Large-scale spatial distribution of virioplankton in the Adriatic Sea: testing the trophic state control hypothesis

Abstract

Little is known concerning environmental factors that may control the distribution of virioplankton on large spatial scales. In previous studies workers reported high viral levels in eutrophic systems and suggested that the trophic state is a possible driving force controlling the spatial distribution of viruses. In order to test this hypothesis, we determined the distribution of viral abundance and bacterial abundance and the virus-to-bacterium ratio in a wide area covering the entire Adriatic basin (Mediterranean Sea). To gather additional information on factors controlling viral distribution on a large scale, functional microbial parameters (exoenzymatic activities, bacterial production and turnover) were related to trophic gradients. At large spatial scales, viral distribution was independent of autotrophic biomass and all other environmental parameters. We concluded that in contrast to what was previously hypothesized, changing trophic conditions do not directly affect virioplankton distribution. Since virus distribution was coupled with bacterial turnover times, our results suggest that viral abundance depends on bacterial activity and on host cell abundance.

FIG. 1.

Sampling area and station locations in the Adriatic Sea. The grey area includes only the coastal (eutrophic) stations.

FIG. 2.

Distribution of hydrological parameters in surface waters of the Adriatic Sea. (a) Temperature; (b) salinity; (c) oxygen concentration; (d) transmittance.

FIG. 3.

Distribution of nutrient concentrations in surface waters of the Adriatic Sea. (a) Nitrates; (b) nitrites; (c) phosphates; (d) N/P ratio.

FIG. 4.

Distribution of microbial densities in surface waters of the Adriatic Sea. (a) Fluorescence; (b) viral abundance (log transformed); (c) bacterial density (log transformed); (d) virus-to-bacterium ratio.

FIG. 5.

Vertical profile of viral and bacterial densities [log (x + 1) transformed] in coastal (solid circles) and offshore (open circles) waters in the Adriatic Sea. (a) Viral abundance; (b) bacterial abundance.

FIG. 6.

Distribution of microbial functional parameters in surface waters of the Adriatic Sea. (a) Bacterial C production; (b) aminopeptidase activity; (c) alkaline phosphatase activity; (d) β-glucosidase activity.

FIG. 7.

Type II regression analysis of bacterial and viral abundance [log (x + 1) transformed] in surface waters of the Adriatic Sea. The slopes of the regressions of the coastal and open sea data sets are shown.

FIG. 8.

Relationship between viral abundance and bacterial turnover time (expressed in hours and calculated by determining the ratio of bacterial biomass to bacterial carbon production) in surface waters of the Adriatic Sea.