Nutrient Limitation in the World's Oceans

Phytoplankton and Nutrient Limitation in Oceans

Nutrients Required by Phytoplankton

• Essential Nutrients:
  • Carbon (C), Nitrogen (N), Phosphorus (P), Iron (Fe), and Sulfur (S) are required by all phytoplankton.
  • Silicon (Si) is specifically required by diatoms.
• Nutrient Availability Impact:
  • The availability of these nutrients affects the production of organic material, influencing food webs.

Nitrogen

• Nitrogen Use:
  • Most phytoplankton use inorganic "fixed" nitrogen for growth.
  • Only a few, called diazotrophs (specialized bacteria and archaea), can use N_2 gas.
  • Some can even utilize organic nitrogen.

Nitrogen Cycle

• Key Processes:
  • Nitrogen fixation: Conversion of N_2 gas to usable forms.
  • Nitrification: Conversion of ammonia (NH4) to nitrite (NO2) and then to nitrate (NO_3).
  • Denitrification: Conversion of nitrate to N_2 gas, removing nitrogen from the system.
• Nitrogen Inputs:
  • River runoff.
  • Upwelling.
• Nitrogen Transformations:
  • Ammonium (NH4) is the preferred form for phytoplankton, followed by nitrite (NO2) and then nitrate (NO_3) (Glibert et al., 2016).

Impact of Nitrogen Limitation

• Surface Area to Volume Ratio:
  • Increases when nitrogen is limiting.
• Cell Size
  • Smaller cells are more common.
• Diazotrophs:
  • More diazotrophs are observed. They fix nitrogen and photosynthesize.
• Diatom-Diazotroph Associations:
  • Some diatoms form associations with diazotrophs, like heterocyst-forming symbionts within Hemiaulus hauckii.

Significance of Nitrogen Fixation

• Contribution to Carbon Flux:
  • N_2 fixation contributes significantly to carbon flux in the open ocean (30-50% near Hawaii).
• Energy Requirement:
  • N_2 fixation is rare because of the triple bond between the two nitrogen atoms, requiring a lot of energy to break.
  • Nitrate (NO3) and ammonium (NH4) are more convenient due to lower energy expenditure.

Importance of N Fixation

• Growth Sustenance:
  • Allows organisms to continue growing when nitrate and ammonium are depleted.
• Replenishment:
  • Nitrogen-fixers convert inert N2 to usable forms (NO3 and NH_4).
• Fertilization:
  • Fertilizes the ocean, allowing NO3- and NH4-using phytoplankton to grow.
• Source of New Nitrogen:
  • Acts as a source of new nitrogen to the environment.
• Trade-off:
  • Diazotrophs are generally slow growers.

Control of Phytoplankton Abundance

• Regional Control:
  • Nitrogen fixation controls phytoplankton abundance in some regions, which in turn controls the abundance of fish, whales, and other animals.
• N2 Fixation Under Sea Ice:
  • Study of nitrogen fixation under sea ice (Shiozaki et al 2020) shows varying rates of fixation at different stations.
  • Examples:
    • Sta. BP: 13,783 nifH transcription
    • Sta. E: 3,003 nifH transcription
    • Sta. D: 38,896 nifH transcription

Mechanism of N Fixation

• Nitrogenases:
  • Carried out by metalloenzymes called nitrogenases.
• Oxygen Sensitivity:
  • Nitrogenases are sensitive to destruction by O_2, requiring a nearly anoxic environment.

Dealing with Oxygen

• Anaerobic Environment:
  • Live in an anaerobic environment
• Specialized Cells:
  • Develop specialized cells to limit O_2 exposure.
• Example: Cyanobacteria
  • Cyanobacteria separate oxygenic photosynthesis and N_2 fixation spatially (in different cells) or temporally (during the night), or a combination of both.

Heterocysts

• Specialized Cells:
  • Heterocysts are thick-walled, hollow-looking cells larger than vegetative cells.
• Function:
  • Provide an anaerobic environment for N fixation.
• Characteristics:
  • Larger than vegetative cells, hollow-looking, thick-walled (prevents gas entry),
  • Photosynthetically inactive (no CO2 fixation or O2 evolution).
• Triggered by:
  • Formation triggered by low [nitrogen] and [molybdenum].

Separation of N2 Fixation from Photosynthesis

• Non-heterocystous Diazotrophs:
  • Some diazotrophs lack heterocysts.
• Timing:
  • N_2 fixation peaks at mid-day, coinciding with photosynthesis.
• Oxygen Inhibition:
  • Nitrogenase is irreversibly inhibited by oxygen.

