Sasha Tozzi

Publications

• 3.69

  Impact points

  Hydrogen cycling by the unicellular marine diazotroph Crocosphaera watsonii strain WH8501.

  Samuel T Wilson, Sasha Tozzi, Rachel A Foster, Irina Ilikchyan, Zbigniew S Kolber, Jonathan P Zehr, David M Karl

  Applied and environmental microbiology. 10/2010; 76(20):6797-803.

  The hydrogen (H₂) cycle associated with the dinitrogen (N₂) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H₂ production and acetylene (C₂H₂) reduction were continuously measured over the diel cycle with simultaneous measurements ... [more] The hydrogen (H₂) cycle associated with the dinitrogen (N₂) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H₂ production and acetylene (C₂H₂) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H₂ production was coincident with the maximum rates of C₂H₂ reduction. Theoretical stoichiometry for N₂ fixation predicts an equimolar ratio of H₂ produced to N₂ fixed. However, the maximum rate of net H₂ production observed was 0.09 nmol H₂ μg chlorophyll a (chl a)⁻¹ h⁻¹) compared to the N₂ fixation rate of 5.5 nmol N₂ μg chl a⁻¹ h⁻¹, with an H₂ production/N₂ fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H₂ production was hypothesized to be a result of H₂ reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H₂ production rates ∼40-fold to a maximum rate of 3.5 nmol H₂ μg chl a⁻¹ h⁻¹. We conclude that the reassimilation of H₂ by C. watsonii is highly efficient (> 98%) and hypothesize that the tight coupling between H₂ production and consumption is a consequence of fixing N₂ at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H₂ cycle provides unique insight into N₂ fixation and associated metabolic processes in C. watsonii.

• Synergistic effects of iron and temperature on Antarctic plankton assemblages

  J. M. Rose, Feng Y, G. R. DiTullio, R. B. Dunbar, C. E. Hare, P. A. Lee, Lohan M, Long M, W. O. Smith Jr, Sohst B, Tozzi S, Zhang Y, D. A. Hutchins

  Biogeosciences Discussions. 01/2009;

  Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling ar... [more] Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling are unknown. We examined the relative effects of iron addition (+1 nM) and increased temperature (+4°C) on plankton assemblages of the Ross Sea, Antarctica, a region characterized by annual algal blooms and an active microbial community. Increased iron and temperature individually had consistently significant but relatively minor positive effects on total phytoplankton abundance, phytoplankton and microzooplankton community composition, as well as photosynthetic parameters and nutrient drawdown. Unexpectedly, increased iron had a consistently negative impact on microzooplankton abundance, most likely a secondary response to changes in phytoplankton community composition. When iron and temperature were increased in concert, the resulting interactive effects were greatly magnified. This synergy between iron and temperature increases would not have been predictable by examining the effects of each variable individually. Our results suggest the possibility that if iron availability increases under future climate regimes, the impacts of predicted temperature increases on plankton assemblages in polar regions could be significantly enhanced. Such synergistic and antagonistic interactions between individual climate change variables highlight the importance of multivariate studies for marine global change experiments.

• Synergistic effects of iron and temperature on Antarctic phytoplankton and microzooplankton assemblages

  J. M. Rose, Feng Y, G. R. DiTullio, R. B. Dunbar, C. E. Hare, P. A. Lee, Lohan M, Long M, W. O. Smith Jr, Sohst B, Tozzi S, Zhang Y, D. A. Hutchins

  Biogeosciences. 01/2009;

  Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling ar... [more] Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling are unknown. We examined the relative effects of iron addition (+1 nM) and increased temperature (+4°C) on plankton assemblages of the Ross Sea, Antarctica, a region characterized by annual algal blooms and an active microbial community. Increased iron and temperature individually had consistently significant but relatively minor positive effects on total phytoplankton abundance, phytoplankton and microzooplankton community composition, as well as photosynthetic parameters and nutrient drawdown. Unexpectedly, increased iron had a consistently negative impact on microzooplankton abundance, most likely a secondary response to changes in phytoplankton community composition. When iron and temperature were increased in concert, the resulting interactive effects were greatly magnified. This synergy between iron and temperature increases would not have been predictable by examining the effects of each variable individually. Our results suggest the possibility that if iron availability increases under future climate regimes, the impacts of predicted temperature increases on plankton assemblages in polar regions could be significantly enhanced. Such synergistic and antagonistic interactions between individual climate change variables highlight the importance of multivariate studies for marine global change experiments.

