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Many medical problems are more or less specific to the marine environment. Consider jellyfish, fire coral, sea snakes and other venomous marine creatures (vertebrates and invertebrates), decompression sickness, electrogenic and traumatogenic fishes such as electric eels, certain sharks and stone fish, the problems caused by some algae and intoxication resulting from eating poisonous fish. In the following paragraphs we will discuss some specific disorders caused by biotoxins.
The seas and oceans contain very large numbers of plankton. Plankton consists of organisms which drift passively with the ocean currents (Gr. planktos: wandering, floating) and includes several species of unicellular and multicellular organisms. The more animal-like ones are called zooplankton, those which have more plant-like features are known as phytoplankton. Some have characteristics from both: animal-like (active movement, eating) and plant-like (photosynthesis). There are various micro-organisms in the phytoplankton which produce toxins. Toxins originating from algae are known as phycotoxins. Plankton also contains some larger animals, a few centimetres long, such as krill (Euphausia superba).
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Phytoplankton: important elements
Intoxication may result from the blooms of certain diatoms. These small algae belong to the Baccilariophyta and are related to the golden algae (Chrysophyta). They are round, square or triangular ("Centrales" with radial symmetry) or elliptical, spool or feather-shaped ("Pennales" with bilateral symmetry). They are enclosed in two hard shells (frustules) which fit into each other like a box with a lid. This is where their name comes from (Gr. diatomos = cut in two). These shells contain silica, which is often arranged as opal (SiO2.nH2O) in beautiful symmetrical patterns. This gives them a brocade-like outer appearance or makes them look like small art nouveau jewels. During asexual reproduction, each daughter cell keeps half of the box and makes a new half to go with it. The old half is always the larger, the newly formed is always the smaller. Consequently one daughter cell will always be smaller than the parent cell. In some varieties the shell is expandable and the original size is restored. In other varieties, individuals which have achieved 30% of the maximum diameter begin sexual reproduction with meiosis and the formation of 4 sperm cells or an oocyte. The zygote is formed after fertilisation and is known as an auxospore. After mitosis the original morphology and size are resumed. The adult cells have no flagella (although the male gametes do). Sediments which contain many diatoms with tiny siliceous skeletons (diatomaceous earth, Kieselguhr) are quite often used for technical purposes, such as polishing, filtration, absorption, and so on. Pseudonitzschia pungens, P. multiseries, P. australis and P. pseudodelicatissima (called by some the genus Nitzschia) produce domoic acid, a toxic amino acid. The role this substance plays in the metabolism of the bacterium itself is still unclear.
Most Dinophyta bear flagella and are called dinoflagellates. Dinoflagellates are also known to botanists as Pyrrophyta while for zoologists they belong to the Mastigophora. They do indeed have characteristics of both plants and animals. Some 2100 species are known, including approximately 20 which are toxic. Most live in the sea, some live in freshwater. They form an important part of the plankton (nanoplankton, organisms measuring from 5-20 µm). They take their name from the fact that they slip through the fine mesh of a standard plankton net (Gr. nanos = dwarf). Picoplankton includes organisms smaller than 5 µm. Most Dinophyta are unicellular and have two flagella, but some form colonies and others have no flagella. Many species exhibit bioluminescence and are responsible for the beautiful, fine soft glow which is often seen in the waves of the ocean on a moonless night. Certain dinoflagellates are pathogens and are responsible for ciguatera, neurotoxic and paralytic poisoning by shellfish and toxic diarrhoea.
