Unusual marine cyanobacteria/haptophyte symbiosis relies on N2 fixation even in N-rich environments.

The microbial fixation of N2 is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N2-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N2 fixation to DIN is unknown. We used stable isotope incubations, catalyzed reporter deposition fluorescence in-situ hybridization (CARD-FISH), and nanoscale secondary ion mass spectrometry (nanoSIMS), to investigate the N source used by the haptophyte host and sensitivity of UCYN-A N2 fixation in DIN-replete waters. We demonstrate that under our experimental conditions, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate (NO3-) and meet little of their N demands via ammonium (NH4+) uptake. Instead the UCYN-A/haptophyte symbiosis relies on UCYN-A N2 fixation to supply large portions of the haptophyte's N requirements, even under DIN-replete conditions. Furthermore, UCYN-A N2 fixation rates, and haptophyte host carbon fixation rates, were at times stimulated by NO3- additions in N-limited waters suggesting a link between the activities of the bulk phytoplankton assemblage and the UCYN-A/haptophyte symbiosis. The results suggest N2 fixation may be an evolutionarily viable strategy for diazotroph-eukaryote symbioses, even in N-rich coastal or high latitude waters.

Synergistic interactions among growing stressors increase risk to an Arctic ecosystem.

Oceans provide critical ecosystem services, but are subject to a growing number of external pressures, including overfishing, pollution, habitat destruction, and climate change. Current models typically treat stressors on species and ecosystems independently, though in reality, stressors often interact in ways that are not well understood. Here, we use a network interaction model (OSIRIS) to explicitly study stressor interactions in the Chukchi Sea (Arctic Ocean) due to its extensive climate-driven loss of sea ice and accelerated growth of other stressors, including shipping and oil exploration. The model includes numerous trophic levels ranging from phytoplankton to polar bears. We find that climate-related stressors have a larger impact on animal populations than do acute stressors like increased shipping and subsistence harvesting. In particular, organisms with a strong temperature-growth rate relationship show the greatest changes in biomass as interaction strength increased, but also exhibit the greatest variability. Neglecting interactions between stressors vastly underestimates the risk of population crashes. Our results indicate that models must account for stressor interactions to enable responsible management and decision-making.

Publisher Correction: Ecological control of nitrite in the upper ocean.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Ecological control of nitrite in the upper ocean.

Microorganisms oxidize organic nitrogen to nitrate in a series of steps. Nitrite, an intermediate product, accumulates at the base of the sunlit layer in the subtropical ocean, forming a primary nitrite maximum, but can accumulate throughout the sunlit layer at higher latitudes. We model nitrifying chemoautotrophs in a marine ecosystem and demonstrate that microbial community interactions can explain the nitrite distributions. Our theoretical framework proposes that nitrite can accumulate to a higher concentration than ammonium because of differences in underlying redox chemistry and cell size between ammonia- and nitrite-oxidizing chemoautotrophs. Using ocean circulation models, we demonstrate that nitrifying microorganisms are excluded in the sunlit layer when phytoplankton are nitrogen-limited, but thrive at depth when phytoplankton become light-limited, resulting in nitrite accumulation there. However, nitrifying microorganisms may coexist in the sunlit layer when phytoplankton are iron- or light-limited (often in higher latitudes). These results improve understanding of the controls on nitrification, and provide a framework for representing chemoautotrophs and their biogeochemical effects in ocean models.

Productivity in the barents sea--response to recent climate variability.

