Primary Productivity-Lecture Notes

This week we will begin our venture into marine life by first looking at the primary producers-namely, phytoplankton (tiny, floating, single-celled photosynthesizers)-and the little critter that feed on them. The first lecture will focus on how plants and other photosynthesizers turn the energy of the sun in usable chemical energy and the factors related to productivity. The second lecture will be an overview of the major players.

Photosynthesis

Remember being alive means that you transfer energy. One of the ways to do this is to take the energy contained in sunlight and use it to convert inorganic substances to usable compounds. The simplest pathway of this process, or "photosynthesis," is to use sunlight to convert water and carbon dioxide to carbohydrates and the byproduct, oxygen:

6CO2 + 6H20 (add some sunlight energy) -> C6H12O6 + 6O2

Note that the molecules going into the formula are much smaller than the molecules coming out; that is why it takes energy. Plants and other photosynthesizers (algae and bacteria) do this through the aid of pigments called "chlorophyll." Finally, realize that primary productivity is not limited to harvesting sunlight. Some bacteria break down inorganic elements to gain energy. These critters are called "chemosynthesizers."

Controls on Photosynthesis

The rate and success of photosynthesizers depend primarily on the level of sunlight. Not enough sunlight, and the processes cannot take place. Too much, though, and the critters become overwhelmed. Many factors conspire to limit the amount of light including depth in the ocean, cloud cover, clarity of the water, and seasonality. Sunlight is composed of a spectrum of lights of various wavelengths. Visible light is between about 400 nanometers wavelength (violet) to 700 nanometers wavelength (red). Smaller wavelengths are called the "ultraviolet" spectrum and larger wavelengths are "infrared." The longer wavelengths cannot penetrate deep depths which is why the oceans look blue.
Besides light, another limiting factor on primary productivity is the presence of accessory nutrients. If the concentration of a nutrient changes, it is called a "nonconservative nutrient." One of the most important nutrients is free nitrogen. We use the modifier "free"to separate it from the abundant gas form, N2, which cannot be used as a nutrient (but is used in other reactions). Similarly, free oxygen is reactive, but gas oxygen, O2, is not.

Where the Primary Producers are Found

These limitations greatly affect the location of primary producers. Essentially, primary producers need to be near the surface of the ocean, in latitudes with good sunlight (at least part of the time), and where upwelling brings in lots of nutrients. Some photosynthesizers have dealt with these constraints by evolving accessory pigments that trap the light that chlorophyll cannot. Also, living in the coastal waters has its own set of problems due to mixing and waves, so those organisms that live in the upper surface also need to be sturdy.
There are two main groupings of primary producers: planktonic forms and benthic forms. Planktonic forms are tiny and drift about the open ocean at the mercy of the currents. Benthic forms include the large algae and angiosperms and are found along the coast and in nearshore waters.

Major Plankton

Although plankton may control vertical movement, they cannot "swim." We can divide plankton into two distinct groups: the autotrophs (organisms which create their own energy, primarily from sunlight) and heterotrophs (organisms that gain energy by eating previously made organic materials-see below). The following is a summary of the key features of the main groups.

Autotrophs-Phytoplankton
Diatoms--Most common of all of the phytoplankton. Diatoms are unicellular algae that secrete beautiful skeletons of silica (SiO2), called a "frustule." The frustule is composed of two, interlocking valves. Most are disk-shaped, others are boat-shaped. Diatoms, like all unicellar plants, reproduce by nonsexual division. The daughter product is always a little smaller than the dividing diatom. Eventually, the diatom becomes so small it sexually produces an "auxospore." In addition to chlorophyll, diatoms also harbor yellowish-brown pigments called "xanthophylls."

Dinoflagellates--Strange looking autotrophs, and a few heterotrophs, that have "beating whips" that extend from the body, called "flagella." The flagella are used to propel through the water and to rotate to get the most sunlight. Two characteristics of dinoflagellates is the release of light energy as "bioluminescence" and the summer blooms called "red tides." Red tides can be deadly because the dinoflagellates harbor toxins that can be injested. No eating shellfish during a red tide!!

