The ecology and diversity of estuarine virioplankton

Viruses are abundant and active members of the marine microbial loop with the inherent ability to affect host communities through cell lysis. Current estimates of viral-mediated mortality (VMM) and viral impacts on host micro-organisms vary from less than 10 to over 100% of the bacterial community infected by a virus on a given day. To begin to characterize the variability in the role of viruses in aquatic ecosystems and identify possible causes, viral production (VP), a key factor in estimating VMM, was investigated in the Chesapeake and Delaware bays.;Measured with a newly developed technique, tangential flow diafiltration, VP in the Chesapeake Bay from September 2002 - February 2007, averaged 7.9 ± 8.8 x 106 viruses ml-1 h-1, VMM averaged 1.6 ± 1.8 x 105 cell ml-1 h-1, and dissolved organic carbon release averaged 76 ± 85 μg C l-1 d-1. These parameters displayed greater temporal than spatial variation within the estuary and were significantly higher in early spring, late summer, 2003 and 2006. The estimated percentages of bacterial abundance and production lost to viral lysis averaged 2.7 ± 3.0% h-1 and 280 ± 530%, respectively, and displayed the opposite trend of VP with 2 to 3-fold higher values in winter.;Variations in VP were also examined at daily and diel time scales. VP rates did not vary significantly day-to-day in either the Chesapeake Bay or coastal California suggesting that viral activities do not change significantly on this time scale. However, significant variations in VP were detected over a diel cycle in the Chesapeake Bay in four of five studies. The patterns of VP during diel cycles also varied seasonally. Across broader spatial scales, VP increased from off-shore, oligotrophic to eutrophic, estuarine locations. The percentage of bacterial abundance lysed by viruses displayed the opposite trend with higher rates in off-shore vs. estuarine locations. These findings support the hypothesis of increasing VP with trophic status and imply that biogeochemical surroundings, at least indirectly, affect virioplankton production. Changes in physio-chemical parameters and viral and host abundance and diversity with season and geographic location may be responsible for the observed inter-annual, seasonal, monthly, diel, and spatial changes in VP and viral impacts.;To determine if viral assemblage composition varies on the same temporal and spatial scales as viral activity, the richness of virioplankton assemblages using randomly amplified polymorphic DNA (RAPD) - PCR was also monitored in the Chesapeake and Delaware Bays. Viral richness, like VP, was high and dynamic in both estuaries and displayed greater temporal than spatial variation. Similarities in RAPD-PCR banding profiles were found across broad temporal and spatial gradients in the Chesapeake Bay and additional sequencing of RAPD-PCR amplicons verified the presence of viral strains between widely separated sampling locations and dates. Thus, seasonal and spatial variations in viral assemblages may explain the previously observed oscillations in VP in these ecosystems. The similarities between RAPD-PCR profiles, which imply the persistence of viruses within the environment, are a possible mechanism for the re-emergence of repeating seasonal patterns of viral activity.;Estuarine virioplankton assemblages are diverse both genetically and functionally, but these variations can be characterized by long and short-term temporal and spatial patterns that are influenced by and interact with similar patterns in host communities and environmental parameters. Thus, viral diversity and the role of aquatic viruses will always vary as conditions change temporally, spatially and evolutionarily.