Lecture 7 Short stories 11.17.00 AM

Lecture Overview

  • Instructor: Andrew Juhl

  • Topic: HNLC (High Nutrient, Low Chlorophyll) regions and N-fixation

  • Deadline: Review by Tuesday 9/24

Problem 1: Nutrient Availability in Oceans

  • Context: Significant areas in oceans, such as the Southern Ocean, show consistently high levels of nutrients (N + P).

  • Question: Why don’t phytoplankton consume available nutrients in these regions?

Iron Concentrations and Phytoplankton Growth

Fe Concentration Data

  • Data Collection: Techniques for measuring Fe were improved to avoid contamination.

  • Figure 1: Shows vertical distribution of dissolved iron, oxygen, and nitrate at Gulf of Alaska Ocean Station PAPA.

The Redfield Ratio

  • Nutrient Ratios:

    • N:Fe = 100,000:1

    • 106 C:16 N:1 P:0.1-0.001 Fe

    • N:Fe = 160-16,000:1

  • Conclusions: High nutrient areas do not lead to phytoplankton blooms due to Fe limitation despite sufficient N and P.

Iron Deposition

  • Sources:

    • Aeolian dust is a primary Fe source for the surface ocean.

  • Research Findings: Studies show that most Fe in the ocean comes from atmospheric dust deposits.

Iron Addition Experiments

  • Early Findings: Experiments by Martin and Fitzwater (1988) showed that adding iron results in the growth of large diatoms.

  • Significance: These studies indicate that Fe is likely a limiting factor for phytoplankton growth in certain marine environments.

Comment on Iron Limitation

  • Key Study: Martin et al. argued that iron limits phytoplankton production in offshore subarctic Pacific regions.

  • Response to Criticism: The study addresses critiques regarding methods and nutrient availability, emphasizing Fe's role in phytoplankton productivity.

Global Patterns of Nutrient Limitation

Diatom Nutrient Limitations

  • Diatoms are limited by:

    • Nitrogen (55.73%)

    • Iron (27.67%)

    • Other Nutrients (e.g., Phosphorus 1,405%)

  • Importance: Identifying these limitations helps understand phytoplankton dynamics globally.

Nitrogen-Fixing Bacteria

  • Problem 2: Areas with no measurable NO3 -2 yet having PO4 -3 suggest why N-fixation by bacteria is inadequate.

  • Key Species:

    • Trichodesmium spp.: A filamentous cyanobacteria fixing N2, abundant in oligotrophic tropical regions.

Types of Diazotrophic Cyanobacteria

  • Small Diazotrophic Cyanobacteria:

    • Recent recognition of their importance in global N2 fixation areas.

    • Variants include uncultured species and those like Crocosphaera, which can fix nitrogen and carbon.

Symbiotic Associations

  • Diatom-Diazotroph Assemblages: Symbiotic relationships enhance N-fixation under certain conditions requiring silica.

  • UCYN-A: Known for its close associations with other organisms; critical for N-fixation in light conditions.

N2 Fixation Process

  • Mechanism: N2 fixation requires significant energy and certain metals (Fe, Mo, P). It is also sensitive to O2 levels.

  • Measuring N2 Fixation:

    • Methods include acetylene reduction assays and 15N incorporation into organic matter.

Discrepancies in N-fixation Rates

  • NifH Gene Abundance: Discrepancies exist between high nifH gene abundance and low N-fixation rates observed in various environments.

Limitations in HNLC Regions

  • Understanding HNLC:

    • Limited iron hinders growth of regular phytoplankton and diazotrophs.

    • Identifies variations between coastal and offshore regions concerning nutrient availability.

  • Predictions: Establishes expectations for where N-fixation is likely to occur and conditions favoring such processes.