Coccolithophores--Another skeletonized autotroph, but the skeleton is made of disks of calcium carbonate (CaCO3) called "coccoliths." In areas where productivity is high, the skeletons build up on the sea floor (above the carbonate compensation depth, of course) to form "chalk deposits" such as the White Cliffs of Dover.

Nanno- and Picoplankton--Unfathomably small autotrophs (and heterotrophs) that have only been known about for less than twenty years. Included in this group are the cyanobacteria, which used to be called "blue-green algae." The important point about nanno- and picoplankton is that although they are small, they account for a large percentage of primary production-up to 70% in some areas!

Other Primary Producers in the Ocean

Larger Algae--Algae include both unicellular diatoms and the larger, multicellular groups. Larger algae belong to one of three groups: the Phaeophytes (brown algae), Chlorophytes (green algae), Rhodophytes (red algae). The difference in the "color" is due to differing amounts and types of pigments used in light-harvesting. Phaeophytes include the large kelp forests common in our waters. Chlorophytes abound but are limited to the uppermost euphotic zone. Ulva is a common local genus. Rhodophytes are special because they can survive in the lowest reaches of the euphotic zone, down to over 200 m below sea level. Rhodophytes are the most abundant of all of the algae groups and are the most complex. Both chlorophytes and rhodophytes can create skeletons of calcium carbonate adding lots of organic carbon to the sediments.

Angiosperms--Most plants on land belong to the angiosperms, or vascular plants that reproduce with fruits and seeds. The only common angiosperms in the ocean are the mangroves (discussed in the lecture on coastlines) and sea grass. Note that "sea grass" is not a grass at all.

Heterotrophs-Zooplankton

In addition to the above critters, there are also heterotrophs (organisms that process organic substances to gain energy; i.e., EAT). Heterotrophs break down the carbohydrates that other organisms have made to gain energy for themselves:

C6H12O6 + 6O2 -> 6H20 + 6CO2 (release chemical energy)

Note that in this reaction, large molecules are broken down to release energy, and CO2 is a by-product. This energy pathway is called "respiration." Both plants and animals respire, or produce CO2; the plants do so at night.
Those organisms that eat primary producers are called "primary consumers." Some primary consumers also live as plankton (called "zooplankton) and we will discuss that now. One key point is that some heterotrophs spend their entire life as zooplankton, but lots of others only spend part of their life (juvenile stage) as zooplankton. Here is a sampling of some groups:

Copepods--The most abundant (70%) zooplankton. Tiny crustaceans related to crabs and lobsters and look like mini shrimp. Copepods have two antennae and are usually less than one millimeter long.

Foraminifera--Bizarre unicellar heterotrophs that live as both planktonic and benthic forms. Although unicellular, "forams" have evolved complex skeletons and modes of life. Some even harbor smaller algae and use them as a source of energy! Forams are very important in paleooceanography.

Cnidarians--"Jellyfish" are a group of Cnidarians (related to corals) that live in the water column as plankton. We think of plankton as small critters, but some jellyfish can get up to 3.5 meters in diameter!!

Meroplankton--Unlike copepods, forams, and jellyfish, which are "holoplankton" or spend all of their life as plankton, "meroplankton" only spend the juvenile part of their life as plankton. Almost all phyla of animals have members of the meroplankton. Some of the more common ones are molluscs, arthropods, and fish.

Key Points:

1. Autotrophs harvest energy from inorganic substances (e.g., plants, chemosynthetic bacteria). Heterotrophs harvest energy from organic substances (e.g., tuna).

2. During photosynthesis, specialized pigments called "chlorophyll" trap sunlight and use it to convert water and carbon dioxide to carbohydrates and the byproduct, oxygen, through the formula:

6CO2 + 6H20 (add some sunlight energy) -> C6H12O6 + 6O2

3. Other autotrophs harvest energy from inorganic elements, such as iron. These critters are called "chemosynthesizers."

4. Many factors conspire to limit the amount of light including depth in the ocean, cloud cover, clarity of the water, and seasonality. Primary productivity is also limited by the lack of, or overabundance of "nonconservative nutrients."

