Phytoplankton

Microbial and Plant Diversity

Eukaryotic Algae

Introduction to Eukaryotic Algae

• Eukaryotic algae are a vital component of aquatic ecosystems. This section will discuss:

  • What constitutes eukaryotic algae.

  • Their evolutionary history.

  • Defining characteristics that distinguish them.

  • The main phylogenetic groups of eukaryotic algae.

Definition of Algae

• The term "algae" has evolved and is no longer used strictly as a taxonomic label but refers broadly to a diverse group of organisms.

  • According to Fritsch (1935), algae must include holophytic organisms (photosynthesis through chlorophyll a), but do not show the complex structures found in archegoniate plants (like mosses, liverworts, and ferns).

Characteristics of Algae

• Algae cover a vast array of species ranging from unicellular, colonial to multicellular forms.

  • Many classes possess both unicellular and multicellular members.

  • Diverse habitats include marine, brackish, and freshwater environments as well as terrestrial ecosystems like soils and deserts.

  • Notable for their rapid growth rates and remarkable carbon fixation abilities.

  • They play a key role at the base of aquatic food chains

  • Are significant in climate change mitigation.

Evolution of Photosynthesis

• Photosynthesis originated in bacteria

  • Particularly cyanobacteria (blue-green algae) which are oxygenic photosynthesizes(produce oxygen) , in contrast to others that are anoxygenic.

Endosymbiosis in Algal Evolution

• The evolutionary history of eukaryotic algae is largely influenced by endosymbiotic events:

  • The primary event involves the incorporation of a cyanobacterium by a protist.

  • Followed by secondary and tertiary endosymbiotic events.

• Modern photosynthetic organisms have descended from an ancestral cyanobacterium through primary endosymbiosis, where the plastid has two membranes.

Types of Endosymbiotic Events

Secondary Endosymbiosis

• In green and red algae, secondary endosymbiotic events resulted in more intricate relationships:

  • Secondary plastids have three membranes, resulting from a cyanobacterium being incorporated into an algal host.

  • In further evolution, processes continued leading to plastids with four membranes, as seen in various groups:

    • Euglenids - Many contain green plastids.

    • Chlorarachniophytes - Rare marine taxa with a nucleomorph.

    • Haptophytes - Marine flagellates with complex plastid structures.

    • Cryptomonads - Plastids contain a nucleomorph within.

    • Stramenopiles - Includes diatoms and other macroalgae.

Characteristics of Phytoplankton

Structural Features and Membranes

• Algal cell walls vary considerably.

  • Prokaryotes have a double membrane featuring a peptidoglycan layer. Glaucophyta retains this layer while others have lost it.

  • Plastids may have two, three or even four membranes:

    • Two membranes - Green and red algae.

    • Three membranes - Euglenids and dinoflagellates.

    • Four membranes - Cryptophyta and others.

Photosynthetic Pigments

• Algae contain several types of chlorophyll (a, b, c1, c2, d) along with accessory pigments such as carotenoids and phycobiliproteins.

  • The evolution of chlorophyll types is complex and intertwined with lineage-specific adaptations throughout evolutionary history.

Major Groups of Phytoplankton

Derived from Primary Endosymbiotic Event

• Glaucophyta - Retain the peptidoglycan layer and feature chloroplasts known as cyanelles, generally found in freshwater.

• Chlorophyta - Green algae essential for nutrient cycling, with both unicellular and multicellular forms; believed to be ancestors of land plants.

• Rhodophyta - Red algae known for their lack of flagella and contribution to various lineages including dinoflagellates and diatoms.

Derived from Secondary Endosymbiotic Event

• Dinophyta (Dinoflagellates) - Diverse group with toxic potential, engage in symbiotic relationships, crucial in marine ecosystems.

• Bacillariophyceae (Diatoms) - Key role in CO2 fixation, significant primary producers in aquatic environments; have silica-based cell walls.

• Prymnesiophyceae (Haptophytes) - Include important CO2 fixers, covered with calcium carbonate scales, some forms produce toxins.

• Cryptophyceae - Known for low-light photosynthesis and the ability to thrive in nutrient-poor waters, highlighted by their ecological significance.

Other Groups

• Acknowledgment of additional significant algal groups such as Apicomplexa and various others that contribute to ecological dynamics, yet are not extensively covered in this context.