Algae & the Swapping of Photosynthesis in Eukaryotes

Algae & the Swapping of Photosynthesis in Eukaryotes\n\n## Learning Objectives\n* Outline key evidence for the endosymbiotic origin of chloroplasts (and mitochondria).\n* Define “plants” phylogenetically, and explain why many lineages of photosynthetic eukaryotes are not considered “plants”.\n* Explain the general outline of the phylogeny of plants: noting the difference between the book and more recent results.\n* Describe main characteristics and synapomorphy of the following groups: Red algae, Chlorophytes (typical green algae), Streptophytes (including Chara, Coleochaete, “other green algae” and the Land Plants).\n* Recall characters present in land plants that evolved in different algal ancestors, including chlorophyll a & b, starch, oogamy, plasmodesmata, and apical growth.\n\n## Origin of Chloroplasts: Endosymbiosis\n\n### All Chloroplasts Trace Back to One Cyanobacterium\n* It was recently discovered that all chloroplasts can be traced back to one cyanobacterium and mosses.\n* Genes required to make peptidoglycan occur in many plants and may be involved in chloroplast division.\n\n### Primary Endosymbiosis\n* Definition: The eukaryotic chloroplast originated from the symbiotic engulfment of a cyanobacterium by a eukaryotic host cell. This event produced the first photosynthetic eukaryotes.\n* Membranes: The cyanobacterium possessed an outer and inner membrane. The host cell membrane enveloped the cyanobacterium. Over evolutionary history, one of these three membranes was lost, leaving the chloroplast with typically two membranes.\n* Result: Chloroplasts are now considered an “organelle” within photosynthetic eukaryotic cells.\n* “Domestication”: This integration of cyanobacterial cells occurred in a common ancestor of red algae, green algae, and land plants.\n* Evidence: The phylogenetic history of the separate genome present in chloroplasts supports this (a similar story applies to mitochondria, which are older and shared by all eukaryotes).\n* Peptidoglycan: Has been lost in most primary plastid groups, except in glaucophytes, where it is retained between the inner and outer chloroplast membranes.\n* Primary Plastid Groups (resulting from primary endosymbiosis): Glaucophytes, Red algae, Chlorophytes, Charophycean green algae, and Land Plants.\n\n### Secondary Endosymbiosis\n* Definition: Occurs when a host eukaryotic cell engulfs another eukaryotic cell that already contains a chloroplast (from primary endosymbiosis).\n* Process: A chloroplast-containing eukaryotic cell is engulfed by another host eukaryotic cell, with the host membrane (from endocytosis) enclosing the engulfed cell.\n* Membranes: This process typically results in chloroplasts with more than two membranes (often three or four). A trace of the engulfed cell's nucleus, called a nucleomorph, is retained in some groups (e.g., chlorarachniophytes and cryptomonads). The inner of the two additional membranes has been lost in euglenids and dinoflagellates.\n* Separate Events: Each instance of secondary endosymbiosis represents a separate evolutionary event.\n* Examples: Brown algae (chloroplasts derived from red algae), Euglenids (Euglena), Dinoflagellates, Apicomplexans, Cryptomonads, Haptophytes, Diatoms, Oomycetes (many are heterotrophic), Stramenopiles, Chlorarachniophytes, Foraminiferans, and Radiolarians.\n\n### Tertiary Endosymbiosis\n* Definition: Occurs when a eukaryotic cell that obtained its chloroplast through secondary endosymbiosis is subsequently engulfed by another eukaryotic host cell. This complex event is humorously likened to a “Tur-Du-cken.”\n* Examples: Dinoflagellates have captured chloroplasts from many different algae via tertiary endosymbiosis. Karenia brevis is a dinoflagellate that produces brevetoxin, a potent neurotoxin. Dinoflagellates are also known for causing bioluminescent phenomena.\n\n### Phylogenetic Definition of “Plants”\n* Phylogenetic Definition: “Plants” (Plantae) are photosynthetic eukaryotes with a chloroplast that has two outer membranes, arising solely from primary endosymbiosis. This two-membrane chloroplast system is considered a synapomorphy of plants.\n* Exclusion of Other Photosynthetic Eukaryotes: Many lineages of photosynthetic eukaryotes (e.g., brown algae, euglenids, dinoflagellates) are not considered “plants” under this phylogenetic definition because their chloroplasts originated from secondary or tertiary endosymbiosis, resulting in more than two membranes or different ancestral origins.