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Archive for phytoplankton

How does the competition between phytoplankton and bacteria for iron alter ocean biogeochemical cycles?

Posted by mmaheigan

· Friday, August 26th, 2022

Free-living bacteria play a key role in cycling essential biogeochemical resources in the ocean, including iron, via their uptake, transformation, and release of organic matter throughout the water column. Bacteria process half of the ocean’s primary production, remineralize dissolved organic matter, and re-direct otherwise lost organic matter to higher trophic levels. For these reasons, it is crucial to understand what factors limit the growth of bacteria and how bacteria activities impact global ocean biogeochemical cycles.

In a recent study, Pham and colleagues used a global ocean ecosystem model to dive into how iron limits the growth of free-living marine bacteria, how bacteria modulate ocean iron cycling, and the consequences to marine ecosystems of the competition between bacteria and phytoplankton for iron.

Figure 1: (a) Iron limitation status of bacteria in December, January, and February (DJF) in the surface ocean. Low values (in blue color = close to zero) mean that iron is the limiting factor for the growth of bacteria; (b) Bacterial iron consumption in the upper 120m of the ocean and (c) Changes (anomalies) in export carbon production when bacteria have a high requirement for iron.

Through a series of computer simulations performed in the global ocean ecosystem model, the authors found that iron is a limiting factor for bacterial growth in iron-limited regions in the Southern Ocean, the tropical, and the subarctic Pacific due to the high iron requirement and iron uptake capability of bacteria. Bacteria act as an iron sink in the upper ocean due to their significant iron consumption, a rate comparable to phytoplankton. The competition between bacteria and phytoplankton for iron alters phytoplankton bloom dynamics, ocean carbon export, and the availability of dissolved organic carbon needed for bacterial growth. These results suggest that earth system models that omit bacteria ignore an important organism modulating biogeochemical responses of the ocean to future changes.

Authors:
Anh Le-Duy Pham (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Olivier Aumont (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Lavenia Ratnarajah (University of Liverpool, United Kingdom)
Alessandro Tagliabue (University of Liverpool, United Kingdom)

Predators Set Range for the Ocean’s Most Abundant Phytoplankton

Posted by mmaheigan

· Friday, April 1st, 2022

Prochlorococcus is the world’s smallest phytoplankton (microscopic plant-like organisms) and the most numerous, with more than ten septillion individuals. This tiny plankton lives ubiquitously in warm, blue, tropical waters but is conspicuously absent in more polar regions. The prevailing theory was the cold: Prochlorococcus doesn’t grow at low temperatures. In a recent paper, the authors argue ecological control, in particular, predation by zooplankton. Cold polar waters are greener because they contain more nutrients, leading to more life and more organic matter production. This production feeds more and larger heterotrophic bacteria, who then feed larger predators—specifically the same zooplankton that consume Prochlorococcus. If the shared zooplankton increases enough, it will consume Prochlorococus faster than it can grow, causing the species to collapse at higher latitudes. These results show that an understanding of both ecology and temperature is required to predict how these ecosystems will shift in a warming ocean.

Figure 1: Surface populations of Prochlorococcus collapse (dashed lines) moving northward from Hawaii as seen in transects (transect line shown in red on map, lower left) from cruises in April 2016 (black dots) and September 2017 (green triangles). This collapse of the Prochlorococcus emerges in dynamical computer models (lower right, color indicates Prochlorococcus biomass in mgC/m3) when heterotrophic bacteria and Prochlorococcus share a grazer (top schematic). Increased organic production heading poleward first increases the heterotrophic bacterial population, increasing the shared zooplankton population which eventually consumes Prochlorococcus faster than it can grow (dashed contour).

Authors
Christopher L. Follett (MIT)
Stephanie Dutkiewicz (MIT)
François Ribalet (UW)
Emily Zakem (USC)
David Caron (USC)
E. Virginia Armbrust (UW)
Michael J. Follows (MIT)

Ocean Acidification drives shifts in global stoichiometry and carbon export efficiency

Posted by mmaheigan

· Friday, November 19th, 2021

Marine food webs and biogeochemical cycles react sensitively to increases in carbon dioxide (CO₂) and associated ocean acidification, but the effects are far more complex than previously thought. A comprehensive study published in Nature Climate Change by a team of researchers from GEOMAR dove deep into the impacts of ocean acidification on marine biota and biogeochemical cycling. The authors combined data from five large-scale field experiments with natural plankton communities to investigate how carbon cycling and export respond to ocean acidification.

