As most, if not all, phototrophs can utilize inorganic phosphate in their environment, it is widely assumed that the ability to utilize organic phosphate is more restricted. If the supply of phosphate is sufficiently restricted that an organism startsto become phosphorus-limited, one possibility to overcome this is to use inorganic phosphate more efficiently, such as by adopting different uptake pathways (Wagner and Falkner, 2001). Although overlooked by these and many other authors, another possibility is to use the organic phosphate present in the environment. This chapter sets out to assess how widespread this is in cyanobacteria, algae and bryophytes, and to review the methods involved. The focus is on soluble organic phosphate, because this has been the subject of most studies, though some cyanobacteria may be able to utilize insolu-ble organic phosphate (Whitton et al., 1991). A further approach adopted by some chry-sophytes, haptophytes, cryptomonads and dinoflagellates is to feed on otherliving organisms (phagotrophy; see Graham and Wilcox, 2000) and thus obtain phosphorus via this route. As 'soluble' organic phosphorus in the environment has almost always been determined following filtration and molybdate colorimetry, the values obtained can include high-molecular-weight complexes and inorganic polyphos-phate .
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