Plankton Diversity A. Phytoplankton Diatoms Class Bacillariophyceae. Phylum Chrysophyta. • Sometimes called the golden algae, because their characteristic yellow-brown pigments mask their green chlorophyll. • Dominate the phytoplankton in waters from the temperate to the polar zones. • They either occur as single cells or form chains. • They range in size from 2 µm to 1 mm. • Each cell is encased in a transparent silica shell of two valves that fit together much like a pillbox. The shell may be covered with spines, or it may be ornamented with a complex series of pores and ridges. The pores are the only connection between the cell and the external environment. • Planktonic diatoms are usually radially symmetrical (shaped like a pinwheel) and are known as centric diatoms, in contrast to pinnate diatoms, which are bilaterally symmetrical and usually live in sediments (i.e. benthic). • Diatoms reproduce very rapidly by binary cell division, with one valve going to each daughter cell and serving as the larger valve. A smaller valve is then formed. After several successive generations of cell division, cell size usually decreases to a lower threshold, when gametes are often formed. The smaller diatoms may also form an auxospore, which enlarges and casts off the valves, forms new large valves, and then undergoes asexual divisions. Some diatom species can form asexual resting spores with thickened frustle, which settle to the seabed and may be regenerated later as a planktonic form. 88 • Because of their asexual mode of reproduction, diatoms can increase in population size rapidly, when their populations discolor the water, it is known as a bloom. • Diatoms are most important as primary producers. Those that are not consumed by herbivores eventually die and sink to the ocean floor. • The cell walls of diatoms that sink to greater depths build up siliceous sediments called the siliceous ooze. Ooze: If fine sediments from the deep sea are more than 30% fine biogenous sediment by weight, then the sediment is known as an ooze. Dinoflagellates Class Dinophyceae, Phylum Pyrrophyta or Dinoflagellata • Exist at lower light levels than diatoms, because they can both photosynthesize like a plant and ingest organic materials like animals (i.e. mixotrophic). • Some species are parasitic on or live within zooplankton. • Mostly unicellular forms, but some form chains. • They range in size range from 20-200 µm. • They have two flagellae, the transverse flagellum is located in a groove (the girdle) that divides the cell into two subequal parts. The other flagellum is oriented perpen- dicularly to the transverse flagellum and extends toward the posterior. The two flagellae are resulted in the characteristic spinning movement of dinoflagellates. • The organism is generally covered with a series of contiguous cellulose plates or theca. Naked dinoflagellates (without theca) are also known. • Dinoflagellates reproduce asexually by binary fission and have the capacity to reproduce up to several times per day. • Some dinoflagellates are called fire algae, because they glow with bioluminescence at night. Dinoflagellate Blooms (Harmful Algal Bloom) and Red Tides • Red tide is the phenomenon where certain species of dinoflagellate increase rapidly of sufficient magnitude (numbering >108 cells l-1) to color the sea- water a dull red-brown. • The origin of red tides, like many other sudden blooms of phytoplankton, is an incompletely explained phenomenon. Red tides are often associated with 89 sudden influxes of nutrients from land sources into the sea. It is also associated with storms, which may mix nutrients into the surface layer of the ocean and remobilize populations of cysts buried in the bottom sediment, setting the stage for red tides. Along some coasts, the most common times for phytoplankton blooms are in the spring, after winter storms have mixed nutrients into the surface layer of the ocean and when water with warmer temperature, the daylight is longer and the sun light is stronger. toxins. Toxic species are (ﻗﻮيّ) Some dinoflagellate species produce potent • generally not poisonous to shellfish feeding on a bloom, but the toxins are concentrated in the tissue of the shellfish and produce PSP, NSP, or DSP in humans who eat them. Toxins can be transferred through the food web where they affect and even kill the higher forms of life such as fish, birds, marine mammals, and even humans. a) Paralytic Shellfish Poisoning (PSP): Algal cells contain highly lethal saxitoxin; accumulation of toxin in clams, mussel, scallops and fish lead to poisoning of people who eat them. In cases of severe poisoning, muscle paralysis and respiratory failure occur, and death may occur if the breathing centers are affected. The toxin is not affected by