The impact of mixotrophy on planktonic marine ecosystems

Mixotrophic protists, which utilize a nutritional strategy that combines phototrophy and phagotrophy, are commonly found in fresh, estuarine, and oceanic waters at all latitudes. A number of different physiological types of mixotrophs are possible, including forms which are able to use both phototrophy and phagotrophy equally well, primarily phototrophic phagocytic ‘algae’, and predominantly heterotrophic photosynthetic ‘protozoa’. Mixotrophs are expected to have important effects on the trophic dynamics of ecosystems, but the exact nature of these effects is not known and likely varies with physiological type. In order to study the impact that mixotrophs may have on the microbial food web, we developed mathematical formulations that simulate each of the three aforementioned physiological types of mixotrophs. These were introduced into idealized, steady-state open ocean and coastal/estuarine environments. Our results indicate that mixotrophs compete for resources with both phytoplankton and zooplankton and that their relative abundance is a function of the feeding strategy (physiological type and whether or not they feed on zooplankton) and the maximum growth and/or grazing rates of the organisms. In our models coexistence of mixotrophs with phytoplankton and zooplankton generally occurs within reasonable parameter ranges, which suggests that mixotrophy represents a unique resource niche under summertime, quasi-steady state conditions. We also find that the introduction of mixotrophs tends to decrease the primary production based on uptake of nitrogen from the dissolved inorganic nitrogen pool, but that this decrease may be compensated for by mixotrophic primary production based upon organic nitrogen sources. © 2000 Elsevier Science B.V. All rights reserved.