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Dinoflagellates are eukaryotes. They have a nucleus with a nuclear membrane. However, their nucleus and DNA exhibit a very characteristic organisation which is not found elsewhere in the animal or plant kingdoms. The DNA fibrils are very narrow and do not contain histones. During mitosis there is no prophase, metaphase, anaphase or telophase. The chromatin is always condensed and is active in this form (in other organisms condensed DNA is always inactive). Reproduction is via binary division, zoospores or gametes with formation of a zygote. The morphologial forms of dinoflagellates vary widely. Some dinoflagellates are naked, but many others possess armour (theca). This armour contains cellulose and sometimes some silica. The armour consists of two or more small plates which have a particular morphology and position. The armour can be quite complex and each species has its own characteristic shape, with for example apical pore, epicone (upper armour) and hypocone (lower armour). These morphological details can be expressed in a thecal formula. The armour plates lie within the plasma membrane and not outside the cell wall as in the case of many other algae. This armour has two grooves which are at right angles to each other. One flagellum lies like a girdle around the equator (cingulum), the other lies in the longitudinal meridian groove (the sulcus). This flagellum can protrude freely. By moving their flagella the organisms turn like spinning tops. The zygotes of many dinoflagellates form hard, chemically inert and resistant cysts (histrichospheres), which permit the organism to survive unfavourable periods. The morphology of the cyst varies greatly from that of the vegetative form. This is probably why in algae which are only known as fossils, cysts and vegetative forms of the same species are often included in different taxonomic groups, because the relationship has not been recognised.
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Some species of dinoflagellates are photo-autotrophics and can carry out photosynthesis. These species possess chloroplasts with chlorophyl a and c. Other species are heterotrophic and can absorb small particles or other cells. The pigments they contain are carotenes and various xanthines, including peridinin, a unique xanthine pigment specific to this group of organisms. Photosynthesising dinoflagellates accumulate starch as a food reserve. Some species live in close symbiosis with other organisms, such as sea anemones, some jellyfish, sponges, tunicates, octopuses, worms and molluscs, including the giant Tridacna shell. Some dinoflagellates may themselves possess endosymbionts. Certain Ostreopsis species have bacteria belonging to the genera Pseudomonas, Alteromonas, Xanthomonas and Agriobacterium as endosymbionts. Their potential role in the production of toxins needs to be studied further. Some dinoflagellates have an eye spot with carotinoids as pigment. The sedentary Erythropsidinium pavillardii even has a complex ocellus with a tiny lens.
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Many coral species contain symbionts and are called hermatypic corals. Polyps from reef-building corals contain countless golden-coloured zooxanthellae. These are dinoflagellates without a shell which perform photosynthesis and provide carbon in the form of glycerol to the host polyp. In return they receive shelter and nitrogen-containing substances, and also CO2. By means of the latter they help the coral to precipitate chalk (CaCO3). Some corals contain up to 30,000 zooxanthellae per mm3. Other species of coral contain zoochlorellae, symbiotic unicellular green algae, for the same purpose. Since the zooxanthellae need light, the tropical reef-building corals only grow in very pure, clear shallow water. In many coral reefs at present coral bleaching is occurring. This disease is characterised by the expulsion of zooxanthellae, after which the coral dies. Ahermatypic corals which do not contain symbionts are found from the tropics to the polar oceans, even at great depths in the darkness. Since ahermatypic corals do not contain zooxanthellae, they are not dependant upon sunlight (no photosynthesis).