The biological pump is a key mechanism in transferring carbon to the deep ocean and contributes significantly to the oceans’ function as a carbon sink. The carbon-to-nitrogen ratio of sinking biogenic particles, here termed (C:N_export), determines the amount of carbon that is transported from the euphotic zone to the ocean interior per unit nutrient, thereby controlling the efficiency of the biological pump. The authors demonstrate for the first time that ocean acidification can change the elemental composition of organic matter export, thereby potentially altering the biological pump and carbon sequestration in a future ocean (Figure 1).

Figure 1: Until now, the common assumption is that changes in C:N (and biogeochemistry, in general) are mainly driven by phytoplankton. In a series of in situ mesocosm experiments in different biomes (left), Taucher et al., (2020) found distinct impacts of ocean acidification on the C:N ratio of sinking organic matter (middle). By linking these observations to analysis of plankton community composition, the authors found a key role of heterotrophic processes in controlling the response of C:N to OA, particularly by altering the quality and carbon content of sinking organic matter within the biological pump (right).

Surprisingly, the observed responses were highly variable: C:N_export increased or decreased significantly with increasing CO₂, depending on the composition of species and the structure of the food web. The authors found that heterotrophic processes driven by bacteria and zooplankton play a key role in controlling the response of C:N_export to ocean acidification. This contradicts the widespread paradigm that primary producers are the principal driver of biogeochemical responses to ocean change.

Considering that such diverse pathways, by which planktonic food webs shape the elemental composition and biogeochemical cycling of organic matter, are not represented in state-of-the-art earth system models, these findings also raise the question: Are currently able to predict the large-scale consequences of ocean acidification with any certainty?

Authors:
Jan Taucher (GEOMAR, Kiel, Germany)
Tim Boxhammer (GEOMAR, Kiel, Germany)
Lennart T. Bach (University of Tasmania, Hobart, Australia)
Allanah J. Paul (GEOMAR, Kiel, Germany)
Markus Schartau (GEOMAR, Kiel, Germany)
Paul Stange (GEOMAR, Kiel, Germany)
Ulf Riebesell (GEOMAR, Kiel, Germany)

Exploiting phytoplankton as a biosensor for nutrient limitation

Posted by mmaheigan

· Wednesday, September 15th, 2021

In the surface ocean, phytoplankton growth is often limited by a scarcity of key nutrients such as nitrogen, phosphorus, and iron. While this is important, there are methodological and conceptual difficulties in characterizing these nutrient limitations.

A recent paper published in Science Magazine leveraged a global metagenomic dataset from Bio-GO-SHIP to address these challenges. The authors characterized the abundance of genes that confer adaptations to nutrient limitation within the picocyanobacteria Prochlorococcus. Using the relative abundance of these genes as an indicator of nutrient limitation allowed the authors to capture expected regions of nutrient limitation, and novel regions that had not previously been studied. This gene-derived indicator of nutrient limitation matched previous methods of assessing nutrient limitation, such as bottle incubation experiments.

These findings have important implications for the global ocean. Characterizing the impact of nutrient limitation on primary production is especially critical in light of future stratification driven by climate change. In addition, this novel methodological approach allows scientists to use microbial communities as an eco-genomic biosensor of adaptation to changing nutrient regimes. For instance, future studies of coastal microbes or other ecosystems may help communities and environmental managers better understand how local microbial populations are adapting to climate change.