• Eutrophication-induced phosphorus limitation in the Mississippi River plume: Evidence from fast repetition rate fluorometry

  J. B. Sylvan, A. Quigg, S. Tozzi, J. W. Ammerman

  Limnology and Oceanography. 01/2007; 52:2679-2685.

• Interannual variations in nutrients, net community production, and biogeochemical cycles in the Ross Sea

  W. O. Smith, Jr, A. R. Shields, G. Catalano, J.A. Peloquin, S. Tozzi, M. S. Dinniman, A. A. Asper

  Deep-Sea Research II. 01/2006; 53:815-833.
• 29.75

  Impact points

  Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters.

  Kenneth H. Coale, Kenneth S Johnson, Francisco P Chavez, Ken O. Buesseler, Richard T. Barber, Mark A Brzezinski, William P Cochlan, Frank J Millero, Paul G Falkowski, James E Bauer, [......], Jodi Brewster, Nicolas Ladizinsky, Geoffrey Smith, David Cooper, David Timothy, Susan L Brown, Karen E Selph, Cecelia C Sheridan, Benjamin S Twining, Zackary I Johnson

  Science (New York, N.Y.). 05/2004; 304(5669):408-14.

  The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the South... [more] The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

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• Variability in spectral backscatter estimated from satellites and its relation to in situ measurements in optically complex coastal waters

  M. A. Moline, R. Arnone, T. Bergmann, S. Glenn, M. J. Oliver, C. Orrico, O. Schofield, S. Tozzi

  International Journal of Remote Sensing. 01/2004; 25:1465-1468.

• Variability in measured and modelled remote sensing reflectance for coastal waters at LEO-15

  S. Tozzi, O. Schofield, T. Bergmann, M. A. Moline, R. Arnone

  International Journal of Remote Sensing. 01/2004; 25:1469-1472.

  A large database of in situ bio-optical measurements were collected at the LEO-15 (Long-term Ecosystem Observatory) off the southern coast of New Jersey, USA. The data were used to quantify the impact of coastal upwelling on near-shore bulk apparent (AOP) and inherent (IOP) optical properties. There... [more] A large database of in situ bio-optical measurements were collected at the LEO-15 (Long-term Ecosystem Observatory) off the southern coast of New Jersey, USA. The data were used to quantify the impact of coastal upwelling on near-shore bulk apparent (AOP) and inherent (IOP) optical properties. There was good qualitative agreement between the AOPs and IOPs in space and time. The measured IOPs were used as inputs to the Hydrolight radiative transfer model (RTE). Estimated spectral AOPs from the RTE were strongly correlated (generally R2>0.80) to measured AOPs. If optical closure between in-water measurements was achieved then the RTE was used to construct the spectral remote sensing reflectance. The modelled remote sensing reflectances were compared to satellite-derived reflectance estimates from four different algorithms. Quantitative agreement between the satellite-measured and in-water modelled remote sensing reflectance was good but results were variable between the different models. The strength of the correlation and spectral consistency was variable with space and time. Correlations were strongest in clear offshore waters and lowest in the near-shore turbid waters. In the near-shore waters, the correlation was strongest for blue wavelengths (400-555 nm) but lower for the red wavelengths of light. © 2004 Taylor and Francis Ltd.

• Historical climate change and ocean turbulence as selective agents for two key phytoplankton functional groups

  S. Tozzi, O. Schofield, P. Falkowski, S. Tozzi

  Marine Ecology Progress Series. 01/2004; 274:123-132.

