17: Eukaryotic Algae and Phytoplankton Diversity

Eukaryotic Algae and Phytoplankton Diversity

Introduction

• Eukaryotic algae: Definition and characteristics.
• Evolutionary history of eukaryotic algae.
• Key features of algae.
• Overview of main algal groups.

Definition of Algae

• "Algae" is no longer a formal taxonomic term but remains in use among biologists.
• Fritsch (1935) defined algae as holophytic organisms (photosynthetic via chlorophyll a) and their colorless derivatives that do not reach the differentiation level of archegoniate plants (e.g., mosses, liverworts, and ferns).

General Characteristics of Algae

• Encompass a vast range of species, from unicellular and colonial to multicellular forms.
• Many classes include both unicellular and multicellular representatives.
• Inhabit diverse aquatic habitats: marine, brackish, freshwater, hot springs, etc.
• Also found in terrestrial environments: soils, snow, rock, desert soils, etc.
• Exhibit some of the fastest growth rates.
• Demonstrate some of the highest levels of carbon fixation.
• Form the base of all aquatic food chains.
• Play a crucial role in mitigating climate change.

Evolution of Photosynthesis

• Photosynthesis originated in Bacteria.
• Cyanobacteria (blue-green algae) are oxygenic photosynthesizers, producing $O_2$.
• All other photosynthetic organisms evolved from this foundation.

Endosymbiosis

• Evolutionary history of the main algal groups is shaped by endosymbiosis.
• Primary endosymbiosis: engulfment of a cyanobacterium by a protist.
• Subsequent secondary and tertiary endosymbiotic events.

Primary Endosymbiosis

• Modern photosynthetic organisms evolved from the endosymbiosis of an ancestral cyanobacterium.
• Primary endosymbiotic event: the resulting plastid has two membranes.

Secondary Endosymbiosis

• Green and red algae underwent secondary endosymbiotic events.
• Secondary plastids typically have three membranes (nucleus and mitochondria degenerate).
• Plastid membranes:
  • Cyanobacterium inner and outer membranes.
  • Secondary phagosomal membrane (chloroplast ER).

Secondary Endosymbiosis with Four Membranes

• In some cases, secondary plastids have four membranes.
• The nucleus from the primary endosymbiotic event persists as a reduced nucleomorph.
• Cryptomonads: plastid with four membranes, including cyanobacterial inner and outer walls, a red algal wall, and a secondary phagosomal membrane.

Secondary Endosymbiosis - Green Algal Cell

• Euglenids: typically contain green plastids with three membranes.
• Chlorarachinophytes: rare marine amoeboflagellates and flagellates with a nucleomorph; plastid has four membranes.

Secondary Endosymbiosis - Red Algal Cell

• Haptophytes: marine flagellates; plastids have four membranes.
• Cryptomonads: plastid has four membranes and a nucleomorph.
• Stramenopiles: includes diatoms and macroalgae; plastid has four membranes.

Secondary Endosymbiosis - Red Algal Cell (cont.)

• Apicomplexa: obligate intracellular parasites with an apicoplast, a relic plastid.
• Dinoflagellates: plastid has three membranes; may have undergone subsequent plastid loss and reacquisition.

Serial Secondary Endosymbiosis

• Endosymbiosis of a cell derived from a primary endosymbiotic event (green algae; symbiont) being phagocytosed by a host (dinoflagellate) that originated from a secondary endosymbiotic event.
• Dinoflagellate (2°) consumes a green algal cell (1°).

Tertiary Endosymbiosis

• Endosymbiosis of a cell derived from a secondary endosymbiotic event (diatom, etc.; symbiont) being phagocytosed by a host (dinoflagellate) derived from a secondary endosymbiotic event.
• Dinoflagellate (2°) consumes a diatom/haptophyte/cryptomonad (2°).

