Dinoflagellates are eukaryotic plankton, existing in marine and freshwater environments. Previously, dinoflagellates had been grouped into two categories, phagotrophs and phototrophs. Mixotrophs, however include a combination of phagotrophy and phototrophy. Mixotrophic dinoflagellates are a sub-type of planktonic dinoflagellates and are part of the phylum Dinoflagellata. They are flagellated eukaryotes that combine photoautotrophy when light is available, and heterotrophy via phagocytosis. Dinoflagellates are one of the most diverse and numerous species of phytoplankton, second to diatoms. Dinoflagellates have long whip-like structures called flagella that allow them to move freely throughout the water column. They are mainly marine but can also be found in freshwater environments. Combinations of phototrophy and phagotrophy allow organisms to supplement their inorganic nutrient uptake This means an increased trophic transfer to higher levels in food web compared to the traditional food web. Mixotrophic dinoflagellates have the ability to thrive in changing ocean environments, resulting in shifts in red tide phenomenon and paralytic shellfish poisoning. It is unknown as to how many species of dinoflagellates have mixotrophic capabilities, as this is a relatively new feeding-mechanism discovery.
Species
Some dinoflagellates that live as parasites are probably mixotrophic. Karenia, Karlodinium, and Lepidodinium are some of the dinoflagellate genera which are thought to contain peridinin, a carotenoid pigment necessary for photosynthesis in dinoflagellates; however, chlorophyll b has been found in these genera as an accessory pigment. This discovery has led scientists to assume that the pigment chlorophyll b actually came from prey which had been ingested by the dinoflagellates. Some species of mixotrophic dinoflagellate are able to feed on toxic prey such as toxic algae and other toxic organisms. For example, Lingulodinium polyedra and Akashiwo sanguinea are two species of mixotrophic dinoflagellates that are known to feed on the toxic dinoflagellate, Alexandrium tamarense. Certain species of mixotrophic dinoflagellates can be affected by light intensity and nutrient conditions . For example, ingestion rates of Fragilidium subglobosum, Gymnodinium gracilentum, and Karlodinium veneficum increase as light intensity increases up to 75 to 100 μmol photons per m2 per second. In contrast, other species are not affected by light intensity. As well, ingestion rates of the mixotrophic dinoflagellate Ceratium furca are affected by intracellular nutrient concentrations.
Types of feeding Marine dinoflagellate species undergo three major trophic modes: autotrophy, mixotrophy and heterotrophy. Many species of dinoflagellates were previously assumed to be exclusively autotrophic; however, recent research has revealed that many dinoflagellates that were thought to be exclusively phototrophic are actually mixotrophic. Mixotrophic dinoflagellates can undergo both photosynthesis and phagocytosis as methods of feeding. Mixotrophic dinoflagellates with individual plastids that depend mostly on photosynthesis can prey on other cells as their secondary source of nutrients. On the other hand, mixotrophic dinoflagellates with individual plastids that depend mainly on phagocytosis are also photosynthetic due to chloroplasts 'stolen' from their prey (kleptochloroplasts) or because of algal endosymbionts. It was discovered that the mixotrophic dinoflagellates Gonyaulax polygramma and Scrippsiella spp. can engulf small-size prey using their apical horn while larger prey are engulfed via their sulcus, showing that dinoflagellates can have more than one mouth for feeding. Moreover, mixotrophic dinoflagellates belonging to the species Karlodinium armiger, can capture small prey by direct engulfment or can use an extendable peduncle to capture larger prey.
Implications for microbial food webs Mixotroph dinoflagellates belonging to the species Gymnodinium sanguineum feed on nanociliate populations in Chesapeake Bay. Predation on ciliates is advantageous for G. sanguineum as the ciliates provide a source of nitrogen which is limiting to the growth of purely photosynthetic dinoflagellates. By preying on ciliates, these dinoflagellates reverse the normal flow of material from primary producer to consumer and influence the trophodynamics of the microbial food web in Chesapeake Bay Several established ecological models of marine microbial food webs have not included feeding by mixotrophic dinoflagellates. These additions would include feeding by mixotrophic dinoflagellates on bacteria, phytoplankton, other mixotrophic dinoflagellates and nanoflagellates, and heterotrophic protists. The impact of grazing by mixotrophic dinoflagellates will affect particular prey species and be influenced by the abundance of dinoflagellate predators and their ingestion rates. Another consideration would be to include predator-prey relationships of mixotrophic dinoflagellates at a species level due to co-existence in offshore and oceanic waters. The diversity of mixotrophic dinoflagellate species and their interactions with other marine organisms contributes to their diverse roles in different niche environments. For example, mixotrophic and heterotrophic dinoflagellates may act as predators on a wide range of prey types due to their diverse feeding mechanisms. Including mixotrophic dinoflagellates would better explain the control of prey population and cycling of limited materials as well as competition between other organisms for larger prey.
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