💧Limnology Unit 5 – Phytoplankton and primary production

What Are Phytoplankton?

• Phytoplankton are microscopic, photosynthetic organisms that inhabit the upper sunlit layer of almost all oceans and bodies of fresh water
• Consist of a diverse group of organisms including eukaryotic cells and cyanobacteria (blue-green algae)
  • Eukaryotic phytoplankton include diatoms, dinoflagellates, and coccolithophores
  • Cyanobacteria are prokaryotic and include species like Prochlorococcus and Synechococcus
• Range in size from less than 1 micrometer to over 100 micrometers
• Contain chlorophyll and other photosynthetic pigments that allow them to convert sunlight into chemical energy
• Play a crucial role in the global carbon cycle by fixing carbon dioxide into organic compounds through photosynthesis
• Serve as the foundation of the aquatic food web, providing energy for higher trophic levels
• Contribute significantly to the world's oxygen production, with estimates suggesting they produce about 50% of the oxygen in Earth's atmosphere

Phytoplankton's Role in Aquatic Ecosystems

• Phytoplankton form the base of the aquatic food web, providing energy and nutrients for higher trophic levels
  • Zooplankton, small fish, and other organisms directly consume phytoplankton
  • Larger predators then feed on these primary consumers, transferring energy up the food chain
• Engage in a symbiotic relationship with some zooplankton species (e.g., corals and foraminifera) by providing them with energy and nutrients
• Influence the biogeochemical cycles of carbon, nitrogen, phosphorus, and other elements in aquatic ecosystems
  • Fix carbon dioxide through photosynthesis, converting it into organic compounds
  • Assimilate dissolved nutrients like nitrate and phosphate, making them available to other organisms
• Affect water quality and clarity by regulating the amount of dissolved nutrients and particulate matter in the water column
• Contribute to the biological pump, a process that transports carbon from the atmosphere to the deep ocean through sinking of dead phytoplankton and fecal pellets
• Serve as indicators of ecosystem health, with changes in phytoplankton community composition and abundance reflecting shifts in environmental conditions

Types and Classification of Phytoplankton

• Phytoplankton are classified based on their size, morphology, and taxonomic groups
• Size classes include picoplankton (0.2-2 μm), nanoplankton (2-20 μm), and microplankton (20-200 μm)
• Major taxonomic groups include:
  • Diatoms (Bacillariophyceae): characterized by their silica cell walls and unique morphologies
  • Dinoflagellates (Dinophyceae): known for their flagella and ability to produce bioluminescence and toxins
  • Coccolithophores (Prymnesiophyceae): distinguished by their calcium carbonate plates (coccoliths) covering the cell surface
  • Green algae (Chlorophyta): contain chlorophyll a and b, and include both unicellular and colonial forms
  • Cyanobacteria (Cyanophyta): prokaryotic organisms with a wide range of morphologies and ecological roles
• Other groups include chrysophytes, cryptophytes, and euglenoids
• Phytoplankton community composition varies depending on environmental factors such as temperature, nutrient availability, and light intensity
  • Diatoms often dominate in cool, nutrient-rich waters
  • Cyanobacteria thrive in warm, nutrient-poor conditions

Primary Production Basics

• Primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide through photosynthesis or chemosynthesis
• In aquatic ecosystems, phytoplankton are the main primary producers, converting light energy into chemical energy stored in organic molecules
• Gross primary production (GPP) is the total amount of organic matter produced by photosynthesis
  • GPP = Net Primary Production (NPP) + Respiration (R)
• Net primary production (NPP) is the amount of organic matter available for consumption by heterotrophs after accounting for phytoplankton respiration
  • NPP = GPP - R
• Primary production is often measured in terms of carbon fixation rates, expressed as grams of carbon per square meter per day (g C m⁻² d⁻¹)
• Photosynthesis in phytoplankton is driven by the light reactions and the Calvin cycle (dark reactions)
  • Light reactions convert light energy into ATP and NADPH
  • Calvin cycle uses ATP and NADPH to fix CO₂ into organic compounds
• Primary production follows a depth-dependent pattern in the water column, with the highest rates occurring in the euphotic zone where light is sufficient for photosynthesis

Factors Affecting Phytoplankton Growth

• Light availability is a critical factor influencing phytoplankton growth and primary production
  • Phytoplankton require sufficient light energy to drive photosynthesis
  • Light intensity decreases exponentially with depth in the water column (Beer-Lambert Law)
  • Seasonal changes in solar radiation and water column mixing affect light availability for phytoplankton
• Nutrient availability, particularly nitrogen and phosphorus, limits phytoplankton growth in many aquatic ecosystems
  • Liebig's Law of the Minimum states that growth is limited by the nutrient in shortest supply relative to the organism's needs
  • Redfield ratio (C:N:P = 106:16:1) describes the average elemental composition of phytoplankton biomass
  • Nutrient limitation can lead to shifts in phytoplankton community composition and size structure
• Temperature affects phytoplankton growth rates and metabolic processes
  • Optimal temperature ranges vary among phytoplankton species
  • Warmer temperatures generally increase growth rates up to a certain threshold
• Water column stability and mixing influence phytoplankton growth by regulating light and nutrient availability
  • Stratification can lead to nutrient depletion in the surface layer and light limitation in deeper layers
  • Mixing can replenish nutrients in the euphotic zone but may also transport phytoplankton out of the well-lit surface layer
• Grazing by zooplankton and other herbivores can control phytoplankton population dynamics
  • Selective grazing can shape phytoplankton community composition
  • Intense grazing pressure can lead to phytoplankton biomass decline and trophic cascades

