Introduction and significance of Symbiotic algae

Coral Reef
Lichens
Sea sponges

1. Symbiotic algae Coral Reef Lichens Sea sponges Ms. Kushbu.R Kristu Jayanti College

2. Coral Reef The corals and these special cells have a mutualistic relationship.  The coral provides the  zooxanthellae  with a protected environment and compounds they need for photosynthesis . In return, the zooxanthellae produce oxygen and help the coral to remove wastes. Zooxanthellae are  single-celled algae . The algae photosynthesize, turning light and carbon dioxide into food that they share with the coral. Video link: https ://youtu.be/JENUAv0w8Q4 Coral , any of a variety of invertebrate marine organisms of the class Anthozoa (phylum Cnidaria ) that are  characterized by skeletons—external or internal—of a stonelike , horny, or leathery consistency. The term  coral  is also applied to the skeletons of those animals, particularly to those of the stonelike corals.

3. INTRODUCTION Coral reefs are often called the rainforests of the seas . Teeming with life, reefs harbor a broad range of organisms that rely on a complex network of ecological interactions and symbiosis (close and often long-term interaction between two or more different biological species). The diversity and complexity of coral reefs starts at the microscopic level. Within the coral tissue, symbiotic algae ( dinoflagellates ), commonly called zooxanthellae (their scientific name is  Symbiodinium ) , are crucial to the coral hosts. Like leaves from a tree, these microscopic algae harvest light and produce energy in the form of carbon rich compounds. A ssessments of the photosynthetic efficiency and photosystems “health” condition, using a special fluorometer is the recent study .

4. These symbiotic algae remove and use the coral’s waste products ( CO 2 , nitrogen ) for growth and photosynthesis. In return the coral gains energy rich organic carbon, created by the algae through photosynthesis. This symbiotic relationship provides most of the coral’s energy needs and contributes to the production of the coral limestone skeleton. The loss of these symbiotic algae, a process known as coral bleaching , reveals the coral’s white limestone skeleton as the tissue looses all pigmentation. On its own, the coral animal struggles to meet its’ energy requirements, and, if prolonged, this is usually lethal for the coral.

5. The disruption of this obligate symbiosis can be triggered by a number of stress factors, including rapid and extreme temperature changes, high light levels and pollution . The susceptibility to these factors can vary according to the coral and type of symbiotic algae, or zooxanthellae , that they host . Corals can harbor one or more types of zooxanthellae .  The type of symbionts also varies with environmental conditions and geographic region. W idely distributed branching coral found throughout the Pacific and Indian Oceans. So far, the scientific team has found and sampled several  Pocillopora   species that are abundant on the fore reef, but appear to be uncommon and rare on the inner lagoonal reefs. These samples will be taken to the laboratory to assess which symbionts they host. This information about symbiotic algae can help scientists and managers foresee some of the areas that will be more susceptible or more resilient to stress factors, such as increasing temperature.

6. Coral and zooxanthallae

7. Symbiotic association  There are several different mechanisms behind this and depend on whether the coral reproduces asexually or sexually. In the case of an asexually reproducing coral, zooxanthellae transmission takes place through coral budding or fragmentation which form a new coral. The zooxanthellae residing in the donor tissue of clonal coral automatically relocate, thereby colonizing the new cora .

8. In sexually reproducing coral, zooxanthellae are either acquired through  direct / vertical  or  indirect/horizontal transfer. In   direct  or  vertical transfer , the mother coral polyp releases the eggs with zooxanthellae inside ,. But most coral eggs do not have zooxanthellae in them; the eggs have to obtain the zooxanthellae through phagocytosis from the coral polyp's gastrovascular cavity. For the coral larvae that was borne from eggs without zooxanthellae , they can uptake their parent's zooxanthellae before their release into the surrounding seawater . But if they do not have this opportunity, they have to absorb them from the environment. This is called  indirect  or  horizontal transfer . The concentration of free-swimming (motile) zooxanthellae over a reef is normally low but sometimes they show preference to newly settled coral.

