Kagami Maiko, von Elert Eric, Ibelings Bas W, de Bruin Arnout, van Donk Ellen
Department of Foodweb Studies, Netherlands Institute of Ecology (NIOO-KNAW), Centre for Limnology, Rijkstraatweg 6, 3631AC Nieuwersluis, The Netherlands.
Proc Biol Sci. 2007 Jun 22;274(1617):1561-6. doi: 10.1098/rspb.2007.0425.
In food-web studies, parasites are often ignored owing to their insignificant biomass. We provide evidence that parasites may affect trophic transfer in aquatic food webs. Many phytoplankton species are susceptible to parasitic fungi (chytrids). Chytrid infections of diatoms in lakes may reach epidemic proportions during diatom spring blooms, so that numerous free-swimming fungal zoospores (2-3 microm in diameter) are produced. Analysis shows that these zoospores are rich in polyunsaturated fatty acids and sterols (particularly cholesterol), which indicates that they provide excellent food for zooplankters such as Daphnia. In life-table experiments using the large diatom Asterionella formosa as food, Daphnia growth increased significantly in treatments where a parasite was present. By grazing on the zoospores, Daphnia acquired important supplementary nutrients and were able to grow. When large inedible algae are infected by parasites, nutrients within the algal cells are consumed by these chytrids, some of which, in turn, are grazed by Daphnia. Thus, chytrids transfer energy and nutrients from their hosts to zooplankton. This study suggests that parasitic fungi alter trophic relationships in freshwater ecosystems and may be the important components in shaping the community and the food-web dynamics of lakes.
在食物网研究中，寄生虫因其生物量微不足道常常被忽视。我们提供的证据表明，寄生虫可能会影响水生食物网中的营养传递。许多浮游植物物种易受寄生真菌（壶菌）感染。湖泊中硅藻的壶菌感染在硅藻春季大量繁殖期间可能达到流行程度，从而产生大量自由游动的真菌游动孢子（直径2 - 3微米）。分析表明，这些游动孢子富含多不饱和脂肪酸和甾醇（特别是胆固醇），这表明它们为诸如水蚤等浮游动物提供了优质食物。在以大型硅藻美丽星杆藻为食物的生命表实验中，存在寄生虫的处理组中水蚤的生长显著增加。通过捕食游动孢子，水蚤获得了重要的补充营养并得以生长。当大型不可食用藻类被寄生虫感染时，藻类细胞内的营养物质会被这些壶菌消耗，其中一些反过来又会被水蚤捕食。因此，壶菌将能量和营养物质从其宿主转移到浮游动物。这项研究表明，寄生真菌改变了淡水生态系统中的营养关系，可能是塑造湖泊群落和食物网动态的重要组成部分。
