Mixotrophic flagellates in coastal marine sediments: quantitative role and ecological significance
Mixotrophy, which is the combination of autotrophic and heterotrophic modes of nutrition, is a common phenomenon in aquatic food webs. Mixotrophic feeding strategies are species specific and dependent on environmental factors, such as light and nutrient conditions. In contrast to the plankton, nothing is known about the occurrence and the ecological role of benthic mixotrophs. In the present study, mixotrophic nanoflagellates were investigated in coastal marine sediments. Their quantitative importance and their ecological role were investigated in dependence of light and nutrient conditions and along small-scale vertical and horizontal gradients at the main sampling site at Falckenstein Beach in the Western Baltic Sea. Furthermore, the significance of mixotrophs was compared in sediment and plankton in different systems along a salinity gradient and at different geographical sites, as well as in plankton and sea ice of the Greenland Sea. Mixotrophs showed varying abundances and contributions to the flagellate community, which could not be attributed to particular environmental factors in all cases. Disparate responses in temporal and spatial scales were attributed to differences in community composition of mixotrophs and in the relative importance of environmental factors that determine mixotrophic feeding strategies. Abundances of mixotrophs increased with increasing salinity. However, they contributed low portions to the total nanoflagellates (max. 7%), with higher portions to the total phytoflagellates (max. 25%) than to total bacterivores (max. 10%). Highest contributions were found in sea ice and plankton of the Greenland Sea. Mixotrophic feeding strategies were proposed to play a greater role in oceanic plankton and sea ice than in coastal sediments.