9. Chemotaxis is the mode of locomotion of such a zooxanthellae ; much like diffusion of molecules from a region of large concentration to a region of lower concentration, motile zooxanthellae can show positive chemotaxis in the direction of corals with zero or lower concentrations of zooxanthellae (Muller-Parker et al, 2015). Additionally, corals can obtain zooxanthellae indirectly through the ingestion of fecal matter excreted by corallivores (animals that eat coral) and of animals who have eaten prey with zooxanthellae in their cells (prey such as jellyfish and sea anemones). Over the course of their lives, corals are able to obtain multiple different species of zooxanthellae . During a bleaching event the zooxanthellae may be expelled from the coral, and if the coral survives, its tissues can be re-populated by a different species of zooxanthellae .

10. LICHENS Lichens are a small group of plants of composite nature, consisting of two dissimilar organisms, an alga- phycobiont ( phycos — alga; bios — life) and a fungus- mycobiont ( mykes — fungus; bios — life); living in a symbiotic association. Generally the fungal partner occupies the major portion of the thallus and produces its own reproductive structures. The algal partner manufactures the food through photosynthesis which probably diffuses out and is absorbed by the fungal partner.

11. General characteristics 1. Lichens are a group of plants of composite thalloid nature, formed by the association of algae and fungi. 2 . The algal partner-produced carbohydrate through photosynthesis is utilized by both of them and the fungal partner serves the function of absorption and retention of water. 3 . Based on the morphological structure of thalli , they are of three types crustose , foliose and fruticose . 4 . Lichen reproduces by all the three means – vegetative, asexual, and sexual. ( a) Vegetative reproduction : It takes place by fragmentation, decaying of older parts, by soredia and isidia . (b) Asexual reproduction : By the formation of oidia . ( c) Sexual reproduction : By the formation of ascospores or basidiospores . Only fungal component is involved in sexual reproduction .

12. Lichens exist in one of the below-mentioned growth forms. The growth form in which the lichens are leafy or bush-like are termed macrolichens . The other forms are called microlichens . Crustose  grow across the substrate. Foliose  are flat, leaf-like sheets of tissues and not bound closely. Fruticose  are freely available in standing branching tubes. Gelatinous  or jelly-like appearance  Leprose  are lichens with powdery appearance.  Squamulose  are closely clustered and lit flattened pebble units . Likewise, lichens can also be seen in various colours like yellow, orange, red, brown, etc. These colours are due to the presence of special pigment called usnic acid. In the absence of this pigment, they are generally olive gray or green. 

13. The composite plant body of lichen consists of algal and fungal members. : The algal members belong to Chlorophyceae ( Trebouxia , Trentepohlia , Coccomyxa etc.), Xanthophyceae ( Heterococcus ) and also Cyanobacteria ( Nostoc , Scytonema etc .) The fungal members mainly belong to Ascomycotina and a few to Basidiomycotina . Among the members of Ascomycotina , Discomycetes are very common; producing huge apothecia, others belong to Pyrenomycetes or Loculoascomycetes .

14. Contd …. Ascospores are produced in Ascolichen . (a) The male sex organ is flask-shaped spermogonium , produces unicellular spermatia . (b ) The female sex organ is carpogonium (ascogonium), differentiates into basal coiled oogonium and elongated trichogyne . ( c) The fruit body may be apothecia ( discshaped ) or perithecial (flask-shaped) type. ( d) Asci develop inside the fruit body containing 8 ascospores . After liberating from the fruit body, the ascospores germinate and, in contact with suitable algae, they form new lichen. Basidiospores are produced in Basidiolichen , generally look like bracket fungi and basidiospores are produced towards the lower side of the fruit body. The growth of lichen is very sThe growth of lichen is very slow, they can survive in adverse conditions with high temperature and dry condition.

