Marine Algae

Starting with Linnaeus —> $1735$ : Divided nature into Animals, Plants, and Minerals
Moved to Whittaker —> $1969$ : Created five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia
- Monera is made up of prokaryotic organisms (Ex., bacteria), and they lack a nucleus
- Protista, Fungi, Plantae, and Animalia are all composed of eukaryotic organisms
Led to modern genetics (molecular phylogenetics).
- Early classification systems relied primarily on observable physical traits (like grouping things into animals, plants, or minerals).
- Molecular phylogenetics instead compares the actual genetic code of organisms (DNA and RNA)

Protists: A general label for any eukaryotic organism that is not a land plant, fungus, or animal.
- A monophyletic group includes a common ancestor and all of its descendants.
- Protists are paraphyletic (non-monophyletic) because they descend from a common ancestor but exclude some of its descendant lineages —> Animals, plants, and fungi
- In the case of protists, the group includes many eukaryotic organisms descended from a common ancestor, but excludes land plants, fungi, and animals, which also descended from that same common ancestor.

Question: If photosynthesizers are found in many different lineages, did they develop the ability to perform photosynthesis independently?

Cyanobacteria: Bacteria that developed oxygen-producing photosynthesis (2.5-3 billion years ago)
- This triggered the Great Oxidation Event (GOE), causing a mass extinction while making complex eukaryotic life possible.
- As cyanobacteria produced vast amounts of oxygen through photosynthesis, oxygen built up in the atmosphere and oceans, poisoning and wiping out most existing anaerobic microbial life.
- Many non-photosynthetic prokaryotes survived by living in oxygen-free (anoxic) environments, such as deep-sea sediments and hydrothermal vents.

**Endo = Inside, Symbiosis = Close relationship

Primary Endosymbiosis: A single-celled heterotrophic eukaryote swallowed a photosynthetic cyanobacterium, creating double-membraned chloroplasts
- Instead of digesting it, the eukaryotic cell kept it alive
- Over evolutionary time, the cyanobacterium lost its independence and transferred much of its genetic code to the host's nucleus
- Specialized into an organelle inside the host cell—which we now call a chloroplast.
- Chloroplasts first evolved in the ancestors of archaeplastids

Evidence of Endosymbiosis:
- Chloroplasts have their own DNA
- They can replicate on their own through Nucleur Fission
- Manufacture their own proteins
- Chloroplasts have a double membrane

Secondary Endosymbiosis: A photosynthetic eukaryote with a chloroplast is engulfed by another eukaryote.
- It remains alive inside the eukaryote, but its nucleus is lost
- Through this, another eukaryotic lineage developed the ability to perform photosynthesis

Phytoplankton:
- Photosynthetic organisms that live in aquatic environments
- They form the base of almost every ocean food web
- Responsible for 50% of all global photosynthesis
- Ecosystem impact: Phytoplankton are very important because they undergo photosynthesis to produce organic compounds that are then digested by species such as zooplankton. The zooplankton are then eaten by larger species, such as sharks.
- Two Types:
  - Photosynthetic bacteria: Prokaryotes such as cyanobacteria, which do not have membrane-bound organelles
  - Photosynthetic eukaryotes: Species such as diatoms and dinoflagellates, which have a nucleus and membrane-bound organelles
- Diatoms: Diatoms have a silia based cell-coverings called frustules

Brown Algae (Phaeophytes):
- Multicellular ocean seaweeds with a basic body structure called a thallus that lacks a complex vascular system.
- They have chloroplasts with four membranes because they are a result of secondary endosymbiosis
- Their body is called a thallus, which is a body system that lacks a complex vascular system
- Kelp: Kept forests are so important to the ecosystem because they provide a habitat for marine species.

Red Algae (Rhodophytes):
- Belong to archaeplasts
- Most diverse group of marine algae
- Found deeper in the ocean
- Lack flagella
- Possess chloroplasts with 22 membranes formed via primary endosymbiosis
- Use the pigment phycoerythrin for photosynthesis, which gives them their red colour and absorbs deep-penetrating blue light, enabling them to live at great ocean depths
- Ex. Coralline Red Algae
  - Deposit calcium carbonate in their cell walls
  - Play a vital ecological role in coral reefs
Keystone Species: A keystone species is a species that has a major effect on its environment relative to its abundance
Sea otters are keystone species because they eat sea urchins, and sea urchins eat kelp
If sea otters did not eat sea urchins, the sea urchins would eat the kelp, but kelp is extremely essential in marine ecosystems
This is a trophic cascade

Step 1: Sporophyte produces spores
- Originally diploid (2n) sporophyte undergoes meiosis to become haploid (n) spores
Step 2: Spores develop into gametophytes
- Spores undergo mitosis and develop into haploid gametophytes
Step 3: Gametophytes produce gametes
- Undergo another round of mitosis to produce haploid gametes
Step 4: Fertilization
- Two haploid gametes fuse together to form a diploid zygote, which then undergoes mitosis and grows to become a diploid sporophyte