Pelagic Surface Waters Research Articles

Diversity and genomics of giant viruses in the North Pacific Subtropical Gyre.

Large double-stranded DNA viruses of the phylum Nucleocytoviricota, often referred to as "giant viruses," are ubiquitous members of marine ecosystems that are important agents of mortality for eukaryotic plankton. Although giant viruses are known to be prevalent in marine systems, their activities in oligotrophic ocean waters remain unclear. Oligotrophic gyres constitute the majority of the ocean and assessing viral activities in these regions is therefore critical for understanding overall marine microbial processes. In this study, we generated 11 metagenome-assembled genomes (MAGs) of giant viruses from samples previously collected from Station ALOHA in the North Pacific Subtropical Gyre. Phylogenetic analyses revealed that they belong to the orders Imitervirales (n = 6), Algavirales (n = 4), and Pimascovirales (n = 1). Genome sizes ranged from ~119-574 kbp, and several of the genomes encoded predicted TCA cycle components, cytoskeletal proteins, collagen, rhodopsins, and proteins potentially involved in other cellular processes. Comparison with other marine metagenomes revealed that several have broad distribution across ocean basins and represent abundant viral constituents of pelagic surface waters. Our work sheds light on the diversity of giant viruses present in oligotrophic ocean waters across the globe.

Proteomic responses of the coccolithophore Emiliania huxleyi to zinc limitation and trace metal substitution.

Zinc concentrations in pelagic surface waters are within the range that limits growth in marine phytoplankton cultures. However, the influence of zinc on marine primary production and phytoplankton communities is not straightforward due to largely uncharacterized abilities for some phytoplankton to access zinc species that may not be universally bioavailable and substitute zinc with cobalt or cadmium. We used a quantitative proteomic approach to investigate these strategies and other responses to zinc limitation in the coccolithophore Emiliania huxleyi, a dominant species in low zinc waters. Zinc limitation resulted in the upregulation of metal transport proteins (ZIP, TroA-like) and COG0523 metallochaperones. Some proteins were uniquely sensitive to growth under replete zinc, substitution of zinc with cobalt, or enhancement of growth with cadmium, and may be useful as biomarkers of zinc stress or substitution in situ. Several proteins specifically upregulated under cobalt-supported or cadmium-enhanced growth appear to reflect stress responses, despite titration of these metals to optimal nutritive levels. Relief from zinc limitation by zinc or cadmium resulted in increased expression of a δ-carbonic anhydrase. Our inability to detect metal binding enzymes that are specifically induced under cobalt- or cadmium-supported growth suggests cambialism is important for zinc substitution in E. huxleyi.

Particle‐Size Distributions and Solubility of Aerosol Iron Over the Antarctic Peninsula During Austral Summer

AbstractThis study provides new data on the properties of aerosol iron (Fe) over the Antarctic Peninsula, one of the fastest warming regions on Earth in recent decades. Atmospheric deposition delivers Fe, a limiting micronutrient, to the Southern Ocean, and aerosol particle size influences the air‐to‐sea deposition rate and fractional solubility of aerosol Fe. Size‐segregated aerosols were collected at Palmer Station on the West Antarctic Peninsula during austral summer 2016–2017. Results show single‐mode size distribution of aerosol Fe, peaking at 4.4 μm diameter. The average concentration of total aerosol Fe was 1.3 (±0.40) ng m−3 (range 0.74–1.8 ng m−3). High concentrations of total aerosol Fe occurred in January, implying increased Fe source strength then. Total labile Fe varied between 0.019 and 0.095 ng m−3, and labile Fe (II) accounted for ~90% of the total labile Fe. The average fractional solubility for total Fe was 3.8% (±1.5%) (range 2.5–7.3%). Estimated dry deposition fluxes for the study period were 3.2 μg m−2 year−1 for total labile Fe and 83 μg m−2 year−1 for total Fe in aerosols. We speculate that local and regional dust sources in Antarctica contributed to the observed aerosol Fe in austral summer and that warming on the Antarctic Peninsula during the past half century may have increased the formation of dust sources in this region. The potential biogeochemical impact of atmospheric Fe input to the West Antarctic Peninsula shelf waters and adjacent pelagic surface waters of the Southern Ocean may need to be re‐evaluated.