15. ascolichens

16. Basidiolichens

17. There are three views regarding the nature of association of algal and fungal partners in lichen: 1. According to some workers, the fungus lives parasitically, either partially or wholly, with the algal components. This view gets support for the following evidences : ( i ) Presence of haustoria of fungus in algal cells of some lichen. ( ii) On separation, the alga of lichen is able to live independently, but the fungus cannot survive . 2. According to others, they live symbiotically, where both the partners are equally benefitted. The fungal member absorbs water and mineral from atmosphere and substratum, make available to the alga and also protects algal cells from adverse conditions like temperature etc. The algal member synthesises organic food sufficient for both of them. 3 . According to another view, though the relationship is symbiotic, the fungus shows predominance over the algal partner , which simply lives as subordinate partner. It is like a master and slave relationship, termed helotism.

18. External structure of lichens Crustose : These are encrushing lichens where thallus is inconspicuous, flat and appears as a thin layer or crust on substratum like barks, stones, rocks etc. (Fig. 4.112B). They are either wholly or partially embedded in the substratum, e.g., Graphis , Lecanora , Ochrolechia , Strigula , Rhizocarpon , Verrucaria , Lecidia etc . Foliose: These are leaf-like lichens, where thallus is flat, horizontally spreading and with lobes. Some parts of the thallus are attached with the substratum by means of hyphal outgrowth, the rhizines , developed from the lower surface (Fig. 4.112C), e.g., Parmelia , Physcia , Peltigera , Anaptychia , Hypogymnia , Xanthoria , Gyrophora , Collema , Chauduria etc . Fruticose ( Frutex , Shrub): These are shrubby lichens, where thalli are well developed, cylindrical branched, shrub-like (Fig. 4.112D), either grow erect ( Cladonia ) or hang from the substratum ( Usnea ). They are attached to the substratum by a basal disc e.g., Cladonla , Usnea , Letharia , Alectonia etc

19. Internal Structure of Thallus Based on the distribution of algal member inside the thallus , the lichens are divided into two types. Homoisomerous or Homomerous and Heteromerous . Homoisomerous : Here the fungal hyphae and the algal cells are more or less uniformly distributed throughout the thallus . The algal members belong to Cyanophyta . This type of orientation is found in crustose lichens. Both the partners intermingle and form thin outer protective layer (Fig. 4.11 3A), e.g., Leptogium , Collema etc. Heteromerous : Here the thallus is differentiated into four distinct layers upper cortex, algal zone, medulla, and lower cortex. The algal members are restricted in the algal zone only. This type of orientation is found in foliose and fruticose lichens (Fig. 4.113B) e.g., Physcia , Parmelia etc.

20. The detailed internal structure of this type is: ( a) Upper Cortex: It is a thick, outermost protective covering, made up of compactly arranged interwoven fungal hyphae located at right angle to the surface of the fruit body. Usually there is no intercellular space between the hyphae, but if present, these are filled with gelatinous substances . (b) Algal Zone: The algal zone occurs just below the upper cortex. The algal cells are entangled by the loosely interwoven fungal hyphae. The common algal members may belong to Cyanophyta like Gloeocapsa (unicellular); Nostoc , Rivularia (filamentous) etc. or to Chlorophyta like Chlorella, Cystococcus , Pleurococcus etc. This layer is either continuous or may break into patches and serve the function of photosynthesis. ( c) Medulla : The medulla is situated just below the algal zone, comprised of loosely interwoven thickwalled fungal hyphae with large space between them.

21. ( d) Lower Cortex: It is the lowermost layer of the thallus . This layer is composed of compactly arranged hyphae, which may arrange perpendicular or parallel to the surface of the thallus . Some of the hyphae in the lower surface may extend downwards and penetrate the substratum which help in anchorage, known as rhizines . The internal structure of Usnea , a fruticose lichen, shows different types of orientation. Being cylindrical in cross-section, the layers from outside are cortex, medulla (composed of algal cell and fungal mycelium) and central chondroid axis (composed of compactly arranged fungal mycelia).