heat, so cooking the shellfish does not neutralize the poison. b) Neurotoxic Shellfish Poisoning (NSP): algal toxin is brevetoxin (Gymnodinium breve); neurological & gastrointestinal symptoms; aerosols can produce asthma; no deaths reported c) Diarrhetic Shellfish Poisoning (DSP): Okadaic acid, Dinophysis sp.; min after consumption 30 ( ﺗَ ﺸَ ﻨﱠ ﺞَ gastrointestinal symptoms (diarrhea, cramps of toxic shellfish, not lethal, recovery ca. 3 days d) Ciguatera Fish Poisoning (CFP): Ciguatoxin, Symptoms of ciguatera poisoning include headache, nausea, vomiting, abdominal cramps, and reduced blood pressure. Ciguatera poisoning is rarely fatal but in severe cases convulsions, muscular paralysis, and death may occur. Symptoms usually clear in 1 to 4 weeks. Coccolithophores Phylum Chrysophyta • Coccolithophores are unicellular and usually nannoplanktonic (mostly <20 µm). • They are nearly spherical and are covered with a series of calcium carbonate (calcite) plates, or coccoliths. • Like many phytoplankton, coccolithophorids can form immense blooms which settle down to the ocean floor producing chalk deposits called calcareous oozes or more precisely coccolithophore ooze, including chalk used for classroom chalkboards. • Reproduce by simple cell division, rarely sexual reproduction. 90 Silicoflagellates Phylum Chrysophyta • Silicoflagellates are unicellular and biflagellate. • They have numerous chloroplasts and an internal skeleton of silicon dioxide (SiO2) scales or framework. • Their bloom can block sunlight from reaching underlying parts of the ocean. Cyanobacteria Class Cyanophyceae • Also called blue-green algae, although they are prokaryotic bacteria, not true algae. • Occur in and may dominate nearshore waters of restricted circulation, as well as brackish water. • Tiny (roughly 1 µm) unicellular forms have been found ubiquitously throughout the ocean and may be the food of smaller zooplankton. • Cyanobacteria can undergo nitrogen fixation, in which gaseous atmospheric nitrogen is converted to NH4+, or ammonium ion, and is then available for incorporation into amino acids and proteins. Fig. The colonial planktonic blue-green cyanobacterium Trichodesmium thiebautii. Green Algae Class Chlorophyceae Phylum Chlorophyta • The true green algae are rarely marine but can dominate the phytoplankton of enclosed estuaries or enclosed lagoons, especially in late summer and fall. • They can be flagellated with two apical or subapical flagella or nonmotile. phytoplankton blooms associated with (ﻣﺰﻋﺞ) Several species cause nuisance • coastal pollution. 91 Cryptomonad Flagellates Class Cryptophyceae Phylum Cryptophyta • Cryptomonad are a small group of flagellates. They are common in freshwater, and also occur in marine and brackish habitats. • They are a group of tiny flagellates, only around 10-20 µm in size and flattened in shape, with an anterior groove or pocket. • There are typically two slightly unequal flagella inserting nearly parallel next to the pocket. B. Zooplankton Nutritional modes in zooplankton: Herbivores: feed primarily on phytoplankton Carnivores: feed primarily on other zooplankton (animals) Detrivores: feed primarily on dead organic matter (detritus) Omnivores: feed on mixed diet of plants and animals and detritus Crustacean Zooplankton Phylum Arthropoda Fig. Some zooplankton: (a) krill, (b) the cladoceran Podon, (c) a foraminiferan, (d) a radiolarian, (e) a comb jelly, (f) an arrow worm, (g) a scyphozoan jellyfish, and (h) a pleustonic siphonophore, the Portuguese man-of-war Physalia physalis. 92 ﻣﺠﺪاﻓﻴﺎت ا ﻷَ رْ ﺟُ ﻞٌ Copepods • Ö The most abundant zooplankton in all the oceans and marginal seas, “called insects of the sea“; Ö The copepods are the largest and most diversified group of crustaceans, account for 70% of crustaceans. Ö They are either Herbivorous, carnivorous or omnivorous species; Ö They range in length from less than 1 mm to a few millimeters. Ö The calanoid (Suborder Calanoida) are the most of marine planktonic species. They swim by means of rhythmic strokes of the first pair of anten- nae and the five posterior pairs of thoracic appendages. Calanoid copepods .antenna (آَﺸْﻜَﺶَ) are easily recognized by their longer-than-body frilly Ö The harpacticoids usually have benthic adults (epibenthic), but the larvae of some species may dominate the estuarine zooplankton. Their antennae are much shorter than their body length and they look like little torpedoes. Ö Still the cyclopoid species which are mainly freshwater. Cyclopoids are both planktonic and epibenthic. The planktonic copepod Euchaeta norvegica • Krill Family Euphausiidae, Ö Krill are shrimp like creatures ranging from 15 mm up to 5 cm long. Ö The name Krill comes from the Norwegian word krill meaning "young fry of fish". Ö They dominate