Flux variability of phyto- and zooplankton communities in the Mauritanian coastal upwelling between 2003 and 2008

Abstract. Continuous multiyear records of sediment-trap-gained microorganism fluxes are scarce. Such studies are important to identify and to understand the main forcings behind seasonal and multiannual evolution of microorganism flux dynamics. Here, we assess the long-term flux variations and population dynamics of diatoms, coccolithophores, calcareous and organic dinoflagellate cysts, foraminifera and pteropods in the eastern boundary upwelling ecosystem of the Canary Current. A multiannual, continuous sediment trap experiment was conducted at the mooring site CBeu (Cap Blanc eutrophic; ∼20∘ N, 18∘ W; trap depth is ca. 1300 m) off Mauritania (northwest Africa), between June 2003 and March 2008. Throughout the study, the reasonably consistent good match of fluxes of microorganisms and bulk mass reflects the seasonal occurrence of the main upwelling season and relaxation and the contribution of microorganisms to mass flux off Mauritania. A clear successional pattern of microorganisms, i.e., primary producers followed by secondary producers, is not observed. High fluxes of diatoms, coccolithophores, organic dinoflagellate cysts, and planktonic foraminifera occur simultaneously. Peaks of calcareous dinoflagellate cysts and pteropods mostly occurred during intervals of upwelling relaxation. A striking feature of the temporal variability of population occurrences is the persistent pattern of seasonal groups contributions. Species of planktonic foraminifera, diatoms, and organic dinoflagellate cysts typical of coastal upwelling, as well as cooler-water planktonic foraminifera and the coccolithophore Gephyrocapsa oceanica, are abundant at times of intense upwelling (late winter through early summer). Planktonic foraminifera and calcareous dinoflagellate cysts are dominant in warm pelagic surface waters, and all pteropod taxa are more abundant in fall and winter when the water column stratifies. Similarly, coccolithophores of the upper and lower photic zones, together with Emiliania huxleyi, and organic dinoflagellate cysts dominate the assemblage during phases of upwelling relaxation and deeper layer mixing. A significant shift in the “regular” seasonal pattern of taxa relative contribution is observed between 2004 and 2006. Benthic diatoms strongly increased after fall 2005 and dominated the diatom assemblage during the main upwelling season. Additional evidence for a change in population dynamics is the short dominance of the coccolithophore Umbilicosphaera annulus, the occurrence of the pteropod Limacina bulimoides and the strong increase in the flux of calcareous dinoflagellate cysts, abundant in warm tropical oligotrophic waters south of the study area after fall 2005. Altogether, this suggests that pulses of southern waters were transported to the sampling site via the northward Mauritania Current. Our multiannual trap experiment provides a unique opportunity to characterize temporal patterns of variability that can be extrapolated to other eastern boundary upwelling ecosystems (EBUEs), which are experiencing or might experience similar future changes in their plankton community.

Regional and seasonal variability in planktonic photosynthesis and planktonic community respiration in Amazon floodplain lakes

Results from two regional surveys and multi-lake seasonal studies were used to investigate the variability of phytoplankton photosynthesis and planktonic community respiration in central Amazon floodplain lakes. Hypothesized effects of optical and chemical variables on planktonic photosynthesis and respiration were examined statistically. Changes in dissolved oxygen in light and dark bottles distributed along light-gradients in a shipboard incubator or in situ were used to calculate volumetric community respiration (Rc), volumetric gross photosynthesis (P), daily integral gross photosynthesis (Π), and daily integral community respiration rates (Λ). Π varied significantly among all lakes with source-water river stage and source-water river type. Λ also varied significantly with source-water river stage. Variation in maximum depth linked to source-water river stage was a key factor controlling seasonal variations in Π through its influence on total suspended solids and total phosphorus concentrations which affected light extinction and light-saturated photosynthesis, respectively. The predominance of sub-saturated dissolved O2 in the pelagic surface waters of Amazon floodplain lakes was attributed to high integral Rc:P ratios, indicating the existence of large sustained inputs of non-phytoplankton organic carbon to these environments.

