The impact of mixotrophy on planktonic marine ecosystems

Mixotrophic protists, which utilize a nutritional strategy that combines ph ototrophy and phagotrophy, are commonly found in fresh, estuarine, and ocea nic waters at all latitudes. A number of different physiological types of m ixotrophs are possible, including forms which are able to use both phototro phy 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 p hysiological type. In order to study the impact that mixotrophs may have on the microbial food web, we developed mathematical formulations that simula te each of the three aforementioned physiological types of mixotrophs. Thes e were introduced into idealized, steady-state open ocean and coastal/estua rine environments. Our results indicate that mixotrophs compete for resourc es with both phytoplankton and zooplankton and that their relative abundanc e 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 o f 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 summerti me, quasi-steady state conditions. We also find that the introduction of mi xotrophs tends to decrease the primary production based on uptake of nitrog en from the dissolved inorganic nitrogen pool, but that this decrease may b e compensated for by mixotrophic primary production based upon organic nitr ogen sources. (C) 2000 Elsevier Science B.V. All rights reserved.