Watch an illustrated video overview of this research

Authors:
Lucas J. Ustick, Alyse A. Larkin, Catherine A. Garcia, Nathan S. Garcia, Melissa L. Brock, Jenna A. Lee, Nicola A. Wiseman, J. Keith Moore, Adam C. Martiny
(all University of California, Irvine)

How atmospheric and oceanographic forcing impact the carbon sequestration in an ultra-oligotrophic marine system

Posted by mmaheigan

· Wednesday, August 11th, 2021

Sinking particles are a critical conduit for the export of material from the surface to the deep ocean. Despite their importance in oceanic carbon cycling, little is known about the composition and seasonal variability of sinking particles which reach abyssal depths. Oligotrophic waters cover ~75% of the ocean surface and contribute over 30% of the global marine carbon fixation. Understanding the processes that control carbon export to the deep oligotrophic areas is crucial to better characterize the strength and efficiency of the biological pump as well as to project the response of these systems to climate fluctuations and anthropogenic perturbations.

In a recent study published in Frontiers in Earth Science, authors synthesized data from atmospheric and oceanographic parameters, together with main mass components, and stable isotope and source-specific lipid biomarker composition of sinking particles collected in the deep Eastern Mediterranean Sea (4285m, Ierapetra Basin) for a three-year period (June 2010-June 2013). In addition, this study compared the sinking particulate flux data with previously reported deep-sea surface sediments from the study area to shed light on the benthic–pelagic coupling.

Figure Caption: a) Biplot of net primary productivity vs export efficiency (top and bottom horizontal dashed lines indicate threshold for high and low export efficiency regimes). b) Biplot of POC-normalized concentrations of terrestrial vs. phytoplankton-derived lipid biomarkers of the sinking particles collected in the deep Eastern Mediterranean Sea (Ierapetra Basin, NW Levantine Basin) from June 2010–June 2013, and surface sediments collected from January 2007 to June 2012 in the study area.

Both seasonal and episodic pulses are crucial for POC export to the deep Eastern Mediterranean Sea. POC fluxes peaked in spring April–May 2012 (12.2 mg m⁻² d⁻¹) related with extreme atmospheric forcing. Overall, summer particle export fuels more efficient carbon sequestration than the other seasons. The results of this study highlight that the combination of extreme weather events and aerosol deposition can trigger an influx of both marine labile carbon and anthropogenic compounds to the deep. Finally, the comparison of the sinking particles flux data with surface sediments revealed an isotopic discrimination, as well as a preferential degradation of labile organic matter during deposition and burial, along with higher preservation of land-derived POC in the underlying sediments. This study provides key knowledge to better understand the export, fate and preservation vs. degradation of organic carbon, and for modeling the organic carbon burial rates in the Mediterranean Sea.

Authors:
Rut Pedrosa-Pamies (The Ecosystems Center, Marine Biological Laboratory, US; Research Group in Marine Geosciences, University of Barcelona, Spain)
Constantine Parinos (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Anna Sanchez-Vidal (Group in Marine Geosciences, University of Barcelona, Spain)
Antoni Calafat (Group in Marine Geosciences, University of Barcelona, Spain)
Miquel Canals (Group in Marine Geosciences, University of Barcelona, Spain)
Dimitris Velaoras (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Nikolaos Mihalopoulos (Environmental Chemical Processes Laboratory, University of Crete; National Observatory of Athens, Greece)
Maria Kanakidou (Environmental Chemical Processes Laboratory, University of Crete Greece)
Nikolaos Lampadariou (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Alexandra Gogou (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)

A new proxy for ocean iron bioavailability

Posted by mmaheigan

· Monday, July 26th, 2021

In many oceanic regions, iron exerts strong control on phytoplankton growth, ecosystem structure and carbon cycling. Yet, iron bioavailability and uptake rates by phytoplankton in the ocean are poorly constrained.

Recently, Shaked et al. (2020) (see GEOTRACES highlight), established a new approach for quantifying the availability of dissolved Fe (dFe) in natural seawater based on its uptake kinetics by Fe-limited cultured phytoplankton. In a follow up study published in GBC, this approach was extended to in situ phytoplankton, establishing a standardized proxy for dFe bioavailability in low-Fe ocean regions.

