My posts on eukaryotic microorganisms (protists) mostly concern with Ciliates the group I know better. I studied them for more than 40 years! However there are other groups worthy of mention for their notable peculiarities.
I already mentioned Flagellates characterized by the possession of one or more flagella, which are long, tapering, hair-like appendages that act as organelles of locomotion. Among Flagellates a particularly interesting group are certainly Dinoflagellates that are mostly part of the marine plankton, but also can be found in freshwater habitats. They typically posses two flagella: a transverse flagellum running along an equatorial sulcus that beats to the cell's left, and a longitudinal flagellum, that beats posteriorly. Most are covered by a shell called lorica often consisting of cellulose plates. They greatly differ from each other in shape and their size varies from 8 to 500 micrometers.
Many are autotrophs, i.e. they can produce its own food using light, water, carbon dioxide, or other chemicals thus, like plants, they are primary producer in the food chain.
Others are instead hetrotrophs so they need to eat.
Many of them are extremely voracious predators able to ingest very large preys.
The species covered by a rigid lorica possess specific devices to capture and gulp down food organisms. Some draw prey to the sulcal region of the cell (either via water currents set up by the flagella or via pseudopodial extensions) and ingest the prey through the sulcus. Other species extrude a large feeding veil — a pseudopod called the pallium - to capture prey which is subsequently digested extracellularly or feed by attaching to its prey and ingesting prey cytoplasm through an extensible peduncle.
Dinophysis acuminata. Light micrographs of live cells feeding on a ciliate
Preys, attracted through chemical stimuli, are protists or even small hurt metazoa. The latter can be co-digested by a number of predators.
Most planktonic dinoflagellates perform diurnal vertical
migrations. In some cases the behaviour pattern is for the cells to migrate
downwards away from the surface as the light becomes less intense and then to ascend again at the sunrise. In
these migrations they cover up to
their size a distance corresponding to
The significance of this migration is understandable in autotrophic species needing light for their metabolism.
But also a number of heterotophic species behave in the same way. Why?
The term plankton is a collective name for all marine and freshwater organisms nonmotile or too small or weak to swim against the current, existing in a drifting state. The term includes certain protists among which Dinoflagellates, bacteria, crustaceans, molluscs and coelenterates interacting in different ways; in particular prey-predators interactions influence reciprocal behaviours. Thus while small crustaceans, which are the main predators of autotrophic or heterotrophic Dinoflagellates, migrate in the deepwaters during the day to prevent their own ingestion from fishes and return to the surface in the night, it is possible thatDinoflagellates migrate in the opposite sense to escape their engulfement.
Anyway Dinoflagellate migration is a very important phenomenon in the planktonic ecosystem because it creates a massive biomass and nutrients shifting in the water column.
Dinoflagellates are the main eukaryotic protists that are capable of producing light. Within this group, bioluminescence is present in a number of ecologically important species, many of which like Noctiluca form blooms. When the organisms perceive the presence of a predator like for example a little crustacean, emit a flash of cold light lasting 0.1-0.5 seconds. The question is: why? Perhaps even this capacity is sophisticated defensive tool. Fishes that catch their food organisms on sight are attracted by the flash and engulf the more visible crustacean allowing the protist to survive.
Dinoflagellates are often responsible of red tides.
Red tides are common events in warm and polluted coastal oceans. They form when dinoflagellate algae explode to huge population levels. Because the dinoflagellates have red plastids, the waters literally turn red. Dinoflagellates take advantage of harsh environmental conditions that kill off other organisms.
Moreover they produce toxins that kill fishes or gather into filter feeding organisms such as mussels or clams and cause damage to molluscs eaters.
At present occurrence of red blooms is increasing all over the world.
The red tides in some locations appears to be entirely natural (algal blooms are a seasonal occurrence) while in others they appear to be a result of increased nutrient pollution from human activities increased temperature, alteration of ocean currents, increased photosinthesis by elevated co2.
Thus dinoflagellates that are under-known as fundamental planktonic components, primary producers and consumers of the lower food chain levels, are instead infamous why in certain conditions, often caused by human beings, may became dangerous for human health.
Ops! I did not mentioned here that also Symbiodinium, symbiont of coral polyps and not only, is a photosyntetic Dinoflagellate. I mentioned Symbiodinium in the post “Together we can” because their suffering, probably due to the temperature enhancement, causes the suffering of coral reefs.
Anyway I would like to emphasize once again:
of microorganisms in every ecosystem.