Assessment of Phosphorus-microbe Interactions in Lake Ontario by Multiple Techniques

Conventional and newly-developed techniques to determine the phosphorus (P) status of Lake Ontario phytoplankton were employed in September 2003, immediately after the passage of the storm system associated with Hurricane Isabel. Surface water (1–5 m) was collected at 29 stations, with selected stations sampled throughout the water column. Chemical estimates of total P concentrations were compared with proxies of P bioavailability: P enrichment bioassays of lake water, alkaline phosphatase activity (APA), and P-dependent bioreporter assays. Average total P (314 nM) and total chlorophyll-a (2.12 μg/L) concentrations measured in pelagic surface waters from throughout Lake Ontario suggest an oligotrophic status prevailed across much of this lake during the sample period. Autotrophic picoplankton (0.2–2 μm) displayed the highest growth rates and were grazed at the highest rate, whereas P-enrichment bioassays favored the production of autotrophic nanoplankton (2–20 μm) and autotrophic microplankton (> 20 μm) biomass. Average concentrations of bacteria (2.61 × 10 10 cells/L) were higher than those measured during summer in a similar lake (Erie), whereas the average viral density (1.38 × 10 10 virus particles/L) was similar. Pelagic stations exhibited higher APA than coastal stations; cyanobacterial bioreporter responses did not show high correlation with APA suggesting that proxies of P-demand based on residual effects (e.g., enzyme production) were not indicative of shorter-term biological responses related to planktonic growth (bioreporter genetic response). The combination of traditional chemical, biochemical (APA), and cutting-edge biological methods (bioreporter) provided information on nutrient concentrations and primary productivity throughout Lake Ontario, while concurrently allowing real-time assessment of P bioavailability.

Dissolved and particulate organic carbon in the Sea of Okhotsk: Transport from continental shelf to ocean interior

Dissolved and particulate organic carbon (DOC and POC) were measured in water columns of the Sea of Okhotsk together with other hydrological and chemical properties, including phytoplankton pigments (Chl‐a) and stable carbon isotopic ratios (δ13C) of POC. Transects from shelf to slope off the northeast coast of Sakhalin showed the existence of a cold‐water mass in the intermediate layer on the slope area having a density of 26.7–27.0 σθ, which is equivalent to the cold and dense water on the shelf (dense shelf water: DSW). The cold intermediate water mass was rich in DOC, POC, and Chl‐a, and the δ13C of POC was high, similar to that in DSW, indicating that the organic matter in the cold intermediate water is exported from the highly productive shelf area by the outflow of DSW. On the other hand, surface waters near the coast of northern Sakhalin had a very low salinity, which must be affected by the fresh water discharge from the Amur River. The low‐salinity water contained a large amount of DOC that was more than 3 times greater than the pelagic surface waters. The linear regression line between the salinity and the DOC in the surface layer of the studied area indicates that the Amur River water contains approximately 690 μM of DOC, and the input of DOC from the Amur to the Sea of Okhotsk was estimated to be approximately 2.5 TgC/yr. The relationships between the temperature and the DOC and POC along a density surface from 26.7 to 27.0 σθ suggest that DOC and POC in the DSW are transported into the intermediate layer of the pelagic region without being removed at the shelf edge. Simple estimates for additions of organic carbon into the intermediate layer of the area off the east coast of Sakhalin suggests that DSW outflow from the shelf brings into this layer approximately 13.6 and 0.9 TgC/yr of DOC and POC, respectively. These fluxes are much larger than the sinking POC flux from the in situ surface water, and so are fundamental in characterizing the biogeochemical cycles in the Sea of Okhotsk.