As explained in the short video lecture above, Yeala Shaked, Ben Twining, and their colleagues have analyzed large datasets collected during 10 research cruises (including 3 GEOTRACES section and process cruises) in multiple ocean regions. Dissolved Fe bioavailability was estimated through single cell Fe uptake rates, calculated by combining measured Fe contents of individual phytoplankton cells collected with concurrently-measured dFe concentrations, as well as modeled growth rates (Figure). Then the authors applied this proxy for: a) comparing dFe bioavailability among organisms and regions; b) calculating dFe uptake rates and residence times in low-Fe oceanic regions; and c) constraining Fe uptake parameters of earth system models to better predict ocean productivity in response to climate-change.

The data suggest that dFe species are highly available in low-Fe settings, likely due to photochemical reactions in sunlit waters.

Figure 1: The new bioavailability proxy (an uptake rate constant-kin-app) was calculated for ~1000 single cells from multiple ocean regions. For each cell, the iron quota was measured with synchrotron x-ray fluorescence (left panel), a growth rate was estimated from the PISCES model for the corresponding phytoplankton group (right panel), and the dissolved Fe concentration was measured concurrently (middle panel).

Authors:
Y. Shaked (Hebrew University and Interuniversity Institute for Marine Sciences)
B.S. Twining (Bigelow Lab)
A. Tagliabue (University of Liverpool)
M.T. Maldonado (University of British Columbia)
K.N. Buck (University of South Florida)
T. Mellett (University of South Florida)

References:
Shaked, Y., Twining, B. S., Tagliabue, A., & Maldonado, M. T. (2021). Probing the bioavailability of dissolved iron to marine eukaryotic phytoplankton using in situ single cell iron quotas. Global Biogeochemical Cycles, e2021GB006979. https://doi.org/10.1029/2021GB006979

Shaked, Y., Buck, K. N., Mellett, T., & Maldonado, M. T. (2020). Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics. The ISME Journal, 1–12. https://doi.org/10.1038/s41396-020-0597-3

Joint highlight with GEOTRACES – read here.

Using BGC-Argo to obtain depth-resolved net primary production

Posted by mmaheigan

· Friday, July 23rd, 2021

Net primary production (NPP)—the organic carbon produced by the phytoplankton minus the organic carbon respired by phytoplankton themselves—serves as a major energy source of the marine ecosystem. Traditional methods for measuring NPP rely on ship-based discrete sampling and bottle incubations (e.g., ¹⁴C incubation), which introduce potential artifacts and limit the spatial and temporal data coverage of the global ocean. The global distribution of NPP has been estimated using satellite observations, but the satellite remote sensing approach cannot provide direct information at depth.

Figure 1. Panel A. Trajectories of 5 BGC-Argo and 1 SOS-Argo with the initial float deployment locations denoted by filled symbols. The dash-line at 47° N divided the research area into the northern (temperate) and southern (subtropical) regions. Stars indicate ship stations where ¹⁴C NPP values were measured during NAAMES cruises and compared with NPP from nearby Argo floats. Panels B and C. Monthly climatologies of net primary production (NPP, mmol m^-3 d^-1) profiles in the northern and southern regions of the research area, derived from BGC-Argo measurements using the PPM model. The shadings indicate one standard deviation. The red dotted line indicates mixed layer depth (MLD, m), and the yellow dashed line shows euphotic depth (Z_1%, m).

To fill this niche, a recent study in Journal of Geophysical Research: Biogeosciences, applied bio-optical measurements from Argo profiling floats to study the year-round depth-resolved NPP of the western North Atlantic Ocean (39° N to 54° N). The authors calculated NPP with two bio-optical models (Carbon-based Productivity Model, CbPM; and Photoacclimation Productivity Model, PPM). A comparison with NPP profiles from ¹⁴C incubation measurements showed advantages and limitations of both models. CbPM reproduced the magnitude of NPP in most cases, but had artifacts in the summer (a large NPP peak in the subsurface) due to the subsurface chlorophyll maximum caused by photoacclimation. PPM avoided the artifacts in the summer from photoacclimation, but the magnitude of PPM-derived NPP was smaller than the ¹⁴C result. Latitudinally varying NPP were observed, including higher winter NPP/lower summer NPP in the south, timing differences in NPP seasonal phenology, and different NPP depth distribution patterns in the summer months. With a 6-month record of concurrent oxygen and bio-optical measurements from two Argo floats, the authors also demonstrated the ability of Argo profiling floats to obtain estimates of the net community production (NCP) to NPP ratio (f-ratio), ranging from 0.3 in July to -1.0 in December 2016.

