Causative Organism In Algae

The algae are a diverse group of eukaryotic organisms that share some fundamental characteristics but are not related on the phy-logenetic tree. These organisms are studied by algologists in a field known as algology. Algae are organisms that use light energy to convert CO2 and H2O to carbohydrates and other cellular products with the release of oxygen. Algae contain chlorophyll a, which is necessary for photosynthesis. In addition, many algae contain other pigments that extend the range of light waves that can be used by these organisms for photosynthesis. Algae include both microscopic unicellular members and macroscopic multicellular organisms (figure 12.2). Algae, however, differ from other eukaryotic photosynthetic organisms such as land plants in their lack of an organized vascular system and their relatively simple reproductive structures. ■ photosynthesis, p. 156

Algae do not directly infect humans, but some produce toxins that cause paralytic shellfish poisoning. Some of these toxins do not cause illness in the shellfish that feed on the algae but accumulate in their tissues and when eaten by humans cause nerve damage. ■ paralytic shellfish poisoning, p. 303

As one of the primary producers of carbohydrates and other cellular products, the algae are essential in the food chains of the world. In addition, they produce a large proportion of the oxygen in the atmosphere.

Classification of Algae

As just noted, algae is not a strict classification term; nevertheless, organisms considered under the general heading of algae are grouped for identification by a number of properties. These include the principal photosynthetic pigments of each group, cell wall structure, type of storage products, mechanisms of motility, and mode of reproduction. The names of the different algal groups are derived from the major color displayed by most of the algae in that group. Note that the different algal groups lie in different places along the evolutionary tree (figure 12.3).

Some of the general characteristics of the algal groups are summarized in table 12.1, on page 302.

Algal Habitats

Algae are found in both fresh and salt water, as well as in soil. Since the oceans cover more than 70% of the earth's surface, aquatic algae are major producers of oxygen as well as important users of carbon dioxide. Unicellular algae make up a significant part of the phytoplankton (phyto means "plant" and plankton means "drifting"), the free-floating, photosynthetic organisms that are found in marine environments. More oxygen is produced by the phytoplankton than by all forests combined.

Animals

Choanoflagellates

-Fungi-

- Zygomycetes -Ascomycetes -Basidiomycetes

-Chytridiomycetes ■ Land plants

Chlorophytes (green algae)

Cryptomonads

Rhodophyta (red algae)

Stramenophiles-

Chrysophyta (golden brown algae and diatoms)

SPhaeophyta (brown algae) . Oomycetes (water molds)

Ciliates pyr^pihyt^—dinofflagellates t

Apicomplexans Cellular slime molds Acellular slime molds Entamoebids Amoeboflagellates Kinetoplastids

Euglenoids

■ Parabasalians

■ Diplomonads

■ Microsporans

Bacteria

Figure 12.3 Phylogeny of Algae The algal groups are highlighted in green. Note that the algae are not directly related to one another but are found all along the evolutionary continuum.

12.1 Algae 301

Phytoplankton is a major food source for many animals, both large and small. Microscopic animals in the zooplankton (zoo means "animal") graze on this phytoplankton, and then both the zooplankton and phytoplankton become food for the benthic whales, some of the world's largest mammals. The unicellular algae of the phytoplankton are well adapted to this aquatic environment. As single cells, they have large, adsorptive surfaces relative to their volume and can move freely about, thus effectively using the dilute nutrients available.

Because one or more algal species can grow in almost any environment, algae often grow where other forms of life cannot thrive. Frequently, algae are among the first organisms to become established in barren environments, where they synthesize the organic materials necessary for the subsequent invasion and survival of other plants and animals. They are often found on rocks, preparing the surface for the growth of more complex members of the biological community.

Structure of Algae

Algae can be both microscopic and macroscopic. Microscopic algae can be single-celled organisms floating free or propelled by flagella, or they can grow in long chains or filaments. Some microscopic algae such as Volvox form colonies of 500 to 60,000 biflagellat-ed cells, which can be visible to the naked eye (see figure 12.2a).

Macroscopic algae are multicellular organisms with a variety of specialized structures that serve specific functions (figure 12.4). Some possess a structure called a holdfast, which looks like a root system but primarily serves to anchor the organism to

Stipe

Holdfast

Blade

Bladder

Figure 12.4 A Young Nereocystis luetkeana, the Bladder Kelp The alga has a large bladder filled with gas.This bladder keeps the blades floating on the surface of the water to maximize exposure to sunlight.The blades are the most active sites for photosynthesis.The holdfast anchors the kelp to rocks or other surfaces. In a single season, kelp can grow to lengths of 5 to 15 m.

Stipe

Holdfast

Figure 12.4 A Young Nereocystis luetkeana, the Bladder Kelp The alga has a large bladder filled with gas.This bladder keeps the blades floating on the surface of the water to maximize exposure to sunlight.The blades are the most active sites for photosynthesis.The holdfast anchors the kelp to rocks or other surfaces. In a single season, kelp can grow to lengths of 5 to 15 m.

Blade

Bladder

Chapter 12 The Eukaryotic Members of the Microbial World

Table 12.1 Characteristics of Major Groups of Algae

Group and

Representative

Member(s)

Usual Habitat

Principal Pigments (in addition to chlorophyll a)

Storage Products

Cell Walls

Mode of Motility (if present)

Mode of Reproduction

Chlorophyta

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Responses

  • prisco endrizzi
    WHAT IS ALGAE AND IT CAUSATIVE AGENT?
    3 years ago
  • kirsten
    What is causetive organism.?
    2 years ago
  • gemma
    What are the causing agent of algae?
    10 months ago

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