Scavenging of 137Cs, 109Cd, 65Zn, and 153Gd by Plankton of the Microbial Food Web in Pelagic Lake Erie Surface Waters

Inorganic dissolved 137Cs, 109Cd, 65Zn and 153Gd were added, at concentrations well below their respective solubility limits, to screened water (< 210 μm) sampled from the pelagic epilimnion of Lake Erie during the summers of 1994 and 1995. The hypotheses tested were that scavenging occurs in all of the ecologically significant size fractions that comprise the microbial food web (picoplankton, 0.2 to 2 μm; nanoplankton, 2 to 20 μm; microplankton, 20 to 210 μm), and that scavenging by plankton is directly related to the respective particle-reactivity of the elements (Gd > Zn > Cd » Cs). Size-selective filtration at intervals over periods of 22 to 30 h established that picoplankton and nanoplankton were the dominant scavenging phases in this environment. Scavenging of 153Gd, 65Zn, and 109Cd by plankton was more similar than predicted on the basis of their anticipated particle-reactivity and 137Cs was weakly scavenged. Except for the picoplankton, 65Zn was the element most readily scavenged by the plankton size fractions. High accumulation of 109Cd in the picoplankton may reflect the sorption of this element by calcite associated with autotrophic picoplankton. These experiments quantify the partitioning of trace metals within the plankton of the microbial food web (0.2 to 210 μm), a dynamic community of particles that dominates the seston in the pelagic surface waters of Lake Erie during thermal stratification, and suggest that plankton dynamics should be considered in predictions of the geochemical fate of trace metals in this environment.

Trace Metal Cycling in the Surface Waters of Lake Erie: Linking Ecological and Geochemical Fates

The highly productive plankton that comprise the microbial food web are ideally suited for trace metal scavenging, the removal of particle-reactive trace metals from the aqueous phase. Scavenging is considered to be the key factor controlling the concentrations of trace metals in the surface waters of large lakes during thermal stratification. Observed characteristics of the various plankton size fractions in the microbial food web (ability to scavenge trace metals from the dissolved phase; potential to regenerate these metals back into the dissolved phase; population dynamics) were incorporated into a dynamic microbial food web model of trace metal cycling in the surface waters of Lake Erie. The model yields estimates of epilimnetic trace metal residence times under the assumption of steady-state conditions: Cs = 352 d, Cd = 19 d, Zn = 23 d, Gd = 45 d. These trace metal residence times were considerably greater than the residence times predicted when microzooplankton grazing activity was eliminated from the model simulations (Cs, +46%; Cd, +58%; Zn, +53%; Gd, +80%). The increase of residence time by microzooplankton grazing is attributed to the trace metal regeneration that results from incomplete assimilation by the grazer of metal previously scavenged by the prey item. The results illustrate the important influence of the microbial food web activities on the geochemical fates of trace metals in the pelagic surface waters of large lakes during thermal stratification.

Settling Removal Rates of PCBs into the Northwestern Atlantic Derived from 238U−234Th Disequilibria

Surface ocean export fluxes of polychlorinated biphenyls (PCBs) were deduced utilizing 234Th as an indicator of the removal rates of particle-bound species from coastal, continental shelf, and pelagic surface waters. The fraction of the PCB settling flux that reaches the sediments (the ratio of bottom sediment import flux to 234Th-derived surface ocean export flux) decreased with both water column depth and with aqueous solubility of the congeners, presumably as a result of particle decomposition and/or compound desorption. A sharper decrease in 234Th-derived surface ocean fluxes away from the continental source was seen for congeners with increasing chlorination. This suggests that atmospheric washout followed by particle-mediated settling out of the surface ocean are the predominant processes affecting the PCBs, as opposed to atmospheric hydroxyl radical-mediated decomposition. Based on this limited data set, the largest fraction of each PCB is removed into the open ocean, although higher areal fluxes a...

