Aerosolization of soil-dust and organic aggregates in sea spray facilitates the long-range transport of bacteria, and likely viruses across the free atmosphere. Although long-distance transport occurs, there are many uncertainties associated with their deposition rates. Here, we demonstrate that even in pristine environments, above the atmospheric boundary layer, the downward flux of viruses ranged from 0.26 × 10 to >7 × 10 m per day. These deposition rates were 9-461 times greater than the rates for bacteria, which ranged from 0.3 × 10 to >8 × 10 m per day. The highest relative deposition rates for viruses were associated with atmospheric transport from marine rather than terrestrial sources. Deposition rates of bacteria were significantly higher during rain events and Saharan dust intrusions, whereas, rainfall did not significantly influence virus deposition. Virus deposition rates were positively correlated with organic aerosols <0.7 μm, whereas, bacteria were primarily associated with organic aerosols >0.7 μm, implying that viruses could have longer residence times in the atmosphere and, consequently, will be dispersed further. These results provide an explanation for enigmatic observations that viruses with very high genetic identity can be found in very distant and different environments.

The consequences of viral infection within microbial communities are dependent on the nature of the viral life cycle. Among the possible outcomes is the substantial influence of temperate viruses on the phenotypes of lysogenic prokaryotes through various forms of genetic exchange. To date, no marine microbial ecosystem has consistently shown a predisposition for containing significant numbers of inducible temperate viruses. Here, we show that deep-sea diffuse-flow hydrothermal vent waters display a consistently high incidence of lysogenic hosts and harbor substantial populations of temperate viruses. Genetic fingerprinting and initial metagenomic analyses indicate that temperate viruses in vent waters appear to be a less diverse subset of the larger virioplankton community and that these viral populations contain an extraordinarily high frequency of novel genes. Thus, it appears likely that temperate viruses are key players in the ecology of prokaryotes within the extreme geothermal ecosystems of the deep sea.

Accurate estimates of viral production in natural environments are critical for assessing the impacts of viral lysis on bacterial mortality and dissolved organic matter release. Here, viral production was estimated using a tangential flow diafiltration (TFD) dilution method, which reduced viral abundance to about 25% of ambient while maintaining near ambient levels of bacterial abundance. In subsequent incubations, the rate of virus-like particle increase was measured and used to calculate viral production. TFD viral production estimates were compared to those from simultaneous incubations using a vacuum diafiltration procedure. At 4 stations in the Chesapeake and Delaware Bays, viral production averaged 4.8 ± 1.7 × 10 10 and 5.9 ± 4.4 × 10 10 viruses l -1 d -1 as assessed by the TFD and vacuum methods, respectively. The TFD procedure improved upon the vacuum-based method by recovering significantly more of the bacterial community and requiring less sample processing time. Optimization tests of the TFD procedure found that a 0.22 µm pore size filter with a flushing rate of 40 ml min -1 , and a flushing volume 4-fold the initial sample volume gave the best combination of bacterial recovery, viral dilution, and processing time. Based on TFD viral production estimates, viral lysis was responsible for the loss of 14 to 93% of the bacterial standing stock and the release of 22 to 47 µg C l -1 d -1 in the Chesapeake and Delaware Bays. These results indicate that viral lysis is a significant factor for microbial mortality and dissolved organic matter cycling within these estuaries. KEY WORDS: Viral production · Tangential flow diafiltration · Viral-mediated mortality · Dilution method Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 41: [221][222][223][224][225][226][227][228][229][230][231][232] 2005 amount of C, N, and P released into a system by viral cell lysis (Fuhrman 1992, Steward et al. 1996, Gobler et al. 1997, Weinbauer & Höfle 1998, Wilhelm et al. 2002. Approaches to estimation of VP have been numerous and include electron microscopic observation of the frequency of visibly infected cells (FVIC) (Proctor et al. 1993, Steward et al. 1996, Binder 1999, Guixa-Boixereu et al. 1999, Hwang & Cho 2002, Middelboe et al. 2002, Choi et al. 2003, Weinbauer et al. 2003a, contact rates of viruses and bacteria (Murray & Jackson 1992, Suttle & Chan 1994, incorporation of 3 H-thymidine or 32 P into viral DNA (Steward et al. 1992a, Fuhrman & Noble 1995, Kepner et al. 1998), viral decay rates (Heldal & Bratbak 1991, Bratbak et al. 1992, Guixa-Boixereu et al. 1999, Tuomi & Kuuppo 1999, fluorescently labeled viruses (FLVs) as tracers of viral decay and VP (Noble & Fuhrman 2000, Helton et al. 2005, this issue), and dilution methods (Wilhelm et al. 2002, Winter et al. 2004a, Helton et al. 2005. The assumptions, advantages, and disadvantages of each of these approaches to measuring VP are summarized in Table 1. For many of these methods, 1 or more of the assumptions or...