The life cycle of dinoflagellates is multi-staged. Evitt (1985) recognised a six stage cycle in peridiniales dinoflagellates.
- During periods favoured by rapid growth and population expansion, vegetative fission dominates yielding motile haploid schizonts.
- At an unknown trigger, the schizonts act as gametes and pair up and fuse to form diploid zygotes. One or more theca may be lost in the process.
- The diploid zygote constructs a new theca and resumes its motility as a planozygote.
- In several species the zygote theca becomes much thicker and considerably larger than the vegetative theca. Its outline becomes less regular, the protoplast becomes granular and reddish bodies are visible within it, the activity level decreases and after as many as fifteen days the flagella are lost, the cell is then termed a hypnozygote. The protoplast shrinks pulling away from the theca and eventually one or two membranes form a new cyst wall. The thecal plates then break apart or are decayed and the completed cyst is exposed.
- The hypnozygote or resting cyst then behaves as a sedimentary particle and settles to the sea floor.
Following a period of obligate dormancy the protoplast excysts (typically through an archaeopyle), and the cycle closes as meiotic division again produces haploid, thecate, motile cells. In laboratories sexual reproduction may be induced by nutrient, temperature or light reduction. In nature sexual reproduction is known to occur in late summer and autumn and in the late stages of blooms.
Red Tides
Red tides are conditions when a dinoflagellate population increases to huge numbers. This "bloom" may be caused by nutrient and hydrographic conditions, although the environmental conditions which result in red tides are not completely understood. The water is discolored red or brown due to the presence of dinoflagellate cells numbering up to 20 million cells per liter. Red tides are composed primarily of one species of dinoflagellate which has been rapidly growing.
Some red tides are luminescent; check out the bioluminescence stimulated in breaking waves during the May 1997 Gonyaulax polyedra red tide in San Diego. A synopsis of the putative mechanisms responsible for this red tides is kindly provided by Prof. Wolfgang Burger, Interim Director of SIO:
"My understanding is this: if, after an upwelling or mixing event (storm?) there is plenty of sunshine, which warms the water and makes for stable stratification, conditions are right for a bloom. If, in addition, Gonyaulax cysts waiting around on the bottom have been stirred up into the water, and have by some means (change in temperature? light?) detected that the time is good for popping open, sufficient seeds are released to start the process. Rapid reproduction ensues (by cell division; these are unicellular organisms) and crowds out everything else, by taking away the light (the water was brown!) and perhaps also by chemical means."
Some both not all red tides are toxic. In toxic red tides, the dinoflagellates produce a chemical which acts as a neurotoxin in other animals. When the dinoflagellates are ingested by shellfish, for example, the chemicals accumulate in the shellfish tissue in high enough levels to cause serious neurological affects in birds, animals, or people which ingest the shellfish.
The are several types of neurotoxins produced by dinoflagellates. These chemicals may affect nerve action by interfering with the movement of ions across cell membranes, thus affecting muscle activity. The toxin saxitoxin, produced by Protogonyaulax catanella off the west coast of North America, and Gessnerium monilatum off the east coast, accumulates in shellfish. Eating contaminated shellfish causes paralytic shellfish poisoning (PSP), while ciguatera is caused by eating contaminated fish. The worst cases of PSP result in respiratory failure and death within 12 hours. Another toxin which accumulates in shellfish is brevitoxin, produced by the dinoflagellate Ptychodiscus brevis. A toxin produced by the dinoflagellate Dinophysis causes diarrhetic shellfish poisoning (DSP), which results in digestive upset but which is not fatal.
What is Bioluminescence?
Bioluminescence is the production of light by living organisms through an internal chemical reaction. Bioluminescence is found among some insects, mollusks, fish, ctenophores (comb jellies) and annelid worms. Some of the most dramatic light-producing creatures are found in marine environments. Bioluminescence should not be confused with fluorescence, in which light from an outside source is stored and re-emitted. The production of light in bioluminescent organisms results from the conversion of chemical energy to light energy. Organisms use their luminescence in different ways; dinoflagellates, a group of marine algae, produce light only when disturbed, while other animals use their light to communicate or find prey.
There are a number of natural and human-made processes that create light. Chemiluminescence is a broader set of light producing chemical reactions of which bioluminescence is just one. Certain types of chemicals when mixed together produce energy. This energy 'excites' other particles which vibrate and generate light. A chemiluminescent product known to many kayakers are Cylume light sticks. A thin glass vial containing hydrogen peroxide sits within a plastic tube containing a chemical called an ester and a fluorescent dye. When the vial is broken the hydrogen peroxide and ester release energy which excites the fluorescent dye and generates light. Bioluminescence is a form of chemiluminescence and, as the name implies, light produced within an organism.
Another type of light emitting process is fluorescence. This is where external energy (as opposed to internal chemical reactions) are absorbed by a fluorescent material and then immediately remitted. This is the central principle behind the operation of fluorescent light tubes. A form of fluorescence is phosphorescence which is where external energy is stored for longer periods of time and slowly reemitted. Watch faces and glow-in-the-dark toys use phosphorescent materials. Some animals and plants are phosphorescent as opposed to being bioluminescent.
Which Organisms are Bioluminescent?
Bioluminescence is primarily a marine phenomenon. It has been observed in over 700 marine genera ranging from bacteria through to fish, molluscs (such as squid), sponges, jellyfish, echinoderms (starfish family) and crustaceans (crabs and the like). A common myth is that bioluminescence mostly comes from bacteria. In fact, a vast range of single-celled plankton, zoo and gelatinous plankton are bioluminescent. The 'sparks' flying off your paddle is most probably plankton such as copeopods or dinoflagellates. The glowing 'trails' in your wake are billions of scared, hungry or over-sexed plankton (more on this later). Interestingly there are almost no freshwater bioluminescent organisms. Land based bioluminescent organisms include some insects (such as fireflies), fungi, worms ('glow worms'), and some terrestrial micro-organisms.
How is Light Made?
All light in the universe comes from the same basic process. When an electron absorbs energy, it moves to a higher orbit. When the electron falls back down to a lower energy state, a packet of energy, known as a photon, is released. Electrons can get excited in a number of different ways. In the sun, a candle flame or an incandescent light bulb, the electrons are thermally excited, which is why we tend to associate heat and light. In bioluminescence the electrons are excited by a very efficient chemical reaction that generates no heat at all. This is why bioluminescence is sometimes called cold light. This light comes from little packages of chemicals that are spread throughout the cell. This cell gets the energy to make these chemicals from the sun. Like many dinoflagellates it is photosynthetic. There are many bioluminescent dinoflagellates which are not photosynthetic and they get the energy needed to synthesize their chemicals by eating other, smaller plankton. Dinoflagellates are the most common source of brilliant bioluminescence in surface waters.
As mentioned above, bioluminescence results from light-producing chemical reactions. The group of chemicals involved are broadly termed luciferins. Light is produced by a series of oxidation reactions set off by a catalyst called luciferase (a catalyst is chemical accelerant that speeds up reactions). Luciferin is oxidised by luciferase to produce energy, oxygen, oxyluciferin and light. How plants and animals create luciferins varies. Some secrete it from special glands. 'Fresh' luciferin must be captured in the diet or synthesised internally.
Larger animals such as squid and fish have specialized light organs called photophores (photo = 'light' and phore = 'bearing') which are wart or blister-like structures packed full of bioluminescent bacteria. When the animal wants to generate light they 'stir' or stimulate the bacteria in the photophore and hey presto - instant light!