The temporal and spatial dynamics of primary and secondary biomass/production in the Barents Sea since the late 1990s are examined using remote sensing data, observations and a coupled physical-biological model. Field observations of mesozooplankton biomass, and chlorophyll a data from transects (different seasons) and large-scale surveys (autumn) were used for validation of the remote sensing products and modeling results. The validation showed that satellite data are well suited to study temporal and spatial dynamics of chlorophyll a in the Barents Sea and that the model is an essential tool for secondary production estimates. Temperature, open water area, chlorophyll a, and zooplankton biomass show large interannual variations in the Barents Sea. The climatic variability is strongest in the northern and eastern parts. The moderate increase in net primary production evident in this study is likely an ecosystem response to changes in climate during the same period. Increased open water area and duration of open water season, which are related to elevated temperatures, appear to be the key drivers of the changes in annual net primary production that has occurred in the northern and eastern areas of this ecosystem. The temporal and spatial variability in zooplankton biomass appears to be controlled largely by predation pressure. In the southeastern Barents Sea, statistically significant linkages were observed between chlorophyll a and zooplankton biomass, as well as between net primary production and fish biomass, indicating bottom-up trophic interactions in this region.

THE EFFECT OF IRON LIMITATION ON THE PHOTOPHYSIOLOGY OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) UNDER DYNAMIC IRRADIANCE(1).

The effects of iron limitation on photoacclimation to dynamic irradiance were studied in Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) W. Krieg. in terms of growth rate, photosynthetic parameters, pigment composition, and fluorescence characteristics. Under dynamic light conditions mimicking vertical mixing below the euphotic zone, P. antarctica displayed higher growth rates than F. cylindrus both under iron (Fe)-replete and Fe-limiting conditions. Both species showed xanthophyll de-epoxidation that was accompanied by low levels of nonphotochemical quenching (NPQ) during the irradiance maximum of the light cycle. The potential for NPQ at light levels corresponding to full sunlight was substantial in both species and increased under Fe limitation in F. cylindrus. Although the decline in Fv /Fm under Fe limitation was similar in both species, the accompanying decrease in the maximum rate of photosynthesis and growth rate was much stronger in F. cylindrus. Analysis of the electron transport rates through PSII and on to carbon (C) fixation revealed a large potential for photoprotective cyclic electron transport (CET) in F. cylindrus, particularly under Fe limitation. Probably, CET aided the photoprotection in F. cylindrus, but it also reduced photosynthetic efficiency at higher light intensities. P. antarctica, on the other hand, was able to efficiently use electrons flowing through PSII for C fixation at all light levels, particularly under Fe limitation. Thus, Fe limitation enhanced the photophysiological differences between P. antarctica and diatoms, supporting field observations where P. antarctica is found to dominate deeply mixed water columns, whereas diatoms dominate shallower mixed layers.

Massive phytoplankton blooms under Arctic sea ice.

Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Photophysiology in two major southern ocean phytoplankton taxa: photosynthesis and growth of Phaeocystis antarctica and Fragilariopsis cylindrus under different irradiance levels.

The Ross Sea, Antarctica, supports two distinct populations of phytoplankton, one that grows well in sea ice and blooms in the shallow mixed layers of the Western marginal ice zone and the other that can be found in sea ice but thrives in the deeply mixed layers of the Ross Sea. Dominated by diatoms (e.g. Fragilariopsis cylindrus) and the prymnesiophyte Phaeocystis antarctica, respectively, the processes leading to the development of these different phytoplankton assemblages are not well known. The goal of this article was to gain a better understanding of the photophysiological characteristics that allow each taxon to dominate its specific habitat. Cultures of F. cylindrus and P. antarctica were each grown semi-continuously at four different constant irradiances (5, 25, 65, and 125 µmol quanta/m2/s). Fragilariopsis cylindrus produced far less photosynthetic pigment per cell than did P. antarctica but much more photoprotective pigment. Fragilariopsis cylindrus also exhibited substantially lower rates of photosynthesis and growth but also was far less susceptible to photoinhibition of cell growth. Excess photosynthetic capacity, a measure of the ability of phytoplankton to exploit variable light environments, was significantly higher in both strains of P. antarctica than in F. cylindrus. The combination of these characteristics suggests that F. cylindrus has a competitive advantage under conditions where mixed layers are shallow and light levels are relatively constant and high. In contrast, P. antarctica should dominate waters where mixed layers are deep and light levels are variable. These results are consistent with distributions of phytoplankton in the Ross Sea and suggest that light is the primary factor determining composition of phytoplankton communities.