The Biology of Coral: History
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Subjects: Biology
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Galana Siro
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Atanas Pipite
,
Ketan Christi
,
Sathiyaraj Srinivasan
,
Ramesh Subramani

Coral reefs are portrayed as tropical underwater forests, profusely rich in biodiversity and has immense ecological and economical importance. Apart from other marine calcifying organisms, scleractinian corals are the major biogenic contributor of complex bicarbonate structures. Corals are built from a collective group of tiny transparent organisms known as coral polyps. The increasing impact of natural and anthropogenic factors such as variation in temperature, salinity, cyclones, pollution and so forth have exert direct or direct effects on the coral reef, engendering the phenomenon of coral bleaching with severe outcomes. Coral microbiome comprises of diverse microorganisms including symbiotic algae, which play a determining role in coral physiology, immunity and responsiveness. Further, corals are equipped with sophisticate chemical and biological warfare that is used to their advantage in unfavorable conditions. 

  • Scleractinian coral
  • Coral
  • Coral defense
  • Coral symbiosis

1. Coral Morphology and Distribution

Coral reefs are the main, unique and striking constituent of a marine ecosystem described as tropical underwater forests. They represent the most crucial bioconstruction on this planet and are described by both biological and geological entities [1]. Unlike tropical rainforests, coral reefs have higher biodiversity despite occupying only 0.1% of the Earth’s ocean surface. They are largely confined to tropical waters and play a central role in hosting a diverse marine flora and fauna, which is represented by more than 2 million species of marine life [2]. Coral reefs have an unusually complex structure, comprising various species of corals built by thousands of tiny, transparent nocturnal animals called coral polyps [3]. Corals are known for the simplicity of their body structure. A basic coral polyp has a sac-like body and mouth surrounded by a series of retractable tentacles that are concentrated with stinging cells called nematocysts. These stinging tentacles are used on purpose for self-defense or to capture food. Coral polyps have two distinct tissue layers: the gastrodermis and the epidermis, which are mostly separated by a simple gelatinous supporting matrix known as the mesoglea. These cell layers develop from the two germ layers, the endoderm and ectoderm, respectively, during the coral’s life cycle [4]. Corals are sessile invertebrates belonging to the class of Anthozoa within the phylum Cnidaria. They are present in a large numbers of species as well as in an abundance and live in colonies or solitaries and reproduce by sexual and asexual reproduction. Their sexual reproduction involves spawning and broadcast brooding, while asexual reproduction involves budding and fragmentation. There are two types of corals based on their physical texture, such as hard coral and soft coral. Hard corals, also known as scleractinian corals or stony corals, have an outer skeleton made of calcium carbonate (CaCO3) arranged in a crystalline form called aragonite. The stony corals that form the reefs are called hermatypic and they grow by shedding CaCO3 skeletons, a vital part of reef formation and structure. Hermatypic corals are the essential calcifying organisms that contribute to the biodiversity of coral reefs. On the other hand, soft or ahermatypic corals have small spikes of calcium carbonate embedded in their bodies. Unfortunately, they do not secrete CaCO3, so do not make a significant contribution to reef formation [3]. Since the ocean is the primary sink for carbon dioxide (CO2), it plays an important role in removing CO2 from the atmosphere, thereby regulating the earth’s climate and marine health [5]. The ocean, being an efficient CO2 absorber, allows trapped carbon dioxide molecules to be incorporated with calcium ions to form calcium carbonate in a process known as calcification via calcifying organisms. Corals are the main factory for calcium carbonate precipitates as well as other marine organisms, namely, molluscs, calcareous algae, foraminifera, sponges and echinoderms [6]. Corals depend on specific physiological and environmental requirements to support their survival. They live at temperatures ranging from 18 to 30 °C and have a salinity of 32 to 40% [7]. Corals are widely distributed in all of the world’s oceans but are limited between the latitudes of the Tropic of Cancer and the Tropic of Capricorn [3]. Their distribution is mainly determined by biotic factors (corallivores, coral’s intra and interspecific competitions; their reproductive and recovery capacity; and their ability to withstand environmental stress) and abiotic (light, water temperature, pH, salinity, turbidity and depth) [8]. Changing these parameters can have a dramatic impact on the health and survival of corals. Coral communities are found in shallow and deep ocean ecosystems.

2. Coral Significance and Bleaching

Coral reefs are among the most productive ecosystems that are ecologically and economically important to the livelihoods of millions of people and to marine life as a whole. The complex structures of coral reefs are an excellent habitat for marine organisms, as they provide a shelter, nursery and greater retention of nutrients as a food source for most marine species. The population as a whole depends on coral reefs: as a natural shoreline buffer against storm surges and wave erosion, and for protein needs, medicinal purposes and a lucrative source of income through the tourism and fishing industries [7][9][10]. Globally, the sustainability of corals is highly threatened by well-characterized phenomena, including natural and anthropogenic factors. Coral bleaching is a phenomenon that occurs due to a disruption of the symbiosis between corals and symbiotic algae. It is described as a loss of coral coloration when a coral polyp expels its zooxanthellae or there is a reduction in photosynthetic pigment in the zooxanthellae due to stressful triggers such as a variation in salinity, solar radiation, temperature, infection, cyclones, pollution and destructive human practices. Under these stressful conditions, certain functions of the coral are compromised, such as its capacity for growth and fecundity. This could then have lethal or sublethal consequences on the overall coral performance [11][12][13]. The recurrence and severity of these stressful triggers increases dramatically over time around the world. However, most corals have obtained a certain degree of tolerance against coral bleaching due to certain functional mechanisms. For instance, coral’s heat stress tolerance can be supported by heat shock proteins, the enhancement of antioxidant defense, photoprotective molecules (green fluorescent protein-like pigment and mycosporine-like amino acids) and host thermotolerant symbionts (uptake from the environment or reshuffle of the existing symbionts) [14].