Examples of Serial Secondary and Tertiary Endosymbiosis in Dinoflagellates

• Lepidodinium chlorophorum and L. viride: serial secondary endosymbiosis with a green alga.
• Durinskia agilis & D. baltica: tertiary endosymbiosis with a diatom.
• Karlodinium (13 spp.): tertiary endosymbiosis with a haptophyte.
• Dinophysis (265 spp.): tertiary endosymbiosis with a cryptomonad.

Cyanobacteria Characteristics

• Chlorophyll a (Chl a).
• Phycobiliproteins.
• Gram-negative bacteria.
• Circular fibrils of DNA.
• Singular thylakoids.
• All eukaryotic photosynthesizers evolved from primary endosymbiosis with an ancestral cyanobacterium called chloroxybacteria.
• Evolved 2.5 billion years ago (in freshwater?).

Characteristic Features of Phytoplankton

• Cell Wall
• Photosynthetic Pigments
• Thylakoid Arrangement

Cell Wall

• Prokaryotes have a double membrane with a peptidoglycan layer in between.
• These two membranes are retained around the chloroplast.
• In Glaucophyta, the peptidoglycan layer is retained; in all other groups, it has been lost.
• The phagosomal membrane is lost.

Cell Wall During Primary Endosymbiosis

• Loss of the peptidoglycan layer and phagosomal membrane results in two membranes around the plastid.
• Exception: Glaucophyta retains the peptidoglycan layer.

Membranes Around Plastids

• Two membranes: green and red algae.
  1. Cyanobacterial cell membrane.
  2. Cyanobacterial outer membrane.
• Three membranes: euglenids and dinoflagellates.
  1. Cyanobacterial cell membrane.
  2. Cyanobacterial outer membrane.
  3. Phagosomal membrane: chloroplast endoplasmic reticulum.
• Four membranes: cryptophytes, Heterokontophyta, etc.
  1. Cyanobacterial cell membrane.
  2. Cyanobacterial outer membrane.
  3. Red algal cell membrane: chloroplast endoplasmic reticulum.
  4. Nuclear envelope: chloroplast endoplasmic reticulum.

Photosynthetic Pigments

• Algae have four types of chlorophyll: a, b, c ($c<em>1$ & $c</em>2$), d.
• Chl a is always present.
• Variety of accessory pigments:
  • Carotenoids: carotenes and xanthophylls.
  • Phycobiliproteins: phycocyanin and phycoerythrin.

Evolution of Photosynthetic Pigments

• Chl a is believed to have evolved first, followed by Chl b & c.
• The discovery of Chl d caused confusion.
• Chl d is found in some cyanobacteria, such as Acaryochloris marina, and possibly evolved “late”.
• Phycobiliproteins were lost in green algae due to “chance events”.
• Other pigments were lost as a consequence of the evolution of LHCs, which gave rise to the other chlorophylls.

Thylakoid Arrangement

• Single: Rhodophyta and Cyanophyta.
• Paired: Cryptophyta.
• 3-ply: Dinophyceae, Bacillariophyceae.
• Granal stacks: Chlorophyta.

Thylakoid Arrangement: Structure and Algal Groups

Main Groups of Phytoplankton - Primary Endosymbiotic Event

Glaucophyta

• Thought to have evolved before the red and green algal lineages.
• Retain the peptidoglycan layer.
• Contain Chl a, phycobilins, and phycobilisomes.
• Chloroplasts are called cyanelles.
• Comprise 13 species assigned to five genera.
• Rare; found in freshwater.

Chlorophyta

• Green algae found in marine and freshwater environments; can be multicellular or unicellular.
• Contain Chl a and b; possess flagella; thylakoids are three to five banded; cell wall is composed of cellulose.
• Have four classes.
• Charophyceae are thought to have given rise to land plants (embryophytes).

Rhodophyta

• Red algae are among the oldest groups of eukaryotic algae.
• Contain Chl a, phycobiliproteins, lack flagella, and have single thylakoids.
• Mostly multicellular.
• Gave rise to other lineages (may or may not be photosynthetic): dinoflagellates, haptophytes, cryptomonads, diatoms.