The Mehler Reaction

• Mechanism:
  • Non-heterocystous diazotrophs use the Mehler reaction.
  • The oxygen produced by PSII is reduced again after PSI into hydrogen peroxide (H2O2).
• Trade-off:
  • H2O2 can be toxic but does not damage nitrogenase.

Nitrogen Loss

• Denitrification:
  • Key process by which nitrogen leaves the ecosystem.
• ANAMMOX:
  • Anaerobic ammonium oxidation.
  • Denitrification and annammox are the main pathways of N out of the ecosystem

Review Questions

• What process brings N_2 gas into the organic form?
• What process causes organic nitrogen to leave the organic form?
• Which form of fixed inorganic nitrogen do phytoplankton prefer?
• Name one HNLC region and its limiting nutrient.

Phosphorus

• Ocean Phosphate Concentration:
  • Ranges from 0.0 to 1.8 μmol/L (data from NOAA World Ocean Atlas).

Phosphorus Cycling

• Simpler than N cycling:
  • Key processes include atmospheric deposition, river input, uptake by phytoplankton, and upwelling.
• Forms of Phosphorus:
  • DIP (Dissolved Inorganic Phosphorus) – Orthophosphate (PO_4^{-2}) is the most abundant and preferred P source for phytoplankton.
  • DOP (Dissolved Organic Phosphorus) – A large fraction of P in surface waters is DOP, which phytoplankton can use.

Strategies for Low DIP Levels

• Surface to Volume Ratio:
  • Phytoplankton increase their surface to volume ratio.
• High-Affinity Phosphate Binding Proteins:
  • Phytoplankton use these, also seen in viruses that infect phytoplankton.
• DOP Utilization:
  • Phytoplankton can hydrolyze DOP to orthophosphate.

Phosphorus Metabolism

• Forms of Phosphorus:
  • Phytoplankton can take up both inorganic and organic forms.
• Concentrations:
  • DOP concentrations can exceed DIP, especially in the photic zone.
• Molecular Swapping:
  • Phytoplankton can swap out P-containing molecules for others.

DOP Compound Access

• Variability:
  • Not all phytoplankton can access the same DOP compounds.
• Examples:
  • Trichodesmium (cyanobacterium) can use phosphonates, monophosphate esters, and inorganic phosphate.
  • Diatoms cannot use phosphonates.
• Enzymatic Hydrolysis:
  • Alkaline phosphatase can hydrolyze phosphomonoesters into bioavailable phosphate at alkaline pH (Kuenzler and Perras 1965).

Phosphorus Conservation

• Lipid Replacement:
  • Phytoplankton replace P-containing lipids with non-P-containing sulfolipids (sulfur-containing) or betaine lipids (nitrogen-containing).
• Adaptation to Oligotrophy:
  • Adjusting the amount of P in cells and recycling lipid P are important adaptations.

Nutrient Uptake and Redfield Ratio

• Redfield Ratio:
  • Alfred Redfield reported the average elemental ratios of C:N:P in POM and dissolved nutrients are nearly constant at 106:16:1.
  • This has been influential in understanding biogeochemical cycles.
• Variations:
  • There are significant temporal and geographical deviations and differences between phytoplankton species.
• Adaptations:
  • Some organisms adapt to low-resource conditions, while others respond to high-nutrient concentrations with rapid growth.

Iron

• Iron Supply:
  • Coastal upwelling, runoff from continents, atmospheric deposition (dust storms, volcanic ash), and upwelling.
• Essential Role:
  • Synthesis of chlorophyll, nitrate utilization, and N_2 fixation (nitrogenase).

Iron Delivery

• Storm and Weathering:
  • Delivery through storms/ weathering of rocks deliver iron to HNLC areas
• Atmospheric Deposition:
  • Mineral aerosols (dust) are deposited on the ocean surface.
  • Dust contains iron that can dissolve and become available to phytoplankton.

Iron and Nutrient Dynamics

• Chlorophyll and Nutrient Relationship:
  • In the North Atlantic, a spring bloom correlates with decreases in silicate and nitrate as chlorophyll increases.
• Subarctic North Pacific:
  • Data from 1984, 1987, and 1988 show interannual variation in nitrate concentration and chlorophyll crop.

High Nutrient Low Chlorophyll (HNLC) Regions

• Characteristics:
  • High macronutrients (N, P, Si) but low phytoplankton biomass.
  • HNLC regions cover 20-30% of the ocean.
• Iron Limitation:
  • Iron is insoluble in oxygenated seawater and precipitates, limiting phytoplankton growth.

