1. Producer–decomposer interactions within aquatic biofilms can range from mutualistic associations to competition depending on available resources. The outcomes of such interactions have implications for biogeochemical cycling and, as such, may be especially important in northern peatlands, which are a global carbon sink and are expected to experience changes in resource availability with climate change. The purpose of this study was to evaluate the effects of nutrients and organic carbon on the relative proportion of primary producers (microalgae) and heterotrophic decomposers (bacteria and fungi) during aquatic biofilm development in a boreal peatland. Given that decomposers are often better competitors for nutrients than primary producers in aquatic ecosystems, we predicted that labile carbon subsidies would shift the biofilm composition towards heterotrophy owing to the ability of decomposers to outcompete primary producers for available nutrients in the absence of carbon limitation.
2. We manipulated nutrients (nitrate and phosphate) and organic carbon (glucose) in a full factorial design using nutrient‐diffusing substrates in an Alaskan fen.
3. Heterotrophic bacteria were limited by organic carbon and algae were limited by inorganic nutrients. However, the outcomes of competitive interactions depended on background nutrient levels. Heterotrophic bacteria were able to outcompete algae for available nutrients when organic carbon was elevated and nutrient levels remained low, but not when organic carbon and nutrients were both elevated through enrichment.
4. Fungal biomass was significantly lower in the presence of glucose alone, possibly owing to antagonistic interactions with heterotrophic bacteria. In contrast to bacteria, fungi were stimulated along with algae following nutrient enrichment.
5. The decoupling of algae and heterotrophic bacteria in the presence of glucose alone shifted the biofilm trophic status towards heterotrophy. This effect was overturned when nutrients were enriched along with glucose, owing to a subsequent increase in algal biomass in the absence of nutrient limitation.
6. By measuring individual components of the biofilm and obtaining data on the trophic status, we have begun to establish a link between resource availability and biofilm formation in northern peatlands. Our results show that labile carbon subsidies from outside sources have the potential to disrupt microbial coupling and shift the metabolic balance in favour of heterotrophy. The extent to which this occurs in the future will probably depend on the timing and composition of bioavailable nutrients delivered to surface waters with environmental change (e.g. permafrost thaw).

中文翻译：

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碳补贴使北部泥炭地生物膜群落向低营养水平而不是高营养条件下的异养转向

1. 水生生物膜中生产者与分解者之间的相互作用范围从相互联系到竞争，取决于可利用的资源。此类相互作用的结果对生物地球化学循环具有影响，因此，在北部泥炭地中尤其重要，该泥炭地是全球碳汇，预计会随着气候变化而经历资源可利用性的变化。这项研究的目的是评估养分和有机碳对北方泥炭地水生生物膜发育过程中主要生产者（微藻）和异养分解者（细菌和真菌）的相对比例的影响。鉴于与水生生态系统中的初级生产者相比，分解者通常是营养素的更好竞争者，
2. 我们使用阿拉斯加芬中的养分扩散性基质，对养分（硝酸盐和磷酸盐）和有机碳（葡萄糖）进行了全因子设计。
3. 异养细菌受有机碳限制，藻类受无机养分限制。但是，竞争性相互作用的结果取决于背景营养水平。当有机碳含量升高而养分含量仍然较低时，异养细菌能够在藻类竞争中胜过可利用的养分，但是当有机碳和养分含量都通过富集提高时，异养细菌则无法竞争。
4. 仅存在葡萄糖时，真菌生物量显着降低，这可能是由于与异养细菌的拮抗作用所致。与细菌相反，营养物富集后，真菌与藻类一起被刺激。
5. 仅在葡萄糖存在下藻类和异养细菌的解偶联将生物膜的营养状态转移到异养。当营养物与葡萄糖一起富集时，由于藻类生物量随后在没有营养物限制的情况下增加，这种作用被颠覆了。
6. 通过测量生物膜的各个组成部分并获得有关营养状态的数据，我们已开始在北部泥炭地建立资源可用性与生物膜形成之间的联系。我们的结果表明，来自外部来源的不稳定的碳补贴有可能破坏微生物耦合并改变代谢平衡，从而有利于异养。未来发生这种情况的程度可能取决于随环境变化（例如多年冻土融化）而输送到地表水中的生物可利用养分的时间和组成。