Main Groups of Phytoplankton - Secondary Endosymbiotic Event (Three Membranes)

Dinophyta: Dinoflagellates

• Include harmful algal bloom (HAB) forming species.
• Form symbiotic relationships (zooxanthellae in corals).
• Very important group of phytoplankton.
• Contain Chl a and c2, peridinin and neoperidinin (carotenoids).
• Can be autotrophic, heterotrophic, or mixotrophic.
• Biflagellate with cellulosic plates under the plasma membrane.
• Exhibit bioluminescence.
• Possess special organelles: pusule, ocellus, nematocyst, trichocysts, and muciferous bodies.
• Have permanently condensed mesokaryotic chromosomes; undergo “dinomitosis”.
• Many species produce toxins, causing HABs:
  • Diarrhetic shellfish poisoning.
  • Paralytic shellfish poisoning.
  • Ciguatera fish poisoning.

Euglenophyta: Euglenids

• Possess a pellicle – proteinaceous strips beneath the plasmalemma.
• Exhibit metaboly (changing shape).
• Primarily a freshwater group.
• Contain Chl a and b.
• Heterotrophic or mixotrophic (osmotrophy).
• Have a mesokaryotic nucleus and do not undergo sexual reproduction.

Main Groups of Phytoplankton - Secondary Endosymbiotic Event (Four Membranes)

Bacillariophyceae: Diatoms

• Cell wall composed of $SiO_2$ (silica).
• Can produce resting spores/cells under unfavorable conditions.
• Very important fixer of $CO_2$ in temperate waters.
• Very important group of unicellular algae.
• Contain Chl a, $c<em>1$ & $c</em>2$, and fucoxanthin.
• Autotrophic (a few species are heterotrophic, feeding on decaying marine vegetation).
• Cell wall consists of two frustules composed of $SiO_2$:
  • Upper valve = epitheca.
  • Lower valve = hypotheca.
• Resistant to enzymes.
• High amounts of unsaturated fatty acids; when released upon death, they transform into short-chain fatty aldehydes, which are toxic to many larval invertebrates.
• Harmful algal blooms (HABs) can cause amnesic shellfish poisoning due to domoic acid.
• Spring bloom: a bloom of phytoplankton in temperate latitudes.
• Succession of phytoplankton communities: diatoms -> dinoflagellates -> zooplankton.
• Important marine primary producers.
• Grow very quickly but become nutrient-limited and are consumed by dinoflagellates.

Prymnesiophyceae: Haptophytes

• Possess a haptonema (specialized organelle with 6+1 microtubules).
• Very important fixers of $CO_2$ in oligotrophic waters.
• Very important group of unicellular algae.
• Contain Chl a, $c<em>1/c</em>2$, ß-carotene, diato-, and diadinoxanthin.
• Covered in $CaCO_3$ scales (coccoliths).
• Also called coccolithophorids.
• Also produce toxins (e.g., Phaeocystis).

Cryptophyceae

• Have ejectosomes.
• Mixotrophic.
• Marine and freshwater flagellates.
• Contain Chl a, $c_2$, and (unusually) phycobiliproteins.
• Have a nucleomorph – remnant red algal nucleus, between the CER.
• Asymmetrical cells with a “swaying” swimming motion.
• Important as they can photosynthesize at low light levels in low nutrients.
• Dominant algae in freshwater lakes in Antarctica.
• Kick-start primary productivity in low light.
• Form a symbiotic association with the ciliate: Myrionecta rubra.
• Very high primary productivity (PP).

Other Phytoplankton Groups

• Apicomplexa: important endoparasites including Plasmodium and Toxoplasma.
• Heterokontophyta:
  • Chrysophyceae: freshwater group.
  • Synurophyceae: silicified scales.
  • Eustigmatophyceae: aquatic and terrestrial group.
  • Dictyochophyceae: golden browns.
  • Phaeophyceae: seaweeds and kelps.

Quick Recap

• Eukaryotic algae – what are they?
• Evolution of eukaryotic algae.
• Characteristic features of algae.
• Main groups.