  Using a classical physiological model based on nutrient uptake kinetics, we explored the effect of turbulence on resource competition and succession between 2 phytoplankton functional groups on ecological and geological time scales. The 2 groups we considered are silica-precipitating diatoms and car... [more] Using a classical physiological model based on nutrient uptake kinetics, we explored the effect of turbulence on resource competition and succession between 2 phytoplankton functional groups on ecological and geological time scales. The 2 groups we considered are silica-precipitating diatoms and carbonate-precipitating coccolithophorids. Using published experimental laboratory data for parameterization, our model results suggest that diatoms dominate under highly turbulent regimes, while coccolithophorids tend to dominate under stable, nutrient-depleted conditions. We attribute the success of diatoms in highly dynamic systems to luxury uptake of nutrients afforded by the evolution of storage vacuoles. In contrast, coccolithophorids are more successful in resource-depleted waters, due to their lower minimum limiting-nutrient requirement (R*). We examine how these differences in nutrient acquisition strategy potentially explain the long-term trends in the fortunes of these 2 taxa on geological time scales. The fossil record indicates that coccolithophorids rose to ecological prominence in the mid-Jurassic and reached an apex in the mid-Cretaceous, but have declined throughout the Cenozoic. In contrast, diatoms have risen rapidly in the late Cenozoic, especially from early-Miocene time to the present. Based on paleoclimate reconstructions, from Mesozoic times, we hypothesize that the relative success of the 2 functional groups reflects, in part, long-term changes in upper ocean turbulence and its influence on the temporal distribution of nutrients.

• SYMPLEX Experiment: first results on oceanic mesoscale dynamics and related primary production from AVHRR and SeaWIFS satellite data and field experiments

  E. Böhm, B. Buongiorno Nardelli, C. Brunet, R. Casotti, F. Conversano, F. Corato, E. D'Acunzo, F. D'Ortenzio, D. Iudicone, L. Lazzara, [......], G. Mori, I. Nardello, C. Nuccio, M. Ribera d'Alcalà, V. Saggiomo, R. Santoleri, M. Scardi, S. Sparnocchia, S. Tozzi, S. Zoffoli

  The European Symposium on Remote Sensing, Barcelona SPA; 01/1998

• Distribution of chlorophyll a in the eastern Mediterranean in relation to sub-basin structures: analysis of the remotely sensed data

  Sasha Tozzi, B. Buongiorno Nardelli, F. D'Ortenzio

  AIOL, Vulcano, Italia; 01/1997

• Surface layer variability in the Ross Sea, Antarctica as assessed by in situ fluorescence measurements

  Walker O. Smith, Vernon Asper, Sasha Tozzi, Xiao Liu, Sharon E. Stammerjohn

  Progress In Oceanography.

  Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003–2004, 2004–2005, and 2005–2006) to assess the short-t... [more] Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003–2004, 2004–2005, and 2005–2006) to assess the short-term, seasonal and interannual variations in phytoplankton biomass and oceanographic conditions. The seasonal climatologies of physical forcing variables were also determined from satellite measurements, and the data from the two sites compared to the 2000–2009 mean. In situ fluorometers were deployed at three depths at 77°S, 172.7°E and 77.5°S, 180°. Significant differences between the two sites were consistently observed, confirming the anticipated high level of spatial and temporal heterogeneity. Chlorophyll fluorescence was maximal in late December, and generally decreased rapidly to modest levels in January and February. However, during 1 year (2003–2004) a secondary bloom was found, with summer maxima being similar to those observed during spring. Fluorescence displayed a strong diel cycle, with strong quenching during periods of maximum irradiance. The magnitude of this reduction was large (the minimum average fluorescence was 25% of the daily mean) and decreased with depth. Fluorescence varied interannually, with the absolute levels and temporal patterns being different among years. The two sites had different temperature/salinity properties as measured at 24 m, and both variables changed with time. During 2004–2005 we were able to continuously measure the photosynthetic quantum efficiency of PSII (Fv/Fm) at 11 m, which revealed a minimum in December, and an increase in January, whereas the absolute fluorescence (Fo) decreased simultaneously. We suggest that this reflected a mixing event, whereby available irradiance increased, allowing a short period of growth in a more favorable optical environment. While substantial variations from the mean physical forcing were observed, the linkage of these physical variations with fluorescence was not always clear. Short-term (over 24-h) changes in fluorescence occurred, and were likely related to advective events. Wind events altered fluorescence in the surface layer, and these redistributed phytoplankton in the surface. The variability in chlorophyll fluorescence and physical forcing over a variety of scales in the Ross Sea provides insights into temporal–spatial coupling of phytoplankton.

