viral-induced mortality by SER (325%d−1) was likely consistently overestimated. These results indicate that viral lysis is a significant factor for prokaryotic mortality suggesting its implication as an important pathway for the cycling of dissolved organic matter in the Gulf of Trieste.