Concentration, production and turnover of viruses and dissolved DNA pools at Stn ALOHA, North Pacific Subtropical Gyre

Abstract

The concentrations, production rates and turnover times of the components of the dissolved DNA (D-DNA) pool, including viruses, were investigated in a depth profile at Stn ALOHA in the North Pacific Subtropical Gyre. A recently developed centrifugal concentration method was used to quantify the 3 major components of the D-DNA pool: free or enzymatically-hydrolyzable D-DNA (ehD-DNA), D-DNA within viruses and uncharacterized bound D-DNA. The production rates of each of these components of the D-DNA pool were estimated using a dilution technique and the turnover times of each component were calculated. Concentrations of total D-DNA and ehD-DNA were approximately 1.2 and 0.6 ng ml -1 , respectively, throughout the mixed layer (upper 100 m), and decreased with increasing depth to 0.2 and 0.06 ng ml -1 at 500 m, resulting in ehD-DNA constituting 27 to 51 % of the total D-DNA pool. Concentrations of viruses ranged from 0.9 to 1.0 x 10 7 ml -1 within the mixed layer and also decreased with increasing depth to 0.2 x 10 7 ml -1 at 500 m. The average mass of DNA per viral genome was estimated at each depth with viral genome fingerprinting, and ranged from 62.5 to 69.8 ag DNA per virus. Multiplying the concentration of viruses by the average mass of DNA per virus at each depth revealed the concentration of D-DNA within viruses, which ranged from 0.13 to 0.68 ng ml -1 and constituted 49 to 63 % of the total D-DNA pool. There was no measurable concentration of uncharacterized bound D-DNA in the depth profile. The production rates of ehD-DNA and D-DNA within viruses ranged from 0.10 to 0.41 and 0.03 to 0.07 ng ml -1 h -1 , respectively, within the mixed layer, but no production of D-DNA could be measured below the mixed layer. The calculated turnover times of ehD-DNA ranged from 0.97 to 6.2 h within the mixed layer and were 3 to 10 times shorter than the turnover times of D-DNA within viruses, which ranged from 9.6 to 24 h. In addition, the amount of phosphorus that was calculated to be produced within the ehD-DNA was able to support the biologically available phosphorus (BAP) demand, based on previously reported measurements of BAP uptake at Stn ALOHA. Using the measured virus production rates, viruses were estimated to lyse 3.2 to 16.5% of the standing stock of bacteria at Stn ALOHA h -1 , resulting in the release of ehD-DNA, which was estimated as 11 to 35 % of the total ehD-DNA production. This research supports the hypothesis that individual components of the D-DNA pool are cycled at different rates and shows that viruses in open-ocean gyre systems may have large impacts on the microbial community there, including viral-induced mortality and subsequent release of cellular contents to the dissolved organic matter pool.