Macroalgae and marine plants convert the sun's energy into food via photosynthesis.

Photosynthesis is essentially the transformation of light energy into chemical energy. In plants, CO₂ and H₂O combine with light to produce carbohydrates such as sugars and starches.

Light energy is trapped by photosynthetic pigments, especially chlorophyll a which absorbs blue and red wavelengths (green wavelengths are reflected... that's why plants look green).

 

Other pigments found in marine algae absorb light in other, slightly "greener" wavelengths (see below).

This allows marine algae to use more of the visible light for photosynthesis.

 

Light is obviously important for all marine plants, yet light intensity diminishes exponentially with depth in sea water and different wavelengths of light attenuate at different rates (refer back to a figure from lecture 3)

Red wavelengths attenuate especially fast below about 1-3 m

 

Also, remember that photosynthesis is inhibited at very high light intensities (i.e. near the surface).

 

Thus, there a relatively narrow band near the surface where meaningful photosynthesis occurs

 

The compensation depth is where O₂ production via photosynthesis equals O₂ consumption via respiration.

This depth changes with season, latitude, and water clarity.

Shading by other marine plants and animals (e.g. corals) may also influence local light levels.

 

1) They produce O₂ and carbohydrates, they are important sources of primary productivity.

Many also have chemical and physical defenses to deter herbivory... this also influences feeding patterns and community structure

 

2) Given their relatively large size, macroalgae (seaweeds) provide shelter for other organisms (especially invertebrates)

 

3) The presence of seaweeds and seagrasses may prevent erosion and encourage sediment deposition

 

4) The skeletons of many tropical algae are important sources of sediment

 

5) Encrusting forms of tropical algae also cement substrate together and reinforce reefs

 

For additional information, The Univeristy College at Galaway Seaweed Home Page provides an excellent overview of many aspects of marine macro algae

Seaweeds are typically associated with the bottom in some fashion, although some, like sargassum can be free floating.

 

The "classic" seaweed: A thallus consisting of a holdfast, stipe, and blades

These come in a diverse array of forms: from single cell sheets (Ulva) to giant kelps (Macrocystis)

Larger seaweeds often have gas filled floats (pneumatocysts) to keep blades near the surface.

Unlike terrestrial plants

Macroalgae take up nutrients directly from the water

Photosynthesis occurs in most cells and there is no differentiation between the tops and bottoms of blades (leaves)

Macroalgae don't require extensive structural components for support

Light, rather than nutrient availability is often limiting

 

Seaweeds are classified by the pigments they use for photosynthesis (all have chlorophyll a), the way they store energy, and their cell wall structure (in addition to cellulose)

Brown Seaweeds (Phaeophyta) also, see the UC Berkeley Brown Seaweed Website

Pigments are chlorophyll c, and fucoxanthin

Energy storage is Laminarin, a glucose polymer

Cell walls include Alginate (an important emulsifier used in everything from toothpaste to ice cream)

Examples:

Large kelps: Macrocystis, Laminaria

Robust intertidal species: Fucus, Postelsia

Red Seaweeds (Rhodophyta)

Pigments are phycoerythrin and phycocyanin and some with chlorophyll d.

Energy storage is Floridean starch (a glycogen-like glucose polymer)

Cell walls include agar (culture medium and food uses) and carrageenan (food thickener)

Examples:

Filamentous forms: Gracilaria, Chondrus, Porphyra

coralline forms: Porolithon

Green Seaweeds (Chlorophyta)

Pigments similar to higher plants

Energy storage in the form of starch (chlorophyll b and carotenoids)

Many lack crosswalls and thus represent some of the worlds biggest (multinucleated) cells

Examples:

sheet-like algae Ulva, Enteromorpha

erect green algae: Penicillus, Halimeda, Codium

 

Seaweeds are also known for their extremely diverse reproductive strategies

Alternating haploid (N chromosomes) and diploid (2N chromosomes) generations are common (an example from the green alga, Ulva)

Gametophytes produce haploid gametes

gametes are generally motile in brown algae and non-motile in red algae (although gametes may still be dispersed by water currents)

Gametes fuse to produce zygotes that may yield diploid Sporophytes. These produce diploid spores.

Seaweeds can be monoecious (both sexes) or dioecious (separate sexes) depending on species.

The relative size of sporophyte and gametophyte stages varies between species

In Ulva, the stages are identical while in kelps (Laminariales) the sporophyte is bigger than the gametophyte. These differences may promote survival through seasonal changes or reduce herbivory by reducing specialization.

Spore and gamete release are often linked to specific times or environmental conditions (lunar phase, tide, turbulence)

Vegetative (asexual) reproduction is also an extremely important form of reproduction

Seagrasses appear to have reinvaded marine habitats from the land

Like terrestrial plants, seagrasses contain chlorophyll a and require lots of light

Thin-bladed seagrasses occur in wave swept areas of temperate coasts and some light-limited tropical habitats

Examples: the temperate surf grass: Phyllospadix and tropical species, Syringodium

 

Wide bladed grasses

Examples: eel grass, Zostera and turtle grass, Thalassia, and others such as Halodule

 

The seeds of seagrasses are produced seasonally, but vegetative reproduction appears to be the dominant way of maintaining populations.