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Chapter 30

BLG 144 Chapter 30: Chapter 30 - Green Algae

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Ryerson University
BLG 144
Michael Arts

Chapter 30 – Green Algae Facts: - Green algae are an important source of oxygen and provide food for aquatic organisms. They could get into new habitats very easily by moving though dust or air. They were super weeds - Land plants release oxygen, hold soil and water in place, build soil, moderate extreme temperature, and winds, and provide food for terrestrial organisms - Land plants were the first multicellular organisms that could live with most of their tissues exposed to the air. A series of key adaptions allows them to survive on land. In terms of total mass, plants dominate today’s terrestrial environments - Once plants could grow on land, a sequence of important evolutionary changes made it possible for them to reproduce efficiently even in extremely dry environments. Introduction: - Green plants: Green algae and Lan plants - Green algae: Important photosynthetic organisms in freshwater habitats - Land plants: Key photosynthesizes in terrestrial environments - Green algae are protists but they are studied along with land plants because they are the closest living relatives to land plants and the transition from aquatic to terrestrial life occurs when land plants evolved from green algae - Why study green plants? People rely on them for: food, oxygen, fuel, building materials, clothing, paper, rope and basket fibres. - Ecosystem services: an ecosystem consists of all the organisms in an area along with physical components such as atmosphere, precipitation, surface water, sunlight, soil, and nutrients. Green algae and land plants enhance the life-supporting attributes of said components of the ecosystem. Green Plants benefit other organisms: o Producing oxygen via oxygenic photosynthesis o Build soil by providing food for decomposers o Hold soil and prevent nutrients from being lost to erosion by wind and water (revegetation leads to erosion) o Hold water in the soil o Moderate the local climate by providing shade and reducing the impact of wind on landscapes - Land plants are primary producers and provide the base of the food chain - Plants are eaten by herbivores which are eaten by carnivores (other organisms are omnivores that eat both plants and animals) - Land plants and green algae have a key role in the carbon cycle o They reduce atmospheric CO to2make sugar - Artificial selection for plants with certain properties have lead to dramatic changes in plant characteristics o Maize(corn) How to study green plants? - 3 complementary strategies: o Compare fundamental morphological features o Analyze fossil record o Asses similarities and differences in DNA sequence from homologous genes to estimate phylogenetic tree - Analyzing morphological traits: o Green algae include species that are unicellular, colonial, and multicellular and that live in marine or freshwater habitats o Green algae lineages hypothesized to be most closely related to land plants: Coleochaetophyceae (coleochaetes) and Charophyceae (stoneworts - Morphological differences: 3 major categories based on morphology o Non-vascular plants (Bryophytes): lack vascular tissue – specialized groups of cells that conduct water and dissolve nutrients throughout the plant body o Seedless vascular plants: have well developed vascular tissues but do not make seeds (consist of an embryo and a store of nutritive tissue surrounded by a tough protective layer) o Seed plants: have a vascular tissue and also make seeds - Examples: non-vascular  mosses, seedless vascular  ferns, seed plants  flowering plants Seeded plants: 2 major lineages - Gymnosperms that produce seeds and do not develop in an enclosed structure - Angiosperms (flowering plants) develop seeds inside a protective structure called a carpel Using the Fossil Record - For green algae, the fossil record begins 700-225 mya and or land plants 475 mya (supports the hypothesis that land plants are derived from green plants) Origin of land plants: - Earliest fossils are microscopic - Characteristics that suggest they grew on land o A watertight cuticle to prevent drying out (desiccation) o Fossilized spores are surrounded by a sheet like coating that is almost identical to the protective sporopollenin (encases spores and pollen in modern plants) o Early spores are found in association with sporangia (spore producing structure similar to those found in modern non-vascular plants) - Silurian-Devonia Explosio: in rocks dated 416-359 mya, fossils from most major plant lineages were found - Characteristics that allow plants to occupy dry land: o Water-conducting tissue o Roots o Stomata o Leaves - Carboniferous Period: o Rocks dated 39-299 mya have extensive deposits of coal (carbon-rich rocks packed with fossil spores, branches, leaves and trunks o Coal formation is thought to start only in the presence of water which means the carboniferous fossils indicate the presence of extensive forested swamps Diversification of gymnosperms - Major groups of gymnosperms: o Cycadphyta (cycads) o Ginkgophyta (ginkgos) o Gnetophyta (gnetophytes) o Ponophyta (pines, spruces and firs) o Other cone-bearing species (redwoords, junipers, yews) - Grow readily in dry habitats, both wet and dry environments became blanketed in green plants for the first time in this interval Diversification of Angiosperms - Fossil record dates to 150 mya - Plants that produced the first flowers are ancestors of today’s grasses, orchids, daisies, oaks, maples, and roses Summary: - Green algae appear first, followed by nonvascular plants, seedless vascular plants, and seed plants Molecular phylogenies: 1. Green plants are monophyletic (single ancestor raised all green algae) 2. Splitting events on the tree, near the root, leads to lineages of green algae (land plants evolved from green algae) 3. Green algae a paraphyletic (green algae include some but not all descendants of a single common ancestor) 4. Charophyceae are the closest living relatives to land plants and thus land plants evolved from a multicellular green alga that lived in freshwater habitats 5. Land plants are also monophyletic (proving there was only one transition form freshwater to land) 6. Bryophytes are the earliest branching group among land plants and thus the non- vascular plants are the most ancient living group of plants 7. Non-vascular plants form a sequence of lineages which means they are paraphyletic 8. Seedless vascular plants are monophyletic and thus the vascular tissue evolved only once 9. Seed plants are monophyletic; seed only evolved once 10. Gymnosperms and angiosperms are monophyletic groups so among seed plants there was a major divergence in seed development - Main point: land plants had adaptations that allowed photosynthetic organisms to move from aquatic to terrestrial environments - They had to reproduce in a dry environment but how did they do that? o Cuticles: waxy watertight sealant that covers the aboveground parts of the plants and gives them the ability to survive in dry environments o Stoma: gas exchange is accomplished though a stoma, which consists of an opening called a pore surrounded by specialised guard cells. The pore opens and closes as the guard cells change shape Upright Growth - Early plats were not rigid and grew close to the ground, they also needed to grow in moist soil to gain water - Strong directional selection allowed plans to grow above ground in order to gain more nutrients (sunlight) - Growth above ground had 2 main problems: transporting water against gravity and lack of rigidity made the plants susceptible to gravity and wind Vascular tissue - Allowed plants to transport water from roots and support erect stems - Primary adaptation: cell walls in early plants with water-conducting cells made that ha d a substance called lignin which is a structural polymer made from six-carbon rings. - Water conducting cells called Tracheids evolved. o Trachieds have lignin containing secondary cell wall in addition to cellulose based primary cell wall o Pits in the sides and ends of the cell wall which allow water to flow efficiently between tracheids - The secondary cell walls gave trachieds the ability to provide better structureal support but water could still move trhough the cells easily because of the pits - Most specialized water-conducting cells appeared which were called vessel elements: o Shorter and wider than trachieds and have gaps on both ends. These characteristics reduce resistance and makes water movements extremely efficient o Vessel elements line up end to end to form a continuous pipe-like structure - Summary: primary water conducting cells  first vascular tissue  trachieds  vessel elements Evolutionary Changes on the Phylogenetic Tree: - Cuticles, vascular tissues, pores, stomata were all key adaption sand fundamentally important to dry conditions. They all evolved just once. - Convergence: o Water-conducting cells evolved independently in mosses and in the vascular plants
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