Measuring Primary Production

• Primary production can be measured using various methods, each with its own advantages and limitations
• Light and dark bottle method (oxygen method)
  • Measures changes in dissolved oxygen concentration in light and dark bottles incubated in situ
  • Provides estimates of net community production (NCP) and community respiration (CR)
• ¹⁴C method (radiocarbon method)
  • Uses radioactive carbon-14 to trace carbon fixation rates in phytoplankton
  • Provides estimates of net primary production (NPP) over short time scales (hours to a day)
• Chlorophyll fluorescence techniques
  • Measure the fluorescence of chlorophyll a to estimate photosynthetic rates and efficiency
  • Pulse Amplitude Modulated (PAM) fluorometry is commonly used to assess photosynthetic parameters
• Remote sensing and bio-optical models
  • Estimate primary production using satellite-derived data on chlorophyll concentration, light availability, and sea surface temperature
  • Algorithms relate these variables to photosynthetic rates and carbon fixation
• Stable isotope tracers (e.g., ¹³C, ¹⁵N, ¹⁸O)
  • Used to track the incorporation of carbon, nitrogen, and oxygen into phytoplankton biomass
  • Provide insights into nutrient uptake and assimilation rates
• Each method has its own assumptions, uncertainties, and spatiotemporal scales of measurement
  • Combining multiple approaches can provide a more comprehensive understanding of primary production dynamics

Ecological Impacts and Importance

• Phytoplankton are the foundation of aquatic food webs, supporting a wide range of organisms from zooplankton to large marine mammals
  • Changes in phytoplankton community composition and productivity can have cascading effects on higher trophic levels
  • Phytoplankton blooms can lead to increased zooplankton abundance and fish production
• Phytoplankton play a crucial role in the global carbon cycle and climate regulation
  • Oceanic phytoplankton absorb about 50 gigatons of carbon per year through photosynthesis
  • The biological pump transports carbon from the surface to the deep ocean, effectively sequestering it from the atmosphere
• Phytoplankton contribute to the production of dimethylsulfide (DMS), a gas that can influence cloud formation and climate
• Some phytoplankton species (e.g., coccolithophores) produce calcium carbonate shells that contribute to the marine carbonate cycle and ocean alkalinity
• Harmful algal blooms (HABs) can have negative impacts on aquatic ecosystems and human health
  • HABs can produce toxins that affect marine organisms and accumulate in seafood
  • Bloom events can lead to oxygen depletion (hypoxia) and fish kills
• Phytoplankton are sensitive to environmental changes and can serve as indicators of ecosystem health
  • Shifts in phytoplankton community composition and abundance can reflect changes in water quality, nutrient loading, and climate
  • Long-term monitoring of phytoplankton can inform management strategies and conservation efforts

Current Research and Future Directions

• Advances in genomics and metagenomics are providing new insights into phytoplankton diversity, evolution, and ecological functions
  • High-throughput sequencing technologies enable the exploration of phytoplankton communities at unprecedented resolution
  • Genomic studies reveal the metabolic capabilities and adaptations of different phytoplankton species
• Remote sensing and autonomous underwater vehicles (AUVs) are revolutionizing the study of phytoplankton dynamics at large spatial scales
  • Satellite-based sensors provide global coverage of phytoplankton biomass and primary production estimates
  • AUVs equipped with bio-optical sensors can collect high-resolution data on phytoplankton distribution and environmental variables
• Modeling efforts aim to predict phytoplankton responses to climate change and anthropogenic stressors
  • Coupled physical-biological models simulate the interactions between phytoplankton, ocean circulation, and biogeochemical cycles
  • Ecosystem models incorporate phytoplankton functional types to capture the diversity of ecological roles and responses to environmental change
• Research on phytoplankton-zooplankton interactions and the role of viruses in regulating phytoplankton populations is expanding
  • Studies investigate the coevolution and arms race between phytoplankton and their grazers and parasites
  • Viral infections can influence phytoplankton mortality, diversity, and biogeochemical cycling
• Biotechnological applications of phytoplankton are being explored for biofuel production, wastewater treatment, and high-value product synthesis
  • Microalgae can be cultivated in photobioreactors or open ponds to produce biomass for various purposes
  • Genetic engineering techniques are being developed to optimize phytoplankton strains for specific applications
• Future research will continue to unravel the complex interactions between phytoplankton, their environment, and other marine organisms in a changing world