Lec 17: Life in the Ocean

• Earth is home to millions of different species
• All life on Earth shares the same underlying mechanisms of capturing and storing energy, manufacturing proteins, and transmitting information between generations
• In essence, all life on Earth is the same, it's just packaged in different ways
• Living matter

  Cannot function without energy - the capacity to do work
• Energy transformation in living organisms

  1. Plant: light energy to chemical energy

  2. Animal: chemical energy into energy of movement
• The main source of energy for living things on Earth is the Sun
• Marine Provinces


  • Pelagic (Open Sea) environment: ocean water itself, where drifters and swimmers live out their lives in complex food webs
  • Benthic (Sea Bottom) environment: ocean bottom, where marine algae and animals that do not float or swim spend their lives

  
• Hydrothermal Vent Communities


  • Abundant and large deep-ocean benthos
  • Discovered in 1977
  • Associated with hot vents
  • Archaea produce food using heat and chemicals (separate food web from photosynthetic processes that support vast majority of life on Earth)
• Types of Marine Organisms

  • Plankton (drifters)
  • Nekton (swimmers)
  • Benthos (bottom dwellers)

  
• Photosynthesis


  Light energy from the sun is trapped by chlorophyll in primary producers and changed into chemical energy
• Photosynthesis is reversible "Re-mineralization"

  
• Zones based on availability of sunlight

  • Euphotic zone: Extends from the surface to a depth where there is still enough light to support photosynthesis (usually ~ 100m)
  • Dysphotic zone: Has a small but measurable quantities of light. Extends from euphotic zone to where light can no longer penetrate
  • Aphotic zone: Has no light, usually ~1000 m

  
• Transmission of Light

  • Visible light portion of the electromagnetic spectrum
  • Blue wavelengths penetrate deepest
  • Longer wavelengths (red, orange) absorbed first
  • Chlorophyll is a green pigment that absorbs best red and violet
  • Since very little red light penetrates past 3 meters, most phytoplankton stay near the surface to absorb red light
  • Primary productivity is highest near top of euphotic zone
  • Exception: cyanobacteria which can use blue light

  
• Compensation depth


  "Break even" depth at which net photosynthesis becomes zero. Not fixed, will vary between locations and at different times of day, usually corresponds to depth where 1% of surface light penetrates - bottom of the euphotic zone

  
• Photosynthesis and Remineralization


  1. In surface water, oxygen is abundant due to mixing with the atmosphere and plant photosynthesis, and nutrient content is low as they are consumed by algae

  2. At deeper depths, oxygen decreases as it's consumed by heterotrophic organisms, producing an oxygen minimum layer (OML) coinciding with a nutrient maximum

  3. Below the OML nutrients remain high and oxygen increases as it is replenished with high-oxygen cold water form polar regions and organic matter is re-mineralized at depth

  
• The primary producers on land are mainly macroscopic plants, but in the ocean, the main producers are microscopic phytoplankton
• Plankton

  • All organisms (algae, animals, and bacteria) that drift with ocean currents
  • Cannot determine their horizontal position in the water column against currents (do have some vertical migratory ability)
  • Most of Earth's biomass consists of plankton adrift in the ocean
  • Though 98% of marine species are bottom dwelling, the majority of the ocean's biomass is planktonic

  
• Phytoplankton


  Primary producers
• Zooplankton

  Microscopic consumers
• Ocean Primary Productivity


  • The incorporation of carbon atoms into carbohydrates by photosynthesis
  • Measured in grams of carbon bound into carbohydrates per square meter of ocean surface per year g C/m2/yr

  
• 95-98% of the ocean's biomass relies directly or indirectly on photosynthesis for food

  
• Global net productivity in marine ecosystems is 35-50 billion metric tons of carbon bound into carbohydrates per year

  
• Global terrestrial productivity is similar, 50-70 billion metric tons
• But total producer biomass (mass of living tissue) in the ocean is 1-2 billion metric tons, while living biomass on land is 600-1000 billion metric tons
• Nutrients cycle from producer to consumer and back much more quickly in marine ecosystems

  
• Major Groups of Marine Phytoplankton

  • Diatoms
  • Coccolithophores
  • Dinoflagellates
  • Picoplankton

  
• Diatoms


  Dominant and most productive algae; test (shells) made of silica; Tests accumulate on seafloor
• Coccolithophores


  Small single-celled autotrophs with plates of calcium carbonate; contribute significantly to calcareous sea floor deposits
• Dinoflagellates


  • Produce harmful algae blooms (HABs) ("red tide")
  • Many are mixotrophic (e.g. capable of autotrophy and heterotrophy)
  • Natural conditions may stimulate dinoflagellate productivity
  • Some produce toxins consumed by fish and shellfish, causing paralytic shellfish poisoning (PSP) in humans
• Picoplankton


  • Encompasses most other plankton types, which are very small (0.2 – 2 µm) and extraordinarily abundant and productive
  • May account for 80% of photosynthetic activity in some parts of the open ocean – especially in areas with low nutrients
  • Example: Prochlorococcus, probably the most abundant photosynthetic organism on Earth, has a chlorophyll variant that allows it to absorb blue light at low light intensities

  
• Classic N-P-Z Marine Food Web


  Most basic model of a pelagic ecosystem, examining the relationship between quantities of nutrients, phytoplankton, and zooplankton

  
• Microbial (Heterotrophic) Plankton


  • Soluble organic materials spilled released into the ocean by plankton
  • Material is consumed by small heterotrophic bacteria
  • Microbial Loop: pathway by which dissolved organic carbon is returned to higher tropic levels via incorporation into bacterial biomass and then coupled with the classic phytoplankton-zooplankton-nekton food chain

  
• Marine primary producers are tiny plankton living in the photic zone
• Photosynthesis and respiration (CO2 & O2 cycles)

  
• Oxygen minimum zones are the result of excessive respiration of organic carbon

  
• Limiting factors and regional productivity (polar, tropical, temperate oceans)