Marine Primary Production and Nutrients

Course Introduction and Context

• Instructor Introduction: Jeff Wright, a seaweed biologist and ecologist, is facilitating the first three weeks of the unit. His research focuses on the primary production of seaweed and the drivers of production in the ocean.

• Curriculum Scope: The initial three-week block covers:

  • Different types of primary production.

  • Environmental drivers and factors impacting production (light regimes, nutrients).

  • Links between production factors and the growth of algae.

  • Learning Outcome 1: Describing the processes of marine primary production.

• Required Reading: All content is derived from Chapter 2 of the textbook Kaiser et al.

• Practical Component: The associated practical laboratory session focuses on solving problems in marine and Antarctic science by applying practical skills, specifically examining photoacclimation and pigment extraction.

Foundations of Marine Primary Production

• Definition of Primary Production: The formation of organic matter through the trapping of light energy and the assimilation of inorganic elements.

  • This is driven fundamentally by photosynthesis.

  • It is performed primarily by photoautotrophs (photosynthetic organisms).

  • Limiting factors include light (quantity, type, and wavelength) and nutrients (different forms of nitrogen and carbon).

• Definition of Productivity: The rate of the net incorporation of carbon into organic compounds.

  • This process involves the harvesting and transfer of inorganic carbon into organic carbon, fixed via energy derived from the light reactions of photosynthesis.

  • Standard Unit of Measurement: Gross productivity is ideally measured in kilograms of carbon fixed per meter squared per year ($kg\,C\,m^{-2}\,yr^{-1}$).

  • Estimation in the ocean is complex due to the open, three-dimensional nature of the environment compared to two-dimensional terrestrial systems.

• Production Balance: Production is the balance between growth (biomass accumulation via photosynthesis) and the loss of carbon/energy (conversion to carbohydrates or metabolic expenditure).

Marine vs. Terrestrial Primary Production

• Global Share: Marine primary productivity accounts for approximately $40\%$ of the Earth's total primary production. Some sources estimate this figure slightly higher, but $40\%$ is the established baseline for this unit.

• Comparison Metrics:

  • Net Primary Productivity (NPP): Marine and terrestrial systems are in the same "ballpark" or order of magnitude.

  • Total Biomass: Marine systems have significantly lower total biomass than terrestrial systems.

  • Turnover Rate: Marine systems exhibit much faster turnover compared to terrestrial systems.

• Explanations for Marine/Terrestrial Differences:

  • Generation Time: Phytoplankton cells may double their population daily. Seaweeds typically live for one to two years, whereas terrestrial trees can live for decades or centuries.

  • Carbon Storage: Terrestrial systems store carbon long-term in woody biomass. Marine systems are in a state of continual turnover.

  • Carbon Leakage: Marine carbon often "leaks" as particulate organic carbon or through the erosion of biomass into the water column.

Taxonomy of Marine Primary Producers

• Phytoplankton (Microalgae): Defined as being less than $2\,mm$ in size, though most are microscopic. All contain chlorophyll and perform photosynthesis.

  • Size Classes: Classified based on dimensions, with various names for different size groups.

  • Examples: Cyanobacteria (photosynthetic bacteria), dinoflagellates, coccolithophores, and diatoms.

• Macroalgae (Seaweed): Categorized into three main phyla based on pigmentation:

  • Rhodophyta (Red Algae): Contain chlorophyll a, sometimes c, and phycobilins (which provide the red color).

  • Chlorophyta (Green Algae): Contain chlorophyll a and chlorophyll b.

  • Ochrophyta / Phaeophyceae (Brown Algae): Contain chlorophyll a and c, and brown pigments such as fucoxanthin.

• Pigment Function: Colors are determined by the relative abundance of pigments. Pigments allow species to harvest light at different wavelengths, providing advantages in low-light or specialized environments.

The Mechanisms of Photosynthesis

• General Reaction Equation:     $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• The Flow of Biological Energy:

  1. Light energy is captured in the chloroplasts by pigments.

  2. Photosynthesis produces energy-rich carbohydrates and releases oxygen.

  3. Carbohydrates are split into $CO_2$ and water, making energy available for cellular functions (respiration).

• The Light Reaction:

  • Pigments harvest light.

