Elevated CO₂ and Phosphate Limitation Favor Micromonas pusilla through Stimulated Growth and Reduced Viral Impact

Growth and viral infection of the marine picoeukaryote Micromonas pusilla was studied under a future-ocean scenario of elevated partial CO2 (pCO(2); 750 mu atm versus the present-day 370 mu atm) and simultaneous limitation of phosphorus (P). Independent of the pCO(2) level, the ratios of M. pusilla cellular carbon (C) to nitrogen (N), C: P and N: P, increased with increasing P stress. Furthermore, in the P-limited chemostats at growth rates of 0.32 and 0.97 of the maximum growth rate (mu(max)), the supply of elevated pCO(2) led to an additional rise in cellular C: N and C: P ratios, as well as a 1.4-fold increase in M. pusilla abundance. Viral lysis was not affected by pCO(2), but P limitation led to a 150% prolongation of the latent period (6 to 12 h) and an 80% reduction in viral burst sizes (63 viruses per cell) compared to P-replete conditions (4 to 8 h latent period and burst size of 320). Growth at 0.32 mu(max) further prolonged the latent period by another 150% (12 to 18 h). Thus, enhanced P stress due to climate change-induced strengthened vertical stratification can be expected to lead to reduced and delayed virus production in picoeukaryotes. This effect is tempered, but likely not counteracted, by the increase in cell abundance under elevated pCO(2). Although the influence of potential P-limitation-relieving factors, such as the uptake of organic P and P utilization during infection, is unclear, our current results suggest that when P limitation prevails in future oceans, picoeukaryotes and grazing will be favored over larger-sized phytoplankton and viral lysis, with increased matter and nutrient flow to higher trophic levels