\n* Earliest Lineages: Most of the earliest divergences in the plant phylogenetic tree have led to aquatic lineages, collectively referred to as ALGAE.\n\n## Plant Phylogeny: Insights from Recent Research\n\n### Traditional View vs. Recent Results\n* Book’s View: The textbook's phylogeny of plants has been updated by more recent scientific findings.\n* Recent Research: Wong et al. 2019, from their paper “Sequencing and Analyzing the Transcriptomes of a Thousand Species Across the Tree of Life for Green Plants” (Ann. Rev. Plant Biol. 2020. $71:1.1–1.25$), provides a refined phylogeny.\n\n### Key Groups and Their Characteristics\n\n#### 1. Glaucophytes\n* Often considered the earliest-diverging lineage of plants.\n* Unique for retaining peptidoglycan between their chloroplast membranes, serving as strong evidence for their direct cyanobacterial origin.\n\n#### 2. Red Algae (Rhodophyta)\n* Characteristics:\n * Vary from single-celled to elaborately branched multicellular organisms.\n * Cell walls: Composed of cellulose.\n * Photosynthetic pigment: Primarily chlorophyll a (similar to Cyanobacteria).\n * Red color: Attributed to phycoerythrin pigments, which absorb green and blue light, enabling them to thrive in deeper waters.\n * Coralline Red Algae: Represent one of the earliest lineages of eukaryotes observable in the rock record (dating back approximately $600 ext{ Ma}$). Their cell walls are calcified with calcium carbonate ($ext{CaCO}_3$). They are ecologically vital as reef-builders and a food source for reef organisms.\n * Commercial Importance: Industrial source of carrageenan (Chondrus) and used as food (e.g., nori from Porphyra).\n\n#### 3. “Green Plants” (Viridiplantae) Clade\n* Synapomorphies:\n * Chloroplasts containing chlorophyll a and b.\n * Store carbohydrates as starch (red algae produce “floridean starch,” which differs by lacking one of the main polysaccharides found in green plant starch).\n\n#### 4. Chlorophytes (Typical Green Algae)\n* Description: Correspond to “most green algae” in the Wong et al. 2019 phylogeny.\n* Habitat: Primarily found in marine or freshwater environments.\n* Examples: Ulva, Oocystis, Volvox.\n* Note: The term “green algae” is paraphyletic, as it includes Chlorophytes and Streptophytes, but excludes land plants, which are nested within Streptophytes.\n\n#### 5. Streptophytes\n* Definition: A clade that encompasses some green algal lineages (e.g., Coleochaete, Chara, Spirogyra, Desmids) plus all land plants.\n* Shared (Ancestral) Characters/Synapomorphies (evolved in algal ancestors before the emergence of land plants):\n * Phragmoplasts & Plasmodesmata:\n * Phragmoplasts: Complex structures that act as a scaffold for cell plate assembly, oriented perpendicular to the plane of the future cell plate. They are thought to facilitate three-dimensional growth in algae and lead to the formation of cell plates with plasmodesmata.\n * Plasmodesmata: Channels that penetrate the cell walls of adjacent cells, creating cytoplasmic connections that enable direct intercellular communication and transport.\n * Parenchyma: The fundamental tissue type in streptophytes, characterized by cells interconnected by plasmodesmata.\n * Oogamy:\n * Definition: A form of sexual reproduction where the female gamete (egg) is large and non-motile, while the male gamete (sperm) is small and motile. This contrasts with isogamy, where gametes are similar in size and motility.\n * Life Cycle Example (Chara, Coleochaete): These plants are predominantly haploid. The fusion of egg and sperm produces a diploid zygote, which immediately undergoes meiosis to produce four haploid spores.\n * Apical Growth: Growth occurring at the tips of the organism.\n * Phylogenetic Note: While Chara (“Stoneworts”) were traditionally considered sister to land plants due to their branched apical growth, improved phylogenies suggest this feature may have evolved multiple times or been lost in certain lineages within the streptophytes.\n* “Other Green Algae” within Streptophytes: This broad category includes diverse forms such as Coleochaete, “stoneworts” (Chara), filamentous organisms like Spirogyra, and Desmids (Zygnematophyceae). It’s important to note that not all streptophytes exhibit all of these synapomorphic traits (e.g., some may have lost apical growth, plasmodesmata, or oogamy).