  • Light energy is converted into chemical energy in the form of Adenosine Triphosphate ($ATP$) molecules.

• The Dark Reaction:

  • Uses the $ATP$ created in the light reaction.

  • Fixes carbon dioxide into sugar via the carbon cycle (Calvin Cycle).

  • Utilizes the enzyme Rubisco for carbon fixation.

• Chlorophyll a Structure: The primary photosynthetic pigment. It is embedded in the thylakoid membrane of the chloroplasts.

Light Dynamics in the Marine Environment

• Light Measurement: Measured as irradiance in units of micromoles of photons per meter squared per second ($\mu mol\,photons\,m^{-2}\,s^{-1}$). This is also referred to as Photon Flux Density.

• Depth Effects:

  • Attenuation: Irradiance decreases with depth as light is absorbed or scattered by water and particles.

  • Quality Change: Wavelengths change with depth. Red light is absorbed first (shallow). Blue light penetrates the furthest, creating a blue-dominated environment in the deeper photic zone.

• Water Clarity: Turbidity (dirtiness) caused by particles from estuaries or plankton blooms impacts light quality. Particles can either absorb or scatter light, altering the available wavelength spectrum regardless of depth.

Pigment Absorption and the "Green Window"

• Chlorophyll a Absorption: Exhibits two primary peaks at approximately $430\,nm$ (blue region) and $650\,nm$ (red region). There is very little absorbance between these peaks.

• The Green Window: The spectral gap where chlorophyll a does not absorb light. Accessory pigments (fucoxanthin, phycobilins) evolve to harvest light within this window.

• Advantages: Accessory pigments allow algae to maximize light harvesting across the spectrum, specifically in low-light environments.

Photoacclimation and Photosynthetic Curves

• Definition of Photoacclimation: A non-evolutionary response where an organism alters its photosynthetic apparatus (pigment number, pigment type, or reaction center density) within its lifetime to adapt to its light environment.

• The Photosynthetic Unit (PSU):

  • Pigments: Harvest/capture light energy.

  • Reaction Centers: Convert harvested light energy into chemical energy.

• P vs. I Curves: A plot of Photosynthesis ($P$) versus Irradiance ($I$). Features include:

  • X-axis: Irradiance ($\mu mol\,photons\,m^{-2}\,s^{-1}$).

  • Y-axis: Photosynthetic rate (measured via $O_2$ production).

  • Three Parts of the Curve:

    1. Light Limited Phase: Photosynthesis increases linearly with light.

    2. Light Saturated Phase ($P_{max}$): The rate reaches a plateau where more light does not increase photosynthesis.

    3. Photoinhibition: At very high light, the photosynthetic rate declines due to cellular damage.

• Key Photosynthetic Parameters:

  • $P_{max}$: Maximum photosynthetic rate.

  • $\alpha$ (Alpha): The initial slope of the relationship; indicates efficiency at low light.

  • $E_k$: The saturation irradiance ($\frac{P_{max}}{\alpha}$).

  • $R_d$: Dark respiration rate (measured in the absence of light).

  • $E_c$: Compensation irradiance; the light level where photosynthesis exactly balances respiration ($Net\,P = 0$).

Nutrient Dynamics and Productivity Status

• Nutrient Classifications for Seawater:

  • Oligotrophic: Low concentrations of essential nutrients and low carbon/primary production. Typical of the East Coast of Tasmania due to the East Australian Current (EAC), which brings nutrient-poor, warm tropical water.

  • Eutrophic: High rates of organic matter influx, usually when a limiting nutrient becomes non-limiting.

  • Hypertrophic: Extremely high nutrient/production levels.

• Nutrient Flux: Eutrophication can be natural (storms, sedimentation) or anthropogenic (pollution, aquaculture runoff). While sometimes viewed negatively, it represents an increase in production.

Principles of Nutrient Uptake and Limitation

• Liebig’s Law of the Minimum: Growth is not limited by total nutrient availability, but by the specific essential nutrient available in the smallest quantity relative to the organism's requirements.

