Viral production, decay rates, and life strategies along a trophic gradient in the North Adriatic Sea

Abstract

Although the relationships between trophic conditions and viral dynamics have been widely explored in different pelagic environments, there have been few attempts at independent estimates of both viral production and decay. In this study, we investigated factors controlling the balance between viral production and decay along a trophic gradient in the north Adriatic basin, providing independent estimates of these variables and determining the relative importance of nanoflagellate grazing and viral life strategies. Increasing trophic conditions induced an increase of bacterioplankton growth rates and of the burst sizes. As a result, eutrophic waters displayed highest rates of viral production, which considerably exceeded observed rates of viral decay (up to 2.9 x 10(9) VLP liter(-1) h(-1)). Viral decay was also higher in eutrophic waters, where it accounted for ca. 40% of viral production, and dropped significantly to 1.3 to 10.7% in oligotrophic waters. These results suggest that viral production and decay rates may not necessarily be balanced in the short term, resulting in a net increase of viruses in the system. In eutrophic waters nanoflagellate grazing, dissolved-colloidal substances, and lysogenic infection were responsible together for the removal of ca. 66% of viral production versus 17% in oligotrophic waters. Our results suggest that different causative agents are primarily responsible for the removal of viruses from the water column in different trophic conditions. Factors other than those considered in the past might shed light on processes responsible for the removal and/or decay of viral particles from the water column.

FIG. 1.

Sampling area and stations location along the transect in the north Adriatic basin (Mediterranean Sea).

FIG. 2.

Active and lysogenic bacterial fractions in the four sampling stations along a transect in the northern basin of the Adriatic Sea.

FIG. 3.

Temporal variations over 24 h of viral abundance (A), bacterial abundance (B), and bacterial carbon production (C) in samples where bacterivores were removed (bacterivore-free treatments).

FIG. 4.

Simplified scheme of the fate of viral production under different trophic conditions. Eutrophic (top panel) and oligotrophic waters (bottom panel) are depicted. Arrow dimensions are roughly proportional to the relative importance of the process described.