5. The longer wavelengths of sunlight cannot penetrate deep depths which is why the oceans look blue.

6. Primary producers are found near the surface of the ocean, in latitudes with good sunlight (at least part of the time), and where upwelling brings in lots of nutrients.

7. Some photosynthesizers have dealt with these constraints by evolving accessory pigments that trap the light that chlorophyll cannot.

8. Planktonic forms are tiny and drift about the open ocean at the mercy of the currents and benthic forms are found along the coast and in nearshore waters.

9. Phytoplankton are tiny, floating, single-celled photosynthesizers. Zooplankton are floating critters that eat phytoplankton.

10. Diatoms are the most common of all of the phytoplankton. Diatoms are unicellular algae that secrete beautiful skeletons of silica (SiO2), called a "frustule."

11. Dinoflagellates have "beating whips" that extend from the body, called "flagella" that are used to propel through the water and to rotate to get the most sunlight. Dinoflagellates release light energy as "bioluminescence" and are the culprits of the summer blooms called "red tides

12. Coccolithophores are autotrophs that have skeletons made of disks of calcium carbonate (CaCO3) called "coccoliths

13. Nanno- and Picoplankton are extremely small autotrophs (and heterotrophs), including cyanobacteria. Nanno- and picoplankton account for up to 70% of primary production.

14. Other primary producers in the ocean include larger algae and angiosperms.

15. Larger algae belong to either the Phaeophytes (brown algae=kelp forests), Chlorophytes (green algae=live in the uppermost euphotic zone), or Rhodophytes (red algae=can live at the bottom of the euphotic zone). Both chlorophytes and rhodophytes can create skeletons of calcium carbonate.

16. Angiosperms, or vascular plants that reproduce with fruits and seeds are found in the ocean as mangroves and sea grass.

17. Heterotrophs break down the carbohydrates that other organisms have made to gain energy for themselves through the pathway: C6H12O6 + 6O2 -> 6H20 + 6CO2 (release chemical energy). This energy pathway is called "respiration

18. Some primary consumers (feed on primary producers) live as zooplankton.

19. Copepods, the most abundant zooplankton, are small crustaceans.

20. Foraminifera are unicellar heterotrophs that live as both planktonic and benthic forms.

21. "Jellyfish" are Cnidarians that live as "macrozooplankton"-up to 3.5 meters in diameter.

22. "Meroplankton" only spend the juvenile part of their life as plankton and have representatives in almost all phyla of animals.

Web Sites:

*Note: for this lecture, there are lots and lots of pages with good pictures and/or information. I merely picked the first decent ones I could find, but there are a lot of other good ones out there.

http://www.geo.utexas.edu/illite/index.html
Site for the research group working on nannobacteria. Lots of pictures!

http://www.calacademy.org/research/diatoms/
Site for the California Academy of Sciences Diatom collection.

http://calspace.ucsd.edu/birch/ocean/diatom/images/
Another diatom site. Such cool pictures!!

http://www.paulsmiths.edu:80/aai/phyto.html
Another phytoplankton image library! Great pictures including phytoplankton blooms.

http://opp.gsfc.nasa.gov/index.html
Home page for the NASA Goddard Space Flight Center Ocean Primary Productivity Science Computing Facility.

http://www.marine.rutgers.edu/opp/
Good site maintained by Rutgers University on primary productivity.

http://lifesci.ucsb.edu/~biolum/
More than you ever wanted to know about bioluminescence! From my alma mater…

http://siolib-155.ucsd.edu/mlatz/Biolum_intro.html
The Scripps version of biolumninescence.

http://www.biobay.com/
One more…this is a tourist spot that has neat bioluminescence!

http://www.mote.org/~mhenry/rtlinks.phtml
The "Red Tide" link page… go wild!