Viruses have been implicated as major players in aquatic nutrient cycling, yet few data exist to quantify their significance. To determine the effect of viruses on ammonium regeneration by bacteria, experiments were carried out in the oligotrophic Indian Ocean and productive False Creek, Vancouver, Canada. Bacteria were concentrated and then diluted with virus-free water to reduce virus abundance, or with virus-replete water to restore natural virus abundances. Virus-replete treatments showed increased ammonium concentrations compared to treatments with viruses removed (differences of 0.287 ± 0.14 and 1.44 ± 0.73 µmol l −1 , mean ± SD, in the Indian Ocean and False Creek, respectively). Bacterial abundances were lower, while phytoplankton abundances and chlorophyll a (chl a) concentrations were greater in the virus-replete treatments, consistent with the increased availability of ammonium in the presence of viruses. These data demonstrate that viral lysis leads to ammonium production, likely through the liberation of dissolved organic N that is remineralised by uninfected bacteria. In turn, the released ammonium fuels primary production. These results show that viruses play a critical role in the marine N cycle, and suggest that viral lysis likely supplies a significant portion of the global N requirements of phytoplankton.

Abstract. Lytic infection of bacteria by viruses releases nutrients during cell lysis and stimulates the growth of primary producers, but the path by which these nutrients flow from lysates to primary producers has not been traced. This study examines the remineralisation of nitrogen (N) from Vibrio lysates by heterotrophic bacterioplankton and its transfer to primary producers. In laboratory trials, Vibrio sp. strain PWH3a was infected with a lytic virus (PWH3a-P1) and the resulting 36.0 µmol L −1 of dissolved organic N (DON) in the lysate was added to cultures containing cyanobacteria (Synechococcus sp. strain DC2) and a natural bacterial assemblage. Based on the increase in cyanobacteria, 74 % (26.5 µmol L −1 N) of the DON in the lysate was remineralised and taken up. Lysate from Vibrio sp. strain PWH3a labeled with 15 NH + 4 was also added to seawater containing natural microbial communities, and in four field experiments, stable isotope analysis indicated that the uptake of 15 N was 0.09 to 0.70 µmol N µg −1 of chlorophyll a. The results from these experiments demonstrate that DON from lysate can be efficiently remineralised and transferred to phytoplankton, and they provide further evidence that the viral shunt is an important link in nitrogen recycling in aquatic systems.

<p><strong>Abstract.</strong> Lytic infection of bacteria by viruses releases nutrients during cell lysis and stimulates the growth of primary producers, but the path by which these nutrients flow from lysates to primary producers has not been traced. This study examines the remineralisation of nitrogen (N) from bacterial lysates by heterotrophic bacteria and its transfer to primary producers. In laboratory trials, <i>Vibrio</i> sp. strain PWH3a was infected with a lytic virus (PWH3a-P1) and the resulting 36.0 μM of dissolved organic N (DON) in the lysate was added to cultures containing cyanobacteria (<i>Synechococcus</i> sp. strain DC2), and a natural bacterial assemblage. Based on the increase in cyanobacteria, 74 % (26.5 μM N) of the DON in the lysate was remineralised and taken up by <i>Synechococcus</i> sp. strain DC2 cells. Lysate from <i>Vibrio</i> sp. strain PWH3a labeled with <sup>15</sup>NH<sub>4</sub><sup>+</sup> was also added to seawater containing natural microbial communities, and in four field experiments, stable isotope analysis indicated that the uptake of labeled N was 0.09 to 0.70 μmol N per μg of chlorophyll <i>a</i>. The results from these experiments demonstrate that DON from bacterial lysate can be efficiently remineralised and transferred to phytoplankton, and provides further evidence that the viral shunt is an important link in nitrogen recycling in aquatic systems.</p>

Lysis of marine bacteria by viruses releases a range of organic compounds into the environment, including D-and L-amino acids, but the uptake of these compounds by other bacteria is not well characterized. This study determined that Photobacterium sp. strain SKA34 (Gammaproteobacteria) increased in abundance following uptake of D-and L-amino acids from viral lysate of Cellulophaga sp. strain MM#3 (Flavobacteria). Ammonium and dissolved free amino acids were taken up almost to detection limits, suggesting that the C:N ratio of bioavailable organic matter in the lysate was high for Photobacterium sp. growth, thus causing a net uptake of ammonium. In contrast, only 1.51 µmol l −1 of the 4.77 µmol l −1 of the total dissolved combined amino acids (DCAAs) were taken up, indicating that a fraction of lysate-derived DCAAs were semi-labile or refractory to bacterial uptake. Both D-and L-amino acid uptake rates were approximately proportional to their concentrations, indicating similar availability for each enantiomer and unsaturated uptake rates. These results imply that under high C:N conditions, both D-amino acids (mainly found in bacterial cell walls) and L-amino acids (found in proteins of the rest of the cell) are equally available for bacterial growth, and support arguments that viruses are key players in marine nitrogen cycling.

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