Viruses, bacterioplankton, and phytoplankton in the southeastern Gulf of Mexico: distribution and contribution to oceanic DNA pools

Little is known concerning the factors which might control the distribution of viral abundance in oceanic environments and the relationship of viruses to the oceanic DNA pool. We have measured the distribution of viruses, bacterioplankton and phytoplankton in the subtropical southeastern Gulf of Mexico and related these parameters to the distribution of DNA (dissolved and particulate) in these waters. Viral direct counts were 4.6 to 27 X 106ml-' in Tampa Bay (Florida, USA), 3.8 to 8.5 X 105 ml-' in all oceanic euphotic zone samples and 1.4 to 4.7 X 104 ml-' in deep (200 to 2500 m) waters, and were highly correlated with chlorophyll a concentrations (r = 0.97), particulate DNA (r = 0.96) and bacterial direct counts (BDC, r = 0.94). A vertical profile indicated a subsurface euphotic zone maximum in viral direct counts that corresponded with maxima for particulate and dissolved DNA, and picocyanobacterial direct counts. For all stations, the vertical distribution of viruses most closely followed the distribution of particulate DNA. Bacterioplankton made the largest contnbution ( z 50 %) to the particulate (> 0.2 pm) DNA pool while phytoplankton averaged 8 %. A predictive model for particulate DNA was determined to be: Particulate DNA (pg 1-') = 4.94 X 10-"BDC I ' ) + 2.31 [chlorophyll a (pg I-')] + 2.77. DNA in viral particles was estimated to comprise only ca 4 % of the dissolved DNA pool. These results suggest that the distribution of viruses is tightly coupled to the distribution of microbial biomass in subtropical oceanic water column environments. Viruses may be instrumental in the production of dissolved DNA, but themselves are not a significant component of the dissolved DNA.