Iron Enrichment Experiments

• Mesoscale Experiments:
  • Experiments like those in the equatorial Pacific examine community-level responses to iron.
  • Iron is added as acidic iron sulfate with an inert tracer (SF6).

Impact of Iron Enrichment

• Phytoplankton Community Changes:
  • Specific components of the phytoplankton community increase in biomass.
  • Diatoms respond strongly to iron addition (SERIES and SOIREE experiments).

Diatom Response

• Rapid Growth and Disappearance:
  • Diatoms grow rapidly but then aggregate and sink, leading to a decrease in cell numbers.

Lessons from HNLC Experiments

• Iron Supply:
  • Low iron supply maintains HNLC conditions, suppressing phytoplankton growth and biomass production.
• Cell Size:
  • Low iron favors smaller cells (picoplankton).
• Grazing:
  • Active microzooplankton grazing keeps picoplankton biomass low and relatively invariant.
  • Results in a highly regenerative upper ocean with rapid NH_4^+ cycling.

HNLC Regions Summary

• Prevalence:
  • HNLC waters cover 30% of the open ocean.
• Cause:
  • Iron supply causes the HNLC condition, though light, grazing, or silicic acid supply can also influence regions.
• Biomass Control:
  • Biomass levels are set by grazing pressure, which resupplies iron.

Sulfur

• Essential Component:
  • Essential for proteins (cysteine and methionine) and therefore essential for life.
  • Average cell contains ~1% S by dry weight.
• Largest Reservoirs:
  • Earth's crust: gypsum (CaSO4) and pyrite (FeS2).
  • Ocean: sulfate anions, dissolved hydrogen sulfide gas, and elemental sulfur.
• Role in Ecosystem:
  • Plays a role in food web dynamics and weather.

Sulfur Cycling

• Key Components:
  • Oceanic sulfate, atmospheric photochemistry, volcanic gases, hydrothermal circulation, sedimentary pyrite formation.
• Conditions:
  • Aerobic and anaerobic conditions drive various sulfur transformations.
• Processes:
  • Microbial oxidation, reduction, assimilation, and decomposition.

Chemolithoautotrophs

• Energy Source:
  • Use inorganic electron donors for energy and reducing power.
• Mechanism:
  • Electron transport chain to oxidize inorganic molecules and generate energy for ATP synthesis.
• Importance:
  • Important in water column, sediments, and hydrothermal vents.

Phytoplankton, Weather, and the Sea

• DMSP Production:
  • Phytoplankton produce Dimethylsulfoniopropionate (DMSP).

DMSP and DMS

• Role in Climate:
  • DMS (dimethyl sulfide) influences cloud formation (CCN) and increases albedo.
• Ecological Roles:
  • Chemoattraction for zooplankton, seabirds, and marine mammals.
• Bacterial Processes:
  • DMS is consumed by bacteria and converted into various compounds.

Eutrophication

• Nutrient Status:
  • Oligotrophic: extremely low nutrient waters.
  • Eutrophic: high nutrient waters.

Causes of Eutrophication

• Major Causes:
  • Fertilizer runoff, land use changes, sewage release.

Consequences of Eutrophication

• Dead Zones:
  • Can lead to hypoxic (low oxygen) or anoxic (no oxygen) conditions.
• Process:
  • Algal cells die, organic carbon sinks, and bacteria respire, consuming oxygen.
  • C6H{12}O6 + 6O2 + 6H2O → 12H2O + 6CO_2

Gulf of Mexico Dead Zone

• Area:
  • 41% of the US area drains into the Gulf of Mexico.
• Hypoxia:
  • Region of hypoxic waters (less than 2 ppm dissolved oxygen) at the mouth of the Mississippi River.
• Size:
  • Can exceed 8,000 square miles.
• Location:
  • Occurs at the Mississippi River delta and extends westward to the upper Texas coast.

Economic Impacts of Gulf of Mexico Dead Zone

• Costs:
  • Costs U.S. seafood and tourism industries ~$82 million a year.
• Seafood Industry:
  • Impacts Gulf's seafood industry, which accounts for > 40% of US seafood.
  • Louisiana is 2nd in seafood production (Alaska is 1st).
• Fishing Practices:
  • Fishermen travel farther from land, increasing time and money.
• Species Impact:
  • Species that can't move die off, leading to the name "dead zone."