Regeneration, recycling, and trophic transfer of trace metals by microbial food‐web organisms in the pelagic surface waters of Lake Erie

Rapid regeneration of 109Cd and 65Zn from their picoplankton prey into the dissolved phase by microzooplankton was observed in water sampled from the pelagic surface waters of Lake Erie (summer 1994 and 1995). Trace metals were added to grazing (lake water &lt;210 µm) and control (lake water &lt;0.2 µm) treatments in the form of radiolabeled Synechococcus. Picoplankton (0.2–3 µm) were grazed heavily by consumers in the nanoplankton (3–20 µm) and microplankton (20–210 µm) size classes (collectively referred to as microzooplankton) as confirmed by dilution assays used to independently measure grazing activity. Most consumed trace metals were regenerated into the dissolved phase (&lt;0.2 µm), but some trophic transfer of 109Cd and 65Zn from radiolabeled prey into the nanoplankton and microplankton did occur: 65Zn was transferred 2.5 times more efficiently into the microplankton and 2.9 times more efficiently into the nanoplankton than was 109Cd. Recycling o:f regenerated 109Cd back into plankton biomass was greater than that for 65Zn. Grazing by microzooplankton influenced the molecular size distribution of regenerated trace metal in the dissolved phase (77±6% 109Cd &lt;5,000 MW; 8 ± 24% 65Zn &lt;5,000 MW). These results show that microzooplankton grazing tends to prolong the residence times of metals such as Cd and Zn in the pelagic surface waters of large lakes.

Specific growth rates and grazing mortality of chroococcoid cyanobacteria Synechococcus spp. in pelagic surface waters in the sea

Specific growth rates and grazing mortality of phycoerythrin-dominant (PE-dominant) Synechococcus spp. populations have been evaluated in oceanic surface waters off Japan. Diel changes in cell numbers and frequency (patterns) of dividing cells (FDC) of the Synechococcus population indicated that cell divisions occurred throughout the day and that a maximum is reached at dusk. The specific growth rate of the Synechococcus population determined by diffusion chamber cultures in which the micrograzers were removed by 1-μm Nuclepore filters, and by the FDC method, was as high as 0.10·h −1 which can double the population size in < 1 day. The grazing mortality by the small-sized (2–5 μmn) colorless flagellates and by the other large (> 10 μm) and less abundant heterotrophs (mostly ciliates) determined by diffusion chamber cultures was 0.04 and 0.06 · h −1, respectively. As a result, the growth rates and grazing mortality are expected to be balanced and the PE-dominant Synechococcus maintained its population size at a certain low level in the study areas.

Experimental studies on an omnivorous microflagellate: implications for grazing and nutrient regeneration in the marine microbial food chain

A phagotrophic marine microflagellate Paraphysomonas imperforata was found to graze on a wide assortment of phytoplankton species as well as bacteria; it also resorted to cannibalism when food was in short supply. Growth rates of the microflagellate were higher than those of the phytoplankton prey, although in some cases lower measured growth rates were found when there was aggregation of cells. This aggregation seemed to be bacterially mediated. The ratio of predator to prey cell length varied from about 2 for the phytoplankton prey to 7 for bacteria. Nitrogen regeneration by the microflagellate, primarily as NH 4 +, never exceeded 50% of the nitrogen originally incorporated by the phytoplankton and bacterial prey. The role of bacteria in regenerating nutrients was found to be minimal relative to microflagellates. These results imply that both omnivory by small protozoa and a greater flexibility in the relationship between predator and prey sizes should be incorporated into the contemporary “microbial food loop” concept. In addition, to achieve nutrient regeneration efficiencies in pelagic surface waters of 80–90%, as is generally believed to occur, requires that the microbial food web be exceedingly complex with a hierarchy of at least several grazing steps. Alternatively, nutrient regeneration efficiencies in surface waters may be lower than envisioned.