This work highlights the utility of float bio-optical profiles in comparison to traditional measurements and indicates that environmental conditions (e.g. light availability, nutrient supply) are major factors controlling the seasonality and spatial (horizontal and vertical) distributions of NPP in the western North Atlantic Ocean.

Authors:
Bo Yang (University of Virginia, UM CIMAS/NOAA AOML)
James Fox (Oregon State University)
Michael J. Behrenfeld (Oregon State University)
Emmanuel S. Boss (University of Maine)
Nils Haëntjens (University of Maine)
Kimberly H. Halsey (Oregon State University)
Steven R. Emerson (University of Washington)
Scott C. Doney (University of Virginia)

Wildfire impacts on coastal ocean phytoplankton

Posted by mmaheigan

· Wednesday, February 24th, 2021

Wildfire frequency, size, and destructiveness has increased over the last two decades, particularly in coastal regions such as Australia, Brazil, and the western United States. While the impact of fire on land, plants, and people is well documented, very few studies have been able to evaluate the impact of fires on ocean ecosystems. A serendipitously planned research cruise one week after the Thomas Fire broke out in California in December 2017 allowed the authors of this study and their colleagues to sample the adjacent Santa Barbara Channel during this devastating extreme fire event.

In a recent paper published in Journal of Geophysical Research: Oceans, the authors describe the phytoplankton community in the Santa Barbara Channel during the Thomas Fire. Phytoplankton community composition was described using a combination of images of phytoplankton from the Imaging FlowCytobot (McLane Labs) and phytoplankton pigments. Dinoflagellates were the dominant phytoplankton group in the surface ocean during the Thomas Fire, according to both methods (Figure 1).

Figure 1. (A) The fraction of total particle volume imaged by the Imaging FlowCytobot (IFCB) comprised of phytoplankton (green) and detritus (brown). Example IFCB images of ash (counted as part of detritus) particles are outlined in brown. (B) The phytoplankton fraction is then further divided by taxonomy, showing the abundance of nano-sized phytoplankton and especially dinoflagellates during the week of sampling. Example IFCB images of Gonyaulax (outlined in dark green), Prorocentrum (outlined in light green), and Umbilicosphaera (outlined in purple) cells are also shown.

While this study was not able to demonstrate a causal relationship between the Thomas Fire and the presence of dinoflagellates, this result is quite different from previous winters in the Santa Barbara Channel, when picophytoplankton and diatoms typically dominate the winter community. The incidence of dinoflagellates in the Santa Barbara Channel in December 2017 was correlated with the warmer-than-average water temperature during this study, which matched observations from other areas along the Central California coast that winter.

At the time this study was conducted, the Thomas Fire was the largest wildfire in California history. Since then, California fires have increased in danger, destruction, and human mortality; the Mendocino Fire complex (summer 2018) and five separate wildfires in summer 2020 exceeded the impacts of the Thomas Fire. With wildfire severity and frequency increasing not only in California but in coastal regions worldwide, this study gives an important first look at the impact of wildfire smoke and ash on oceanic primary productivity and community composition.

Authors:
Sasha Kramer (University of California Santa Barbara)
Kelsey Bisson (Oregon State University)
Alexis Fischer (University of California Santa Cruz)