Algal growth is of course a natural process. Excessive growth on the other hand, may have unfavourable consequences for humans and for the environment. In certain situations the organisms may multiply unhindered and give rise to algal bloom or "red tides". This happens not only in tropical regions, but also for example, in American coastal waters or in colder seas, e.g. the Baltic. The following circumstances promote algal blooms: (1) a calm sea, (2) increased temperature, (3) low salt content, due for example to recent rains, (4) a lot of sunshine, (5) increased nitrogen and phosphorous content (run-off from fields, due to both animal dung and artificial fertiliser), (6) increased iron content (iron is often a bottleneck element in the growth of algae) as when a great deal of dust and sand containing iron is blown from the land by winds passing over dry regions, (7) sometimes also due to downwelling (a downwards sea current) or upwelling of sediments after storms. The algal bloom may take days or weeks. When the organisms die, they decompose and thereby reduce oxygen levels, so that the water becomes anoxic. Sometimes large amounts of foam can be seen on the sea in the surf, which originates from plant material derived from dead algae. This is of course, together with the stench, disturbing for regions which depend on tourism. The seawater may exhibit red, green, yellow, purple or other tints, depending on the pigments in the dominant micro-organism. Certain algae may cause physical injury to fish, e.g. damage their gills, by both secretion of mucus (Thalassiosira sp., Phaeocystis pouchetii) and by sharp protuberances which penetrate the gills (Chaetocerus convolutus). Some dinoflagellates produce toxins. These algal blooms regularly cause massive mortality in fish, seabirds and sea mammals via accumulation of poison in the food chain. In 1987 approximately 50% of the dolphins in the west of the North Atlantic Ocean were killed by poisonous Gymnodinium breve. Some species of algae such as Heterosigma akashiwo and Prymnesium parvum secrete toxins directly into the water, which kill fish. Fish may absorb these toxins via their gills, leading to haemolysis. If toxic species of dinoflagellates are abundant, crayfish and crabs as well as mussels and oysters will become temporarily unfit for human consumption. These animals are resistant to a number of toxins, but they do concentrate them in their bodies. In 1987 after a bloom of Rhinosolenia chunii, the mussels could not be sold for 7 months. Some algae and diatoms give crustaceans a bitter taste. The dinoflagellate Hematodinium causes a bad taste in crabs in Alaska (bitter crab disease).
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Various types of algal bloom:
Eutrophy is in fact rather a misnomer since it means normal nutrition. It seems strange, but the waters in a crystal clear mountain stream or the clarity of a coral sea are due to the very low concentrations of elementary nutrients, so that very low numbers of algae are present (desert-like). If the concentrations of nutrients increase, algal growth also increases, and the water will become cloudy. Eutrophication means excessive numbers of plants and algae appear, which occurs when the level of nutrients in the water (nitrogen, phosphorus and trace elements such as iron) no longer limits their growth. Sometimes heavy rainfall is responsible for the introduction of large amounts of rich silt and organic material, but sometimes this is due to human factors (over-manuring, soaps containing phosphate, lack of purification of waste water). In view of the serious consequences to the natural fauna and flora, attempts should be made to counteract eutrophication.
Algal blooms can be monitored in several complementary ways. There are a number of ways of measuring the primary production of plankton. One of these is using a spectrophotometer, an apparatus which can measure the light intensity at different wavelengths. By using a spectrophotometer it is possible to determine how much light the algae absorb. At present there are satellites which monitor the ocean specifically at a number of wavelengths (evaluation of chlorophyl-content of seawater). Another way of measuring primary production is by monitoring the oxygen concentration in the water. From these measurements, the course of primary production can be determined during several years. It is also possible to determine, for example, whether primary production is equally distributed across the oceans, by comparing measurements from coastal regions with those in mid-ocean.
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Note: Eutrophication of fresh water
Fresh waters with a low pH are often oligotrophic (poor in nutrients such as nitrates and phosphates). Waters with pHs of 7-9, on the other hand, are often eutrophic (more or less rich in nitrates and phosphates), allowing a greater biomass of algae to develop. In addition there are dystrophic waters, such as found in very acid bog pools, where the water is poor in nutrients and coloured brown by dissolved peaty humic material. These are very general categories disguising much other important variation in water quality. The Danish hydrobiologist Nygaard developed a formula, which makes it possible to convey an overall impression of the trophic status of a fresh waterbody. His formula describes t (t for trophic). It goes as follows : t = (Nr of species of Cyanobacteria + centric diatoms + Chlorococcales / Nr of species of Desmidiales).
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The algae of the Orders of the Chlorococcales and the Desmidiales belong to the Phylum Chlorophyta. In some classification schemes the diatoms belong to the Class Bacillariophyceae, Phylum Heterokontophyta. Some of the Desmidiales algae ("desmids") which thrive in such inhospitable places as cold peat bogs, are special in the sense that they concentrate the chemical element barium in prominent vacuoles in their bodies, storing it as heavy barite crystals (barium sulphate). Maybe they use those crystals as part of gravity sensors to orient themselves.