• Interannual variations in nutrients, net community production, and biogeochemical cycles in the Ross Sea

  Walker O. Smith, Amy R. Shields, Jill A. Peloquin, Giulio Catalano, Sasha Tozzi, Michael S. Dinniman, Vernon A. Asper

  Deep Sea Research Part II: Topical Studies in Oceanography.

  The Ross Sea continental shelf is dominated by the seasonal appearance of a large phytoplankton bloom. This bloom is regularly dominated by diatoms and the haptophyte Phaeocystis antarctica, and significant nutrient (nitrogen and silicon) reductions within the water column occur during the growing s... [more] The Ross Sea continental shelf is dominated by the seasonal appearance of a large phytoplankton bloom. This bloom is regularly dominated by diatoms and the haptophyte Phaeocystis antarctica, and significant nutrient (nitrogen and silicon) reductions within the water column occur during the growing season (early November to late February). Diatoms mediate silicic acid removal, whereas both taxa remove nitrate. Dissolved and particulate nitrogen and silica concentrations were collected from a series of cruises to the southern Ross Sea over 3 years. Simple, one-dimensional nutrient budgets were generated for nitrogen and silica, and estimates of vertical flux were derived from these budgets. Substantial variations among years are observed to occur in seasonal community production, assemblage composition, Si:N uptake ratios, and export, and standard deviations are equal to ∼30% of the mean. During 2003–2004 a large Phaeocystis antarctica bloom occurred in December, and was followed by a bloom of diatoms. This secondary bloom was equal in magnitude to that of the initial P. antarctica bloom. In contrast, no secondary bloom was observed in 2001–2002. Continuous fluorescence measurements suggested that the spatial–temporal mosaic of phytoplankton dynamics in the Ross Sea is far more complex than previously thought. We hypothesize that variations occur between years not only in terms of both magnitude and composition of the bloom, but also in the controlling mechanisms.

• Phytoplankton photosynthetic pigments in the Ross Sea: Patterns and relationships among functional groups

  Walker O. Smith, Michael S. Dinniman, Sasha Tozzi, Giacomo R. DiTullio, Olga Mangoni, Monica Modigh, Vincenzo Saggiomo

  Journal of Marine Systems.

  Phytoplankton assemblages of the Ross Sea are generally dominated by two functional groups: diatoms and haptophytes (Phaeocystis antarctica). Within this “normal” pattern of dominance, there is a substantial amount of temporal (over months, seasons and years) and spatial variability. Such variabilit... [more] Phytoplankton assemblages of the Ross Sea are generally dominated by two functional groups: diatoms and haptophytes (Phaeocystis antarctica). Within this “normal” pattern of dominance, there is a substantial amount of temporal (over months, seasons and years) and spatial variability. Such variability has a significant impact on several biogeochemical cycles, such as the carbon and sulfur cycles, at the regional and global scales. We compiled all available accessory pigment data for the southern Ross Sea as a means to quantify the prevalence and dominance of each group, and generated a seasonal “climatology” of assemblage composition. The climatological pattern of phytoplankton pigments shows that haptophytes normally grow and accumulate early in the season, and largely in the southern Ross Sea polynya. Diatoms reach a biomass maximum later, and reach most extensive concentrations closer to the coast of Victoria Land. While the pattern of spring growth of P. antarctica followed by an increase in diatom abundance is found frequently, deviations from that pattern were observed. Two periods – November, 2006 and January, 2004 – illustrated that variations of up to one order of magnitude can occur relative to the climatology. These deviations may provide insights into the dominant control mechanisms of the two functional groups.

