Life is Controlled by the Limiting Nutrient

Incubation Experiment

We all know that we are supposed to eat a balanced diet containing the Recommended Dietary Allowance of vitamins and minerals to maintain our health.  Humans, and all other organisms, require major and minor nutrients in well-defined quantities for healthy growth.   Marine phytoplankton, free floating marine plants, harvest the energy of the sun to fuel their growth, but they still need nutrients as the building blocks of their new cells.   The basic building blocks for marine plankton are carbon dioxide, water, nitrate, and phosphate.   The ocean generally has plenty of water and carbon dioxide, but can often be depleted in nitrate or phosphate.   If we think about photosynthesis as a balanced chemical reaction it is easy to use chemical stoichiometry to understand this limitation,

106 CO2 + 120 H2O + 16 HNO3 + 1 H3PO4 → light →  (CH2O)106(NH3)16PO4 + 137O2

where (CH2O)106(NH3)16PO4 is the chemical formula for an average phytoplankton.  Each mole of plankton requires 106 moles of carbon dioxide, 120 moles of water, 16 moles of nitrate (written as nitric acid), and 1 mole of phosphate (written as phosphoric acid).   The ratio of carbon to nutrients is often described as the Redfield Ratio after the biologist that first proposed the stoichiometry.

Nutrient Profiles. Phosphate is scaled up by 10.

We only need small amounts of the nutrients nitrate and phosphate to support photosynthesis, but they are always required, and are required in well-defined molar ratios.   In most freshwater lakes phosphate is limiting.  In the ocean, nitrate is most often limiting.   One of the questions that is being investigated on this cruise is what limits the growth of coccolithophores in the Southern Ocean.   In addition to nitrate, light and iron could also be limiting phytoplankton growth.   Light is required to drive photosynthesis.   Clearly, no phytoplankton can grow in the dark, but phytoplankton do grow under cloudy conditions and some species can grow very well.   Iron is a micronutrient that is also required of all plankton, but at very low levels – on the order of 0.001 moles Fe to a mole of phytoplankton.  To help answer the nutrient limitation questions, plankton incubation experiments are being performed on the deck of the ship.   Different samples of water are spiked with nutrients, metals, CO2, and the plankton growth rates are being measured at different light levels.  These experiments will take months of analysis before the question of  phytoplankton limitation in this area of the ocean is understood.   In mean time, take a look at a “typical” Southern Ocean nutrient profile to see if you can spot evidence for nutrient limitation.   What do you think drives the profiles of the nutrients?   Use the oxygen profile as additional information.    The fluorescence data can be used as a proxy for phytoplankton biomass.

Our next blog will include video of whales around the ship.   The Redfield ratio is remarkably conserved in these huge creatures.   Using a daily feeding rate of 2000 kg of plankton a day, estimate how much phosphate and nitrate are released per day in the upper ocean by whale excretion?

– Whitney

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