MEPS 164:189-197 (1998)  -  doi:10.3354/meps164189

Protozoan size influences algal pigment degradation during grazing

Suzanne L. Strom*, T. Aaron Morello**, Kelley J. Bright

Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Rd, Anacortes, Washington 98221, USA
**Present address: School of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195, USA

Degradation of algal pigments during grazing by planktonic protozoa was examined using batch cultures of 6 protozoan species (3 ciliates, 3 heterotrophic dinoflagellates). Pigment losses due to grazer activity were compared with algal cell losses to determine pigment degradation efficiencies. Chlorophylls a and c were degraded (i.e. converted to phaeopigments or to colorless compounds) with high efficiencies (>=94%) in nearly all experiments. Degradation efficiencies for carotenoids were variable (reaching maxima of 38 to 103%), indicating that chlorophylls are intrinsically more labile than carotenoids in the presence of grazers. Algal species and carotenoid identity appeared to have little effect on degradation efficiency. Rather, grazers were the main source of variability, with small (<25 µm) protozoa (Amphidinium sp., Gymnodinium sp., Uronema sp.) able to degrade carotenoids more efficiently than large (>80 µm) protozoa (Favella sp., Strombidinopsis acuminatum, Noctiluca scintillans). Regardless of grazer size, little algal pigment was found in the dissolved (<0.7 µm) phase, demonstrating that pigment removed from the particulate phase was truly degraded and not simply repackaged as colloidal (or smaller) material. Degradation of carotenoids varied during experiments, increasing as much as 5-fold over time. Such increases suggested that, during the later stages of incubations, protozoa were reingesting pigmented waste particles or digesting pigments more extensively in response to decreasing food concentrations. These data indicate that small protozoa degrade ingested algal carotenoids and, by extension, ingested organic matter more extensively than large protozoa. Microzooplankton size structure may have a strong influence on the efficiency of organic matter recycling in microbially dominated planktonic systems.

Chlorophyll · Carotenoid · Digestion · Ciliate · Dinoflagellate · Microzooplankton

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