Phagotrophy by pigmented flagellates is known from the literature but has recently been rediscovered in the context of microbial
food webs. Particle ingestion rates were found to be equivalent for pigmented and nonpigmented microflagellates in both field
and laboratory studies. Ingestion rates of the chrysophytes
Ochromonas danica, O. minuta, and
Poterioochromonas malhamensis, the dinoflagellate
Peridinium inconspicuum, and the cryptophytes
Cryptomonas ovata and
C. erosa were compared with those of two nonpigmented
Monas species using 0.57 µm polystyrene beads as a food source. Ingestion rates were 0.31 to 3.17 beads/cell/h and filtration rates
were 10
−7 to 10
−8 ml/cell/h with no detectable difference between pigmented and nonpigmented forms. Ingestion rates in unpigmented
Monas species showed a linear increase with increasing particle concentration from 1.9 × 10
6 to 1.6 × 10
7 beads/ml.
Light and DOC levels in the range of those encountered by phytoflagellates in the field also influenced laboratory measurements
of bead ingestion by Poterioochromonas malhamensis. Ingestion rates decreased and photosynthesis increased over the natural PAR light range from 0 to 1800 microeinsteins/s/m2. At 40 microeinsteins/s/m2 maximum ingestion rates and high rates of photosynthesis occurred simultaneously. Ingestion rates decreased above 4 mgC/l
supplied as glucose. DOC levels commonly occurring in Lake Oglethorpe range from 3.5 to 10.0 mgC/l. These studies suggest
that mixotrophy, the trophic utilization of particulate food and dissolved organic matter as well as photosynthetically fixed
organic matter, is a balanced process that can be regulated by environmental conditions.
In field studies during a chrysophyte bloom, phytoflagellate grazing exceeded heterotrophic microflagellate grazing and constituted
up to 55% of the bactivory of all microflagellates, ciliates, rotifers, and crustaceans combined. Neither bacterial abundance,
light nor temperature were good predicters of grazing rates for the phagotrophic phytoflagellate association as a whole during
this unstratified period. Phagotrophs are often most abundant at the metalimnetic plate during stratification.
Key words phagotrophy - myxotrophy - mixotrophy - selectivity - cannibalism - flocculation - phagotrophic phytoflagellates - heterotrophic microflagellates - chrysophytes - cryptophytes - dinoflagellates -
Ochromonas
-
Poterioochromonas
-
Peridinium
-
Cryptomonas
-
Monas
- microbial food web - metalimnetic plate