Eco-evolutionary dynamics of bacteria and phage in contrasting resource environments

Background/Question/Methods

Bacteria typically live in environments which are depleted in resources such as nitrogen (N) and phosphorus (P).  These organisms have advanced nutrient uptake and retention mechanisms to cope with nutrient limiting situations, but these same structural features may be sites of attack by viral predators (i.e., phage). Phage play an important role in the ‘top-down’ regulation and structuring of bacterial communities, but also influence the evolutionary trajectory of host populations on ecologically relevant time scales. The interaction between virus predation and nutrient limitation, however, has not been extensively examined within an evolutionary context. Using the marine cyanobacterium, Synechococcus sp. WH 7803 and a T4-like phage, we explored the fitness tradeoffs and eco-evolutionary dynamics associated with phage attack and nutrient limitation. Through an eight month chemostat experiment, we manipulated virus exposure and the N:P supply ratio to create N-limiting and P-limiting environments (10:1 and 40:1 ratios, respectively). 

Results/Conclusions

The nutrient and phage addition treatments interacted strongly to affect Synechococcus population dynamics.  Synechococcus densities were more stable, exhibiting fewer cycles and lower amplitude fluctuations, under P-limitation versus N-limitation.  These differences may have been influenced, in part, by the evolution of Synechococcus to phage in N and P environments, since virus resistance evolved rapidly in all of the chemostats.  We tested this hypothesis by quantifying the relative fitness of Synechococcus isolates obtained at various time points throughout the chemostat experiment. The P-limited bacterial isolates had higher relative fitness than the N-limited isolates, demonstrating that the cyanobacterial cells did not readily adapt to N-limiting conditions. In contrast, we observed no differences in the relative fitness of Synechococcus that evolved in the presence or absence of phage.  Consistent with some theoretical predictions, our results suggest that although phage are critical for understanding bacterial dynamics, they may not mediate the evolutionary response to nutrient limitation.