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."