Marine and Antarctic Ecosystems - Primary Production 1: Photosynthesis & Light

Primary Production

  • Formation of organic matter through trapping light energy and assimilating inorganic elements.

  • Mostly via photoautotrophs (macro and micro-algae, seagrass, mangroves).

  • Limited by light, CO_2, and nutrients.

Productivity

  • Rate of net incorporation of carbon into organic compounds.

  • Energy from light reaction fixes inorganic carbon into organic carbon.

  • Mass of carbon fixed per unit area per unit time (e.g., kg^{-1} C m^{-2} y^{-1}).

  • Net production is the balance of growth and loss (e.g., respiration).

Marine Primary Production

  • Marine primary production accounts for 40% of Earth's primary production.

  • Net primary productivity: Marine (35-50 10^{15} g y^{-1}), Terrestrial (50-70 10^{15} g y^{-1}).

  • Total plant/algal biomass: Marine (1-2 10^{15} g), Terrestrial (600-1000 10^{15} g).

  • Turnover time: Marine (0.02-0.06 years), Terrestrial (9-20 years).

Marine Primary Producers: Phytoplankton

  • Size classes of phytoplankton:

    • Picoplankton (0.2 - 2 µm): Cyanobacteria.

    • Nanoplankton (2 – 20 µm): Small dinoflagellates, coccolithophores.

    • Microplankton (20 – 200 µm): Diatoms, large dinoflagellates.

    • Mesoplankton (200 µm – 2 mm): Chain-forming diatoms.

Coccolithophores

  • Primary production, carbon cycling, carbon sequestration.

Diatoms

  • Silica cell wall.

  • High rates of growth and production.

  • Important in carbon cycling.

Dinoflagellates

  • Coastal; form large blooms.

  • Can be toxic.

  • Red tides and harmful algal blooms.

  • Some are fish-eating (e.g., Pfisteria).

Dinoflagellates: Zooxanthellae

  • Microscopic photosynthetic dinoflagellates in coral polyp tentacles.

  • Also in sea anemones, giant clams, and sponges.

  • Fix carbon via photosynthesis, providing food for coral.

Marine Primary Producers: Benthic

  • Seagrasses and mangroves (tropical and sub-tropical).

  • Seagrasses (tropical to temperate).

  • Macroalgae (= seaweeds).

Macroalgae (= Seaweeds)

  • Categories:

    • Green: Phylum Chlorophyta.

    • Red: Phylum Rhodophyta.

    • Brown: Phylum Ochrophyta.

Macroalgae: Ecological Importance

  • Marine forests with many creatures depending on kelp.

Products from Seaweed

  • Carrageenans:

    • Food additives (gelling and emulsifying properties).

    • From red algal species (Eucheuma, Kappaphycus).

  • Alginates:

    • Stabilizers or emulsifiers (textile industries).

    • Gelling agents/thickeners for food.

    • In shampoos, cosmetics.

    • From large brown algae (Macrocystis, Ascophyllum, Laminaria, Durvillaea spp.).

  • Agar:

    • Solid substrate for bacteria growth.

    • Gelling agent in food.

    • From red algal species (Gelidium, Gelidiella, Gracilaria, Pterocladiella, Pterocladia).

  • Pyropia (= nori = sushi wrapper).

    • Japan produces 400,000 tonnes per year.

    • US$1.5b (FAO 2013).

Photosynthesis

  • Light energy + CO2 + H2O → Carbohydrate + O_2.

  • Light reactions:

    • Photosynthetic pigments in chloroplasts harvest light.

    • Light energy converted to chemical energy (ATP).

  • Dark reactions:

    • ATP fixes CO_2 as sugar via the Calvin cycle (RuBisCO).

Photosynthetic Pigments

  • Chlorophyll a and accessory pigments.

  • Embedded in thylakoid membranes of the chloroplast.

Light Attenuation and Transmittance

  • Irradiance: µmoles photons m^{-2} s^{-1} (photon flux density - PFD).

  • Irradiance decreases with depth (attenuation).

  • Wavelength changes with depth (red light absorbed first, blue light transmitted further).

  • Photic zone: depth at which 1% of incident light remains.

Depth of Photic Zone

  • Depends on water quality.

  • Scattering and absorption by particles and plankton.

Chlorophyll a and Accessory Pigments

  • Chlorophyll a absorbs light in red and blue regions.

  • Accessory pigments absorb light in the ‘green window’.

  • Maximizes light harvesting across the spectrum underwater.

  • Light is key limiting factor.

Accessory Pigments

  • All algae have Chl a.

  • Accessory pigments:

    • Chlorophylls (b,c,d), carotenoids, and fucoxanthins within the thylakoid membrane.

    • Phycobiliproteins (cyanobacteria and Rhodophyta) on the thylakoid membrane surface.

Macroalgae: Pigments and Compounds

  • Chlorophyta: chlorophylls a and b.

  • Rhodophyta: chlorophylls a, c, and phycobilins (red).

  • Phaeophyceae: chlorophylls a and c, fucoxanthin (brown).

Photosynthetic Unit

  • RC = pigment molecule (chlorophyll a and accessory pigments).

  • Responsible for light harvesting.

  • Reaction center enables conversion of light energy to chemical energy.

Photosynthesis vs Irradiance Curve

  • Measures photosynthesis rate (P) at various irradiances (E).

  • Photosynthetic parameters determine acclimation to light environment and rates of net primary production.

Photosynthetic Parameters

  • Pmax: Maximal photosynthetic rate.

  • Alpha (α): Initial slope.

  • Ek: PFD that saturates photosynthesis (µmol m^{-2} s^{-1}).

  • Ec: Compensation irradiance (µmol m^{-2} s^{-1}).

  • Rd: Respiration rate (units as for Pmax).

Acclimation to Low vs High Light

  • Increase light harvesting by:

    1. Increasing the number of photosynthetic units.

    2. Increasing accessory pigments associated with existing PSUs.

Seaweeds = Macroalgae

  • Three colors: Brown, Green, Red.

  • No true roots.

  • Most need rocky substrate for holdfast.

  • Some have root-like rhizoids for soft sediment.