Latitudinal variation in virus-induced mortality of phytoplankton across the North Atlantic Ocean

Abstract

Viral lysis of phytoplankton constrains marine primary production, food web dynamics and biogeochemical cycles in the ocean. Yet, little is known about the biogeographical distribution of viral lysis rates across the global ocean. To address this, we investigated phytoplankton groupspecific viral lysis rates along a latitudinal gradient within the North Atlantic Ocean. The data show large-scale distribution patterns of different virus groups across the North Atlantic that are associated with the biogeographical distributions of their potential microbial hosts. Average virusmediated lysis rates of the picocyanobacteria Prochlorococcus and Synechococcus were lower than those of the picoeukaryotic and nanoeukaryotic phytoplankton (that is, 0.14 per day compared with 0.19 and 0.23 per day, respectively). Total phytoplankton mortality (virus plus grazer-mediated) was comparable to the gross growth rate, demonstrating high turnover rates of phytoplankton populations. Virus-induced mortality was an important loss process at low and mid latitudes, whereas phytoplankton mortality was dominated by microzooplankton grazing at higher latitudes (456°N). This shift from a viral-lysis-dominated to a grazing-dominated phytoplankton community was associated with a decrease in temperature and salinity, and the decrease in viral lysis rates was also associated with increased vertical mixing at higher latitudes. Ocean-climate models predict that surface warming will lead to an expansion of the stratified and oligotrophic regions of the world's oceans. Our findings suggest that these future shifts in the regional climate of the ocean surface layer are likely to increase the contribution of viral lysis to phytoplankton mortality in the higher-latitude waters of the North Atlantic, which may potentially reduce transfer of matter and energy up the food chain and thus affect the capacity of the northern North Atlantic to act as a long-term sink for CO 2 .

Temperature is a key factor in Micromonas–virus interactions

“…For example, increasing viral lysis rates of phytoplankton were recorded from high to low latitudes across the North Atlantic Ocean and, interestingly, correlated positively with temperature and salinity (Mojica et al, 2015). Consistent with this finding, high incidence of lysogeny and low viral lysis rates were shown to occur in low temperature ecosystems such as polar, mesopelagic and deep-sea waters or during cold periods in temperate systems Williamson et al, 2002;Weinbauer et al, 2003;McDaniel et al, 2006;Evans et al, 2009).…”

“…There is, to our knowledge, no report of actual viral lysis rates in Micromonas natural populations to support our speculation. Nonetheless, a recent study observed a latitudinal variation in virus-mediated mortality of phytoplankton across the North Atlantic Ocean and pointed out a correlation between the velocity of viral lysis and in situ temperature that varied between 10 and 25°C (Mojica et al, 2015). Such geographic partitioning of viral lysis processes could have important consequences for the ecology and the biogeochemistry of the global ocean.…”

Alterations in microbial community composition with increasing <i>f</i>CO<sub>2</sub>: a mesocosm study in the eastern Baltic Sea

“…Overall, the combination of growth and losses (grazing and viral lysis) could explain the microbial population dynamics observed in this study. It is noteworthy to mention a recent study in the oligotrophic northeastern Atlantic Ocean, which reported a shift from a grazing-dominated to a viral-lysis-dominated phytoplankton community with strengthening vertical stratification (shoaling the mixed layer depth and enhancing nutrient limitation; Mojica et al, 2016). Thus, we highly recommend that future research on OA combine mesocosm studies focusing on changes in microbial community composition and activity with experiments aimed at understanding the effects of OA on food web dynamics, i.e.…”

“…Microzooplankton grazing and viral lysis of phytoplankton was determined using the modified dilution assay based on reducing grazing and viral lysis mortality pressure in a serial manner allowing for increased phytoplankton growth (over the incubation period) with dilution (Mojica et al, 2016). Two dilution series were created in clear 1.2 L polycarbonate bottles by gently mixing 200 µm sieved whole seawater with either 0.45 µm filtered seawater (i.e.…”

“…Consequently, bacteria may be affected indirectly by OA through changes in the quality and/or quantity of DOM (Weinbauer et al, 2011). Viral lysis has been found to be as important as microzooplankton grazing to the mortality of natural bacterioplankton and phytoplankton (Weinbauer, 2004;Baudoux et al, 2006;Evans and Brussaard, 2012;Mojica et al, 2016). Thus far, most studies examining the effects of OA on microzooplankton abundance and/or grazing have found little or no direct effect (Suffrian et al, 2008;Rose et al, 2009;Aberle et al, 2013;Brussaard et al, 2013;Niehoff et al, 2013).…”