• Essential Nutrients: Nutrients required for growth that cannot be replaced by another element. Examples include Nitrogen ($N$), Phosphorus ($P$), Iron ($Fe$), and Silica ($Si$).

• Uptake Mechanisms:

  • Seagrasses: Vascular plants with roots; take up nutrients from sediments.

  • Seaweeds/Phytoplankton: Non-vascular; take up nutrients via molecular diffusion across the cell surface.

• Uptake Kinetics:

  • Passive Diffusion: No energy required; uptake rate is a function of external concentration.

  • Active Transport: Requires energy (ATP); exhibits saturation kinetics.

  • $V_{max}$: Maximum nutrient uptake rate.

  • $K_s$: Half-saturation constant; the concentration where uptake is half of $V_{max}$.

• Competition: Phytoplankton generally have a higher surface-area-to-volume ratio than macroalgae, allowing for a much higher $V_{max}$ and the ability to outcompete seaweed for nutrients.

Nitrogen Acquisition in the Ocean

• Limiting Status: Nitrogen is typically the primary limiting nutrient in marine systems.

• Forms of Nitrogen: Nitrate ($NO_3^-$), Nitrite ($NO_2^-$), Nitrous Oxide ($N_2O$), Ammonium ($NH_4^+$), and molecular Nitrogen ($N_2$).

• New vs. Regenerated Production:

  • New Production: Driven by "new" nitrogen entering the system ($NO_3^-$ from upwelling).

  • Regenerated Production: Driven by recycled nitrogen ($NH_4^+$ from excretion/decomposition).

  • f-ratio: The ratio of new production to total production.

• Uptake Energetics:

  • Ammonium ($NH_4^+$): Taken up passively; energetically cheap. It is the form ultimately assimilated into amino acids.

  • Nitrate ($NO_3^-$): Requires active transport (energy) and must be reduced to ammonium within the cell before use.

Carbon and Ecological Stoichiometry

• Carbon Forms: Dissolved $CO_2$ and Bicarbonate ($HCO_3^-$).

  • $CO_2$ is low concentration but cheap (passive diffusion).

  • $HCO_3^-$ is high concentration but expensive (requires active transport and enzymatic conversion).

• Redfield Ratio: The theoretical stoichiometric balance for phytoplankton: $106C:16N:1P$.

• C:N Ratio as a Tool: A ratio of $C:N$ greater than $6$ in phytoplankton generally indicates nitrogen limitation. In macroalgae, the ratio is usually higher (e.g., $10$ to $13$), but still serves as a diagnostic for algal health.

• Other Nutrients:

  • Silica: Essential for diatom frustules (shells).

  • Sulfur: Used in osmoregulation and chemical defense (e.g., sulfuric acid in some seaweeds).

  • Iron: Limits growth in High Nutrient, Low Chlorophyll (HNLC) regions.

Questions & Discussion

• Question (Audience): Why are there large differences in biomass and turnover between marine and terrestrial producers?

• Answer (Jeff Wright): Generation time is the key. Phytoplankton double rapidly; terrestrial trees store carbon for centuries. Additionally, marine systems experience "carbon leakage" and biomass erosion.

• Question (Audience): How can we determine if photoacclimation is due to increased pigments or increased reaction centers?

• Answer (Jeff Wright): We can extract chlorophyll and accessory pigments in the lab. If the hue of the plant changes or if pigment concentration increases per cell, it suggests an increase in accessory pigments. If photosynthesis increases without pigment change, it suggests an increase in the number of photosynthetic units (PSUs).

• Question (Audience): If increasing PSUs increases both $P_{max}$ and $\alpha$, why would plants ever choose to just increase accessory pigments?

• Answer (Jeff Wright): Energetic efficiency. Building full PSUs, including the necessary enzymes and chemical reaction centers, is metabolically expensive. Increasing only pigment molecules is energetically "cheaper."

• Question (Audience): Can I move to a different practical class?

• Answer (Beth via Jeff Wright): Generally no, unless there is a major clash, due to the high enrollment numbers and specific scheduling requirements.