To be able to fix carbon in photosynthesis is the abiding property separating the majority of photoautotrophic organisms ('plants') from the majority of phagotrophic heterotrophs ('animals'). This division is by no means so obvious among the protists where nominally photosyn-thetic algae show capacities to ingest particu-late matter and bacteria as a facultative or a quite typical feature of their lifestyles. This kind of mixotrophy is seen among the dinoflagellates and certain types of chrysophyte (mostly chromu-lines). Alternatively, the bacterium-like ability to absorb selected dissolved organic compounds across the cell surface ('osmotrophy') is possessed among some chlorophyte algae (of the Chlorococ-cales) and in certain Euglenophyta, and among cryptomonads (Lewitus and Kana, 1994).
Whereas osmotrophy clearly represents a means of sourcing assimilable carbon, without the requirement for its prior photosynthetic reduction, mixotrophy is generally regarded as a facultative ability to supplement nutrients other than carbon (chiefly N or P) under conditions of nutrient limitation of production (Riemann et al., 1995; Li et al., 2000). However, there is a valuable energetic subsidy to be derived too, although the exploitative opportunity varies (Geider and Mac-Intyre, 2002); some marine dinoflagellates are said to be 'voraciously heterotrophic', ingesting other protists. In some lakes, Gymnodinium helvet-icum seems to be predominantly phagotrophic, to the point of abandoning chlorophyll pigmentation. Among the Chromulinales, the resort to phagotrophy seems very strongly associated with a shortage of nutrients; though normally pig-mented, however, cells resorting to bacterivory are much paler and show reduced photosyn-thetic capacity above a threshold prey density (references in Geider and MacIntyre, 2002). On the other hand, some of these species (e.g. Ochromonas) are prominent nanoplanktic bacteri-vores and fulfil a key stage in the microbial loop (see Section 3.5).
An ability of Chlorococcales to take up glucose and other soluble sugars derivatives has been inferred or demonstrated on several occasions (Algeus, 1950: Berman et al., 1977; Lewi-tus and Kana, 1994). Their habitats, which frequently include organically rich ponds, provide the opportunities for assimilating organic solutes but the relationship appears not to be obligate. Though sometimes representing a major step in the carbon dynamics of ponds, the trait is far from being obligate; algal heterotrophy nevertheless can play an important role in the pelagic carbon cycle of large lakes.
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