Abstract
Throughout the Baltic Sea redoxcline, virus production and the frequency of lytically-infected prokaryotic cells were estimated from parallel incubations of undiluted seawater and seawater that contained prokaryotes with substantially reduced numbers of viruses (virus dilution approach), effectively preventing viral reinfection during the incubation period. Undiluted seawater incubations resulted in much higher estimates of virus production (6–35×104 mL-1 h-1) and the frequency of infected cells (5–84%) than the virus dilution approach (virus production: 1–3×104 mL-1 h-1; frequency of infected cells: 1–11%). Viral production and the frequency of infected cells from both approaches, however, cannot be directly compared, as data obtained from undiluted incubations were biased by viral reinfection and other uncontrollable processes during the incubation period. High in situ viral abundance (1–2×107 mL-1) together with low virus production rates based on the virus dilution approach resulted in some of the longest viral turnover times (24–84 d) ever reported for the epipelagial. Throughout a wide range of environmental conditions, viral turnover time and burst size were negatively correlated. Given that viral decay estimated in ultra-filtered water was below the detection limit and the burst size was low (1–17), we conclude that prokaryotic viruses in the Baltic Sea redoxcline are investing most of their resources into stress defense (strong capsids) rather than proliferation (high burst size). In summary, the Baltic Sea redoxcline constitutes an environment where low virus production is found in combination with low viral decay, resulting in high viral abundance.
Highlights
The Baltic Sea is among the largest brackish water systems on Earth
virus production (VP) and frequency of infected cells (FIC) estimated from undiluted incubations were much higher compared to data from virus dilution incubations (Figs 3 and 4), confirming our initial hypothesis
VP and FIC were not correlated between both approaches, indicating that uncontrollable processes in undiluted incubations dramatically affect the results
Summary
The Baltic Sea is among the largest brackish water systems on Earth. The discharge of major rivers (e.g., Neva, Vistula) leads to the establishment of a stable halocline between less saline water at the surface and the deeper more saline water originating from the North Sea. The reduction of sulfate results in high concentrations of hydrogen sulfide (H2S) in deeper waters. The pelagic redoxcline is characterized by a steep redox gradient and is found between the well-oxygenated surface and the deeper sulfidic water. The water column of the Baltic Sea can be divided into four depth zones based on the concentration of O2 and H2S [2]. Episodic mixing between fresher surface and saltier deep waters may occur due to salt water inflows from the North Sea, leading to saline oceanic water filling large areas of the Baltic Sea. The last two reported major inflow events of North Sea water into the Baltic Sea occurred in the years 2003 [3] and 2014 [4], resulting in a complete ventilation of the water column, even of its deep basins
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