Flow cytometric analysis of marine prokaryotes routinely reveals two distinct clusters of heterotrophic cells referred to as high nucleic acid fluorescent (HNA) and low nucleic acid fluorescent (LNA) ...populations. Evidence suggests that these may represent physiologically and ecologically distinct prokaryote populations. According to the “kill the winner” hypothesis, viral lysis reduces the efficiency of the microbial loop by decreasing the biomass and activity of the most abundant and active members of a population (i.e., competition specialist). Thus, viral-induced mortality may vary according to the physiology of HNA and LNA cells, with implications for the marine carbon cycle. Here, the abundance and production of heterotrophic prokaryotic populations were assessed in the North Atlantic during two phases of the annual plankton cycle and related to bottom-up (i.e., organic carbon variability) and top-down processes (i.e., viral abundance and lytic production). Our results demonstrate that the relative abundance of HNA and LNA heterotrophic cells and heterotrophic prokaryote production vary according to organic carbon variability in the water column, which can be strongly influenced by the physical eddy field (i.e., type of eddy: cyclonic, anticyclonic, or no eddy). In addition, the abundance and lytic production of virus subpopulations were correlated with the cellular production and abundance of heterotrophic HNA and LNA prokaryote communities. Our data suggest group-and activity-specific linkages between hosts and viruses (i.e., HNA-V1 and LNA-V2). Specifically, V1 had a greater contribution to total viral production (i.e., 2.6-fold higher than V2 viruses), similar to their putative host. Finally, we explore potential implications of group- and activity-specific linkages between host and virus groups on the flux of carbon through the microbial food web.
Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical ...composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm
) and 33% (36 cm
) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm
) in late-autumn but only 4% (4 cm
) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.
Earth's aquatic food webs are overwhelmingly supported by planktonic microalgae that live in the sunlit water column where only a minimum number of physical niches are readily identifiable. Despite ...this paucity of environmental differentiation, these "phytoplankton" populations exhibit a rich biodiversity, an observation not easily reconciled with broadly accepted rules of resource-based competitive exclusion. This conundrum is referred to as the "Paradox of the Plankton". Consideration of physical distancing between nutrient depletion zones around individual phytoplankton, however, suggests a competition-neutral resource landscape. Application of neutral theory to the sheer number of phytoplankton in physically-mixed water masses yields a prediction of astronomical biodiversity, suggesting the inverted paradox: Why are there so few phytoplankton species? Here, we introduce a trophic constraint on phytoplankton that, when combined with stochastic principals of ecological drift, predicts only modest levels of diversity in an otherwise competition-neutral landscape. Our "trophic exclusion" principle predicts diversity to be independent of population size and yields a species richness across cell-size classes that is consistent with broad oceanographic survey observations.
Successful management of carbon dioxide (CO2) requires robust and sustained carbon cycle observations. Yet key elements of a national observation network are lacking or at risk. A U.S. National ...Research Council review of the U.S. Climate Change Science Program earlier this year highlighted the critical need for a U.S. climate observing system to meet requirements of mitigation policies for improved carbon cycle observations. This Forum highlights the most significant gaps and threats to carbon cycle observations—including observations from satellites; in situ observations of land, ocean, and aquatic systems; and direct atmospheric measurements—and suggests ways to improve the U.S. national effort.
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