Species loss alters ecosystem function in plankton communities

Posted by mmaheigan

· Monday, February 8th, 2021

Climate change impacts on the ocean such as warming, altered nutrient supply, and acidification will lead to significant rearrangement of phytoplankton communities, with the potential for some phytoplankton species to become extinct, especially at the regional level. This leads to the question: What are phytoplankton species’ redundancy levels from ecological and biogeochemical standpoints—i.e. will other species be able to fill the functional ecological and/or biogeochemical roles of the extinct species? Authors of a paper published recently in Global Change Biology explored these ideas using a global three-dimensional computer model with diverse planktonic communities, in which single phytoplankton types were partially or fully eliminated. Complex trophic interactions such as decreased abundance of a predator’s predator led to unexpected “ripples” through the community structure and in particular, reductions in carbon transfer to higher trophic levels. The impacts of changes in resource utilization extended to regions beyond where the phytoplankton type went extinct. Redundancy appeared lowest for types on the edges of trait space (e.g., smallest) or those with unique competitive strategies. These are responses that laboratory or field studies may not adequately capture. These results suggest that species losses could compound many of the already anticipated outcomes of changing climate in terms of productivity, trophic transfer, and restructuring of planktonic communities. The authors also suggest that a combination of modeling, field, and laboratory studies will be the best path forward for studying functional redundancy in phytoplankton.

Figure caption: Examples of the modelled ecological and biogeochemical responses to the extinction of different phytoplankton species.Figure caption: Examples of the modelled ecological and biogeochemical responses to the extinction of different phytoplankton species.

Authors:
Stephanie Dutkiewicz (Massachusetts Institute of Technology)
Philip W. Boyd (Institute for Marine and Antarctic Studies, University of Tasmania)
Ulf Riebesell (GEOMAR Helmholtz Centre for Ocean Research Kiel)

How environmental drivers regulated the long-term evolution of the biological pump

Posted by mmaheigan

· Friday, January 22nd, 2021

The marine biological pump (BP) plays a crucial role in regulating earth’s atmospheric oxygen and carbon dioxide levels by transferring carbon fixed by primary producers into the ocean interior and marine sediments, thereby controlling the habitability of our planet. The rise of multicellular life and eukaryotic algae in the ocean about 700 million years ago would likely have influenced the physical characteristics of oceanic aggregates (e.g., sinking rate), yet the magnitude of the impact this biological innovation had on the efficiency of BP is unknown.

Figure. 1. The impact of biological innovations (left) and environmental factors (atmospheric oxygen level and seawater temperature; right) on the efficiency of marine biological pump (BP). Temperatures are ocean surface temperatures (SST), and atmospheric pO2 is shown relative to the present atmospheric level (PAL). The BP efficiency is calculated as the fraction of carbon exported from the surface ocean that is delivered to the sediment-water interface. The results indicate that evolution of larger sized algae and zooplanktons has little influence on the long-term evolution of biological pump (left panel). The change in the atmospheric oxygen level and seawater surface temperature as environmental factors, on the other hand, have a stronger leverage on the efficiency of biological pump (right panel).

The authors of a recent paper in Nature Geoscience constructed a particle-based stochastic model to explore the change in the efficiency of the BP in response to biological and physical changes in the ocean over geologic time. The model calculates the age of organic particles in each aggregate based on their sinking rates, and considers the impact of primary producer cell size, aggregation, temperature, dust flux, biomineralization, ballasting by mineral phases, oxygen, and the fractal geometry (porosity) of aggregates. The model results demonstrate that while the rise of larger-sized eukaryotes led to an increase in the average sinking rate of oceanic aggregates, its impact on BP efficiency was minor. The evolution of zooplankton (with daily vertical migration in the water column) had a larger impact on the carbon transfer into the ocean interior. But results suggest that environmental factors most strongly affected the marine carbon pump efficiency. Specifically, increased ocean temperatures and greater atmospheric oxygen abundance led to a significant decrease in the efficiency of the BP. Cumulatively, these results suggest that while major biological innovations influenced the efficiency of BP, the long-term evolution of the marine carbon pump was primarily controlled by environmental drivers such as climate cooling and warming. By enhancing the rate of heterotrophic microbial degradation, our results suggest that the anthropogenically-driven global warming can result in a less efficient BP with reduced power of marine ecosystem in sequestering carbon from the atmosphere.

Authors:
Mojtaba Fakhraee (Yale University, Georgia Tech, and NASA Astrobiology Institute)
Noah J. Planavsky (Yale University, and NASA Astrobiology Institute)
Christopher T. Reinhard (Georgia Tech, and NASA Astrobiology Institute)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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