Algae

Archaeplastida

Archaeplastida are a supergroup of protists that comprise red and green algae, which include unicellular, multicellular, and colonial forms.

Learning Objectives

Describe the relationship between red algae, green algae, and land plants

Key Takeaways

Key Points

  • Archaeplastida are typically associated with their relationship to land plants; in addition, molecular evidence shows that Archaeplastida evolved from an endosymbiotic relationship between a heterotrophic protist and a cyanobacterium.
  • Red algae (rhodophytes), are classified as Archaeplastida and are most often characterized by the presence of the red pigment phycoerythrin; however, there are red algae that lack phycoerythrins and can be classified as parasites.
  • Red algae typically exist as multicellular protists that lack flagella; however, they can also exist as unicellular organisms.
  • Green algae are the most abundant group of algae and can be further classified as chlorophytes and charophytes.
  • Charophytes are the green algae which resemble land plants and are their closest living relative.
  • Chlorophytes are the green algae which exhibit a wide range of forms; they can be unicellular, multicellular, or colonial.

Key Terms

  • endosymbiotic: that lives within a body or cells of another organism
  • plankton: a generic term for all the organisms that float in the sea

Archaeplastida

Red algae and green algae are included in the supergroup Archaeplastida. It is well documented that land plants evolved from a common ancestor of these protists; their closest relatives are found within this group. Molecular evidence supports that all Archaeplastida are descendants of an endosymbiotic relationship between a heterotrophic protist and a cyanobacterium. The red and green algae include unicellular, multicellular, and colonial forms

Red Algae

Red algae, or rhodophytes, are primarily multicellular, lack flagella, and range in size from microscopic, unicellular protists to large, multicellular forms grouped into the informal seaweed category. The red algae life cycle is an alternation of generations. Some species of red algae contain phycoerythrins, photosynthetic accessory pigments that are red in color and outcompete the green tint of chlorophyll, making these species appear as varying shades of red. Other protists classified as red algae lack phycoerythrins and are parasites. Red algae are common in tropical waters where they have been detected at depths of 260 meters. Other red algae exist in terrestrial or freshwater environments.

Green Algae: Chlorophytes and Charophytes

The most abundant group of algae is the green algae. The green algae exhibit similar features to the land plants, particularly in terms of chloroplast structure. It is well supported that this group of protists share a relatively-recent common ancestors with land plants. The green algae are subdivided into the chlorophytes and the charophytes. The charophytes are the closest-living relatives of land plants, resembling them in morphology and reproductive strategies. Charophytes are common in wet habitats where their presence often signals a healthy ecosystem.

The chlorophytes exhibit great diversity of form and function. Chlorophytes primarily inhabit freshwater and damp soil; they are a common component of plankton. Chlamydomonas is a simple, unicellular chlorophyte with a pear-shaped morphology and two opposing, anterior flagella that guide this protist toward light sensed by its eyespot. More complex chlorophyte species exhibit haploid gametes and spores that resemble Chlamydomonas.

The chlorophyte Volvox is one of only a few examples of a colonial organism, which behaves in some ways like a collection of individual cells, but in other ways like the specialized cells of a multicellular organism. Volvox colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein secretion. Individual Volvox cells move in a coordinated fashion and are interconnected by cytoplasmic bridges. Only a few of the cells reproduce to create daughter colonies, an example of basic cell specialization in this organism.

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Volvox aureus: Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions.

True multicellular organisms, such as the sea lettuce, Ulva, are represented among the chlorophytes. In addition, some chlorophytes exist as large, multinucleate, single cells. Species in the genus Caulerpa exhibit flattened, fern-like foliage and can reach lengths of 3 meters. Caulerpa species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.

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Caulerpa taxifolia: Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of nuclei.

Protists as Primary Producers, Food Sources, and Symbionts

Protists function as sources of food for organisms on land and sea.

Learning Objectives

Give examples of how protists act as primary producers

Key Takeaways

Key Points

  • Photosynthetic protists serve as producers of nutrition for other organisms.
  • Protists like zooxanthellae have a symbiotic relationship with coral reefs; the protists act as a food source for coral and the coral provides shelter and compounds for photosynthesis for the protists.
  • Protists feed a large portion of the world’s aquatic species and conduct a quarter of the world’s photosynthesis.
  • Protists help land-dwelling animals such as cockroaches and termites digest cellulose.

Key Terms

  • zooxanthella: an animal of the genus Symbiodinium, a yellow dinoflagellate, notably found in coral reefs
  • primary producer: an autotroph organism that produces complex organic matter using photosynthesis or chemosynthesis

Primary Producers/Food Sources

Protists function in various ecological niches. Some protist species are essential components of the food chain and are generators of biomass.

Protists are essential sources of nutrition for many other organisms. In some cases, as in plankton, protists are consumed directly. Alternatively, photosynthetic protists serve as producers of nutrition for other organisms. For instance, photosynthetic dinoflagellates called zooxanthellae use sunlight to fix inorganic carbon. In this symbiotic relationship, these protists provide nutrients for the coral polyps that house them, giving corals a boost of energy to secrete a calcium carbonate skeleton. In turn, the corals provide the protists with a protected environment and the compounds needed for photosynthesis. This type of symbiotic relationship is important in nutrient-poor environments. Without dinoflagellate symbionts, corals lose algal pigments in a process called coral bleaching and they eventually die. This explains why reef-building corals do not reside in waters deeper than 20 meters: insufficient light reaches those depths for dinoflagellates to photosynthesize.

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Corals and dinoflagellates: Coral polyps obtain nutrition through a symbiotic relationship with dinoflagellates.

The protists themselves and their products of photosynthesis are essential, directly or indirectly, to the survival of organisms ranging from bacteria to mammals. As primary producers, protists feed a large proportion of the world’s aquatic species. (On land, terrestrial plants serve as primary producers. ) In fact, approximately one-quarter of the world’s photosynthesis is conducted by protists, particularly dinoflagellates, diatoms, and multicellular algae.

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Protists and aquatic organisms: Virtually all aquatic organisms depend directly or indirectly on protists for food.

Protists do not only create food sources for sea-dwelling organisms. Certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches where they contribute an essential step in the digestion of cellulose ingested by these insects as they bore through wood.