bio2106-2(b)
Chlorophyta Overview
Chlorophyta is a division of green algae that contain chlorophylls a and b.
Storing food as starch in their plastids is a defining characteristic.
Various species under Chlorophyta exist, including unicellular and multicellular forms.
Genome structures within Chlorophyta differ, but common characteristics exist across species.
Habitat diversity includes freshwater, marine, and terrestrial environments.
Conditions for survival:
Light
Carbon
Essential nutrients
Water quality
Temperature
Tidal exposure
Some intertidal species, like ulva (sea lettuce), can tolerate a range of temperatures and survive desiccation at low tide.
While most Chlorophyta are aquatic, some inhabit different environments.
General Characteristics of Chlorophyta
Chlorophyta flourish during summer due to ample light, nutrients, and increased temperatures.
Some filamentous forms may be inedible.
Flagella assist in reducing sinking tendencies of Chlorophyta.
Exhibiting various shapes and sizes, planktonic Chlorophyta share a characteristic: rapid growth potential.
Cell Structure and Metabolism
Predominantly unicellular, with some multicellular species in Chlorophyta.
Types:
Free-living
Colonial
Coenocytic
Filamentous sporophytes have a singular lenticular nucleus embedded in thick cytoplasm.
Characteristically, Chlorophyta possess biflagellated gametes.
Pigments:
Major pigment is chlorophyll b.
Some tropical species contain siphonoxanthin and siphonein.
Storage of starch derived from photosynthesis occurs in double-membrane-bound chloroplasts.
Cell walls are made of cellulose.
Figure 1: Illustrates General Forms of Chlorophyta (Parenchymatous and Siphonocladous).
Photosynthesis
Chlorophyta perform photosynthesis to obtain starch, characterized as autotrophic organisms.
Figure 2: Displays General structure of Chlorophyta.
Reproduction and Life Cycle
Chlorophyta have both sexual and asexual reproduction mechanisms:
Predominantly, reproduction occurs sexually.
Asexual reproduction methods include fission, fragmentation, and zoospores.
Sexual reproduction types:
Isogamous
Anisogamous
Oogamous
Example: Ulva lobata features alternation of generations, alternating between haploid and diploid phases.
In haploid phase: gametes are formed.
In diploid phase: zoospores are formed.
Not all species exhibit this alternation; for some, meiosis occurs in the zygote.
Types of existence:
As single cells (both motile and non-motile)
As multicellular filaments or colonies.
Example of unicellular organism: Chlorella, a fast-growing species that can inhabit animals.
Multi-cellular filaments may involve both asexual and sexual reproduction processes.
Ulva goes through alternation of generations.
Characteristics of parents are recombined through gamete fusion, with reproduction also via spores.
Colonies consist of single cells arranged without specialized functions.
Rapid reproduction occurs with nearby mating cells.
Example of a colony: Volvox, a spherical colony of individual cells bonded by cytoplasm strands, using flagella for motion in water.
Figure 3: Illustrates General Life cycle of Chlorophyta.
Oedogonium Life Cycle
Asexual Reproduction
Oedogonium is a filamentous green algae capable of asexual reproduction, containing both male and female reproductive organs.
Asexual reproduction occurs through fragmentation, producing zoospores
Zoospores navigate through water spontaneously.
On finding a viable substrate, they divide and form new filaments.
Sexual Reproduction: Zygote Production
Oedogonium reproduces sexually, with the filament called antheridium producing and releasing sperm.
Sperm then sinks until it fertilizes the egg, located in the oogonium.
Successful fertilization leads to the production of a zygote.
Sexual Reproduction: Zoospores
The zygote develops into zoospores after being released into water.
Mature zoospores split, attach to the substrate, and cells in zoospores will divide to form new filaments of Oedogonium.
Figure 4: Illustrates Life cycle of Oedogonium.
Ecology
Chlorophyta thrive in shallow water ecosystems, found in both freshwater (90% species) and marine habitats.
Few terrestrial species inhabit rocks or trees.
Some form symbiotic relationships with fungi (lichens) or animals.
Caulerpa racemosa, a Chlorophyta species, was introduced to the Mediterranean Sea in 1990 and first observed in France in 1997, continuing to spread in fishing areas.
Environmental conditions in the Mediterranean favor Caulerpa racemosa growth: ideal climate and substrata.
Spirogyra insignis, known for adapting to harsh conditions, was studied by Flores-Moya et al. (2005) in sulfurous waters.
Initially inhibited growth and reproduction; however, a resistant mutant variant survived, which is crucial for adaptation through spontaneous mutation.
Class Chlorophyceae
Contains the largest number of species within Chlorophyta.
Members typically possess two or more flagella near the cell's apex.
Mitosis involves phycoplasts, microtubular structures that manifest between and separate daughter nuclei, unique to this class.
A range of reproductive techniques: asexual and sexual.
Sexual reproduction results in a zygospore formation, a dormant diploid zygote protected by thick walls, which later undergoes meiosis.
Includes unicellular organisms like the genus Chlamydomonas (two apical flagella) and non-motile organisms like Chlorella (cultivated for dietary use).
Colonial genera include Hydrodictyon ("water net") and the volvocine line of flagellated genera ranging from simple to intricate spherical colonies like Volvox, which can consist of up to 60,000 cells with some cellular specialization.
Class Ulvophyceae
Comprises marine organisms exhibiting various forms: few cells, long filaments, thin sheets, or coenocytic structures.
Typically radially symmetrical.
Exhibits alternation of generations; meiosis occurs in spores, as opposed to the zygotes in other classes.
Features two or more apical flagella (when present).
During mitosis, both the nuclear envelope and mitotic spindle persist similarly to Charophyceae.
Class Charophyceae
Known for their significant fossil presence, with stoneworts dating back to the late Silurian period.
Cells display asymmetry.
Motile algae possess two flagella oriented at right angles near the apex.
Sexual reproduction, similar to Chlorophyceae, involves zygospore formation and zygotic meiosis.
In contrast to other classes, the nuclear envelope disintegrates when mitosis begins alongside a persistent mitotic spindle; some use a phragmoplast for cell plate formation akin to plants.
Notable members include Spirogyra (filamentous algae) and desmids (noted for symmetry and beauty, found exclusively in freshwater), integral to the food chains of nutrient-poor ponds.
The stoneworts have complex branched thalli with erect stems and numerous whorled branches.
References
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