• Interactive effects of iron, irradiance and CO2 on Ross Sea phytoplankton

  Y. Feng, C.E. Hare, J.M. Rose, S.M. Handy, G.R. DiTullio, P.A. Lee, W.O. Smith, J. Peloquin, S. Tozzi, J. Sun, Y. Zhang, R.B. Dunbar, M.C. Long, B. Sohst, M. Lohan, D.A. Hutchins

  Deep Sea Research Part I: Oceanographic Research Papers.

  We conducted a factorial shipboard continuous culture experiment to examine the interactive effects of altered iron, irradiance and CO2 on the summer phytoplankton community of the Ross Sea, Antarctica. After 18 days of continuous incubation, iron enrichment increased phytoplankton biomass, nutrient... [more] We conducted a factorial shipboard continuous culture experiment to examine the interactive effects of altered iron, irradiance and CO2 on the summer phytoplankton community of the Ross Sea, Antarctica. After 18 days of continuous incubation, iron enrichment increased phytoplankton biomass, nutrient drawdown, diatom and Phaeocystis abundance, and some photosynthetic parameters. High irradiance significantly increased the number of Phaeocystis antarctica colonies, as well as P. antarctica abundance relative to diatoms. Iron and light had significant interactive effects on diatom and P. antarctica pigment concentrations, P. antarctica colony abundance, and Si:N, Si:C, and N:P ratios. The major influence of high CO2 was on diatom community structure, by favoring the large centric diatom Chaetoceros lineola over the small pennate species Cylindrotheca closterium. The ratio of centric to pennate diatoms was significantly responsive to changes in all three variables individually, and to all of their possible two- and three-way combinations. These results suggest that shifts in light, iron, and CO2 and their mutual interactions all play a role in controlling present day Ross Sea plankton community structure, and need to be considered when predicting the possible future responses of biology and biogeochemistry in this region.

• Synergistic effects of iron and temperature on Antarctic phytoplankton and microzooplankton assemblages

  J. M. Rose, Y Feng, G. R. DiTullio, R. B. Dunbar, C. E. Hare, P. A. Lee, M. Lohan, M. Long, W. O. Smith, B. Sohst, S. Tozzi, Y Zhang, D. A. Hutchins

  © 2009 The Authors. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 6 (2009): 3131-3147, doi: 10.5194/bg-6-3131-2009 Iron availability and temperature are important limiting factors for the biota in m... [more] © 2009 The Authors. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 6 (2009): 3131-3147, doi: 10.5194/bg-6-3131-2009 Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling are unknown. We examined the relative effects of iron addition (+1 nM) and increased temperature (+4°C) on plankton assemblages of the Ross Sea, Antarctica, a region characterized by annual algal blooms and an active microbial community. Increased iron and temperature individually had consistently significant but relatively minor positive effects on total phytoplankton abundance, phytoplankton and microzooplankton community composition, as well as photosynthetic parameters and nutrient drawdown. Unexpectedly, increased iron had a consistently negative impact on microzooplankton abundance, most likely a secondary response to changes in phytoplankton community composition. When iron and temperature were increased in concert, the resulting interactive effects were greatly magnified. This synergy between iron and temperature increases would not have been predictable by examining the effects of each variable individually. Our results suggest the possibility that if iron availability increases under future climate regimes, the impacts of predicted temperature increases on plankton assemblages in polar regions could be significantly enhanced. Such synergistic and antagonistic interactions between individual climate change variables highlight the importance of multivariate studies for marine global change experiments. This project was supported by US NSF grants ANT 0528715 to JMR, ANT 0741411, ANT 0741428 and OCE 0825319 to DAH, ANT 0338157 to WOS, ANT 0338097 to GRD, and ANT 0338350 to RBD.