**2.1 Mixotrophic ciliates**

One of the most interesting groups of protists are the mixotrophic ones. Some of them may correspond to the old morphological groups of ciliates, flagellates, and ameboebas. Mixotrophy is defined as the ability to combine phagotrophy and phototrophy in a single cell [20]. This group can be divided into constitutive mixotrophs, meaning they have the innate ability to photosynthesize, and the facultative or non-constitutive mixotrophs. These organisms may sequester the plastids after consuming their phototrophic preys or by harboring photosynthetic endosymbionts [20, 21]. Around 23% of planktonic ciliates species (marine and freshwater combined) perform acquired phototrophy, and this ability is present in at least 8 main ciliated taxa: Heterotrichea, Hypotrichia, Oligotrichida, Stichotrichida, Litostomatea, Prostomatea, Peniculia, and Peritrichia. Phototrophy is usually acquired from algae endosymbionts in 7 of these 8 ciliated taxa. Contrastingly, Oligotrichida usually obtains this ability by plastid sequestering [22].

The structures of the mixotrophic ciliates community varies through seasons, depending on the changing water trophic condition. Mixotrophic ciliates dominate in spring and summer, reaching from 58–100% of the ciliates in oligotrophic waters [23–25], but represent only 5% of the total community of ciliates in winter, probably due to the lower water temperatures and nutrients. These conditions restrict the growth of algae, negatively affecting the population of mixotrophic ciliates if their preferred species of algae is missing [24].

The mixotrophic ciliates are mainly from the genera *Mesodinium rubrum* (*Myrionecta rubra*), *Strombidium* spp., *Laboea*, *Lohmaniella,* and *Tontonia.* All of them represented by small species (30–50 μm) [23, 25, 26]. Even *Mesodinium rubrum* and other functionally photoautotrophic ciliates can sometimes contribute significantly to primary production in lakes and oceans [27]. Other species of mixotrophic ciliates are larger; for example, the genus *Stentor* is a "large" cell ~200 μm and is contributed between 49% and 68.8% of the total biomass of zooplankton in the oligotrophic lake at the Northern Patagonia of Chile [28]. *Stentor niger* represented 90% of the total ciliates biomass in Lake McCloud [29] and was the dominant protist of acidic oligotrophic lakes [30]. Some species

#### *Food Webs DOI: http://dx.doi.org/10.5772/intechopen.97252*

of the genus *Stentor* also contributed with more than 50% of the plankton's photosynthesis of oligotrophic Australian lakes [31]. Dominance of this trophic group may be influenced by the limiting conditions for phototrophs, to achieve the same productivity that mixotrophs may obtain by feeding both ways. Grazing allows mixotrophs greater flexibility for balancing the supply and demand of scarce nutrients [32], a clear advantage in times of scarcity [19]. Due to their flexible nutrition, mixotrophic protists dominate in mature or more stable systems (e.g., during mild summer, in established eutrophic systems, and in oligotrophic systems). Furthermore, climate change can be expected to favor mixotrophs in the more stable water columns [32].

Ward and Follows [33] performed a global simulation of the ocean-surface food web, revealing that mixotrophy enhances the transfer of biomass to larger organisms at higher trophic levels, which in turn increases the efficiency of oceanic carbon storage through the production of larger and faster sinking conglomerates of organic molecules. It follows that mixotrophic protists play a key role in modulating the primary production that underlies the food web in aquatic systems [21, 22, 32]. However, their importance has not been fully appreciated because traditional field and laboratory studies focus on strict classifications as phototrophs or phagotrophs [32] because incorporating this flexibility to acquire food is difficult to modelize. Mixotrophy is known to be common in all aquatic systems but its contribution to net community production is difficult to quantify, and the integration of their impact on the global biogeochemical cycles remains to be incorporated.
