Karen G. Porter

Ciliate protozoans as links in freshwater planktonic food chains

THE potential importance of bacteria in freshwater planktonic food webs is recognised1, but the mechanisms of their incorporation are not fully known. Most planktonic bacteria are not attached to particles and are less than 1 µm (refs 2–4) in size—a range inefficiently filtered by crustacean zooplankton5. Daphnia and Diaptomus feed at lower rates on free living bacteria than on algae6,7. Bacterial food sources have been used with varying degrees of success to culture zooplankton8–11. These variations may be due to the unnatural range of concentrations used and the uncontrolled effects of different culture media, laboratory strains and the degree of accessibility to the zooplankton (because of cell clumping and sedimentation). Ciliate protozoan microzooplankton may be important intermediaries which transform ultrafine organic matter, including bacteria, into a particle size range readily available to crustacean zooplankton. Here we present evidence that ciliates are cropped by crustacean zooplankton in the natural environment. We have also measured feeding, filtering, and assimilation rates of Daphnia magna on both a large and small ciliate in the laboratory and have observed feeding behaviour directly. Dissolved organic carbon, bacteria, ciliate protozoans and crustacean zooplankton may represent a significant pathway of carbon flow in the freshwater plankton.

Phagotrophic phytoflagellates in microbial food webs

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 × 106 to 1.6 × 107 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.

Relationship between phototrophy and phagotrophy in the mixotrophic chrysophyte Poterioochromonas malhamensis

The time scales involved in the transition between phototrophic and phagotrophic modes of nutrition were examined in the mixotrophic chrysophytePoterioochromonas malhamensis. Phagotrophy began almost immediately when bacteria were added to phototrophically growing cultures of the alga, and chlorophylla concentration per cell in these cultures decreased over a 24-hour period. Chlorophyll concentrations per cell began to increase when bacteria were grazed to a density of approximately 106 ml−1, but after more than 24 hours they had not returned to the higher chlorophyll concentrations observed in the phototrophically grown cultures. Bacterivory was the dominant mode of nutrition in all cultures containing heat-killed bacteria. Photosynthesis did not contribute more than ≈7% of the total carbon budget of the alga when in the presence of abundant heat-killed bacteria. Bacterial density was the primary factor influencing the ability ofP. malhamensis to feed phagotrophically, while light intensity, pH, and the presence of dissolved organic matter had no effect on phagotrophy. We conclude thatP. malhamensis is capable of phagotrophy at all times. In contrast, phototrophy is inducible in the light during starvation and is a long-term survival strategy for this mixotrophic alga (i.e., it operates on time scales greater than a diel cycle).

Morphology, flow regimes, and filtering rates of Daphnia, Ceriodaphnia, and Bosmina fed natural bacteria

SummaryBody size is the best overall indicator of the abilities of the cladocerans Daphnia magna, D. parvula, Ceriodaphnia lacustris and Bosmina longirostris to filter natural bacteria (<1.0 μm). However, species differences exist which cannot be inferred from differences in size, behavior, or morphology alone. The relationship between filtering rate (FR in ml animal-1h-1) and body length (L in mm) for the cladocerans studied can be described by the power function:

Occurrence of bacterivory in Cryptomonas, a common freshwater phytoplankter

\begin{gathered} FR = 0.538 L^{1.545} \hfill \\ (r^2 = 0.88, F = 168.54, P < 0.001). \hfill \\ \end{gathered}

Relative nutritional value of ciliate protozoa and algae as food forDaphnia

In D. parvula, algal filtering rates are higher and increase more rapidly with increasing body size than do bacterial filtering rates which are 26 to 33% of algal rates. This suggests that different processes may be involved in the capture of these ultrafine particles and that ultrafine particle capture efficiency decreases with increasing body size within a species. Weight specific filtering rates (in μl μg dry wt-1h-1) have a strong negative relationship to body size and show species specific differences. Appendage beat rates intersetular distances, setule diameter, appendage, area, % open space on the filtering appendage, Reynolds number, and boundary layer thickness do not provide simple predictions of bacterial filtering rates for the cladocerans studied. Filtering rates on cultured laboratory bacteria and algae may not indicate filtering rates on natural bacterioplankton because of differences in bacterial size, motility, and surface properties. Uptake of ultrafine particles may be enhanced by the presence of larger, more readily filtered particles through a “piggybacking” phenomenon.

Life history variation within a parthenogenetic population of Daphnia parvula (Crustacea: Cladocera)

Surface chemistry of both particles and animals is important in filter feeding at low Reynolds number. Daphnia magna, fed mixtures of three sizes of polystyrene particles, retained particles that were smaller than the mesh size of the animals (1.0 micrometer) at greater efficiencies than predicted by a sieving model. Retention efficiency of the smallest particles (0.5 micrometer) was increased when negative surface charge on the particles was neutralized, and retention was decreased when a nonionic surfactant was, added to reduce wettability.

The use of DAPI for identifying and counting aquatic microflora1

SummaryBacterivory was detected by incorporation of 0.57 μm diameter, fluorescent polystyrene beads and fluorescently labeled bacteria (FLB) in two cultured species of Cryptomonas (C. ovata and C. erosa), and a population of Cryptomonas sp in a humic, mesotrophic lake. Rates of ingestion and clearance were very low, and similar for the cultures and the in situ population. The in situ population incorporated 0.7–1.7 bacteria cell-1 h-1, thereby ingesting 0.3%–2.0% of the total bacterial numbers present in the water per day, and receiving less than 2% of its carbon content per day through bacterivory. Thus, bacterivory by Cryptomonas was quantitatively important neither as a sink for bacterial biomass, nor as a carbon source for the algal cells. Possibly, it served in the uptake of essential nutrients.