3. Coral Defense Mechanism

Corals have a series of well-established and effective defence mechanisms that protect themselves from sedimentary organisms, sediments, pathogens and other potential threats. One of these defence mechanisms involves the production and release of mucus. The exudation of mucus films as a defence strategy coincides with the ability of corals to trap particles using secreted mucus. Coral mucus is synthesized by phlegm and its composition varies between coral species. Mucus is derived from photosynthesis products produced by symbiotic algae and compounds obtained via heterotrophic feeding. It is composed mainly of carbohydrates and mixtures of lipids, polysaccharides and glycoproteins (mucins). Several studies have characterized coral mucus as a nutrient-rich environment conducive to microbial growth and a potential source of energy for other marine organisms [15][16][17]. Mucus production has a number of protective measures. To begin with, beneficial mucus microbes act as a protective barrier against invading species via resource competition or the secretion of antimicrobial molecules [18][19]. In addition, mucus protects corals against solar radiation by generating proteins or pigments that absorb ultraviolet light [20]. Another relevant strategy is that when corals are exposed to sedimentation or air at low tide, mucus secretion prevents desiccation and suffocation [17]. Although corals are simple organisms, they have a complex immune system, which includes mechanisms capable of healing them from injuries and also the production of melanin as a defensive maneuver to get rid of or confine harmful bacteria [21]. Moreover, a major defence strategy used by corals is the digestion of phagocytic cells. Microscopic organisms are fought or degraded by enzymes and oxygen free radicals [22]. Defence levels vary among different families, genera and species of corals.

4. Coral Symbiosis

Corals harbor a large population of various microorganisms including bacteria, fungi, viruses and archaea as well as its symbiotic algae [23][24]. The coral symbionts offer significant contributions to the coral’s physiology, development, immunity and responses to fluctuating environmental conditions [25]. Recent culture-independent coral studies have shown that the microbial community inhabiting corals is highly diverse, abundant and rich in novel microbial species. Collectively, corals and their symbionts are characterized as a holobiont or coral microbiome. A coral microbiome is found in distinct parts of the coral, including the surface mucus layer, tissues and skeleton. Each coral compartment differs in its richness and microbial diversity [24][26][27]. For example, the microbial population of coral tissues is much more stable than that of surface mucus, which is constantly renewed. Environmental stressors can alter a coral’s microbiome, thereby compromising its immunity and allowing opportunistic microbes to thrive, which in turn drives coral motility [28][29]. A striking example of coral symbiosis includes the mutualistic relationship of corals with dinoflagellates zooxanthellae, symbiotic algal cells belonging to the genus Symbiodinium that inhabit their gastrodermis tissue, which enhances their growth and survival [30][31]. Corals have adapted to different mechanisms and strategies to take up Symbiodinium such as through their parents [32] or from adjacent seawater [33], where the dinoflagellates are constrained by a series of algal membranes embedded in an outermost membrane of coral origin; the entire membrane-bound organelle is called a symbiosome [34]. Vertical transmission of Symbiodinium species by the corals is correlated with a higher specificity in endosymbiosis union than horizontal acquisition [35].
Molecular analysis of the genus Symbiodinium has evidently revealed its diversity [30][36][37]. It is estimated that every square centimeter of the coral’s surface is filled with millions of these single-celled algae [30]. At different ocean depths, a single species of coral can be dominated by a single Symbiodinium type or can host many types of Symbiodinium in its anatomy [33]. There are certain physiological traits that differ among Symbiodinium types and their coral hosts, such as the thermal tolerance, growth rate, host infectivity, photophysiology and translocation of inorganic compounds [33][38].
Symbiodinium species can utilize light energy very efficiently [39]. These photosynthetic algae produce unique protein pigments that capture different wavelengths of light and emit colorful, vibrant colors displayed by diverse coral communities. In addition to this, some corals are biofluorescent under appropriate conditions due to fluorescent proteins [40]. There are corals that have zooxanthellae and others that do not. Corals without zooxanthellae exist in all the oceans of the world and depend entirely on zooplankton or particles apprehended by their tentacles for food. As these corals have no photosynthetic requirements, their growth is slower than those with zooxanthellae. Corals of this trait are able to live in shallow to deep water where there is no light [41]. In contrast, reef-building corals acquire their metabolic needs from microscopic organisms or particles and their symbiosis. Their dual dietary character is widely recognized as autotrophic and heterotrophic. Apparently, endosymbiotic dinoflagellates provide adequate nutrition in the form of organic molecules including glucose, fatty acids, glycerol and amino acids. In return, the corals provide protection, carbon dioxide, nitrogen and phosphorus to the symbiont for photosynthesis and cellular respiration [42][43]. A very vital element synthesized by symbiotic algae is the vast oxygen capacity provided to corals and its associated prokaryotes for efficient respiration [24]. The autotrophic supply of zooxanthellae is very crucial for the survival of corals, especially in nutrient-limiting ecosystems.

This entry is adapted from the peer-reviewed paper 10.3390/microorganisms10071349

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