22. Sea sponges As shown in this coral reef, sea sponges are an important part of coral reef  ecosystems . As members of an ecosystem, like all  animals , they have an important role in their environment. Photosynthesising microorganisms (often green algae or cyanobacteria )can live on and in sponges forming a mutualistic relationship. Sea Sponges- Filter Feeders

23. Ecology of Sponges Sponges are strictly  aquatic  organisms as you may expect considering that their feeding mechanism is based on the filtration of inflowing  water  from  the environment . Almost all sponge  species  are found in the ocean with the exception of about 150 freshwater species. Sponges can be found throughout the  marine  world, in both cold polar waters and warmer  tropical  regions and at both great depths and in shallower areas. Because sponges generally require a  solid  support for attachment, they are often found in rocky marine areas or on the ocean floor . Sponges are critical components of the  ecosystems  of coral reefs, where they provide shelter for a variety of organisms including shrimp, crabs, and  algae . They are also a source of food for many sponge-eating  fish  species. Many sponge species form large colonies or aggregates of individual organisms.

24. Ultra structure of sponge

25. Symbiosis of Sponges Sponges form  symbiotic relationships  with a variety of microorganisms, including  bacteria  and  algae . A symbiotic relationship between organisms is a close ecological association between two  species  that may be mutually beneficial or may benefit one partner at the expense of the other. In the case of sponges, the benefit for the microorganisms may be a sheltered surface area on which to grow, and the benefit for the sponge may be nutrients provided by the metabolism of the microorganisms. Although most sponges are filter-feeders that consume microorganisms or obtain nutrients by  symbiosis , there is one family of sponges that is actually carnivorous . These sponges capture and consume small  crustaceans  using their spicules. One such sponge,  Chondrocladia lyra ,  has been called the " harp sponge ," because its structure resembles a harp or lyre turned on its side.

26. Giant Barrel Sponges Chondrocladia lyra

27. Amoebocytes in symbiosis with sponges AMOEBOCYTES in sponges are  motile, amoeba-like cells that transport nutrients between cells, transform into other cell types, and enable sexual reproduction . In other words, amoebocytes function to help sponges eat, grow, and reproduce. In order to build a structure it requires building materials and a binder to hold the materials together. Amoebocytes are involved in both. Sclerocytes  are specialized amoebocytes that secrete biosilica (silica dioxide), which is a substance that binds calcium. Biolsilica and calcium provide strength and rigidity to the spicule. Archaeocytes  secrete  galectin , which acts as a cellular glue and holds all the parts of the spicule together. Lophocytes  secrete  collagen fibrils , which allow the endoskeleton to be flexible and pliable. Collagen fibrils are not a part of spicules, but they give the mesohyl its gel-like consistency.

28. Pinacocytes  are thin flat cells that line the outer surface of a sponge. These cells can contract, which can change the shape of some sponges. There are specialized pinacocytes called  porocytes . Porocytes regulate water circulation, this is done by acting as pathways for water to travel through the sponge wall. Mesenchyme   cells move freely in the  mesohyl   (jelly like layer below pinacocytes ) and are useful in reproduction, secreting skeletal elements, transporting and storing food, and forming contractile rings around openings in the sponge wall. Choanocytes  lay under the mesohyl , they also line the inner chamber(s) of the sponge. These cells are flagellated and they posses a collar-like ring of microvilli that surrounds a flagellum. The fact that sponges have choanocytes can explain that choanoflagellates ,  a group of protists , are  related .

29. Choanocytes Choanocytes  (not an amoebocyte but a different type of cell) are flagellated cells that capture and digest food in sponges. These cells use their flagella to create a current, bringing food into the pores of the sponge, capturing them, and packaging them into food vacuoles. Once inside a food vacuole, amoebocytes pick up and carry the food to other cells in the sponge. Choanocytes Amoebocyte Given to Sponges as nutrition

30. Asexual REPRODUCTION Fission Fragmentation Budding

31. BUDDING https://youtu.be/Bt7q6eFeHRk

32. sexual REPRODUCTION

33. For reference notes: https ://www.britannica.com/animal/sponge-animal/Natural-history