Response of a Microbial Iron Cycling Ecosystem From a Glacial Meltwater Stream to Perturbations in Organic Matter Availibility.

Abstract:

Microbial iron cycling plays an essential role in biogeochemistry, but iron-cycling microbial ecosystems are mainly studied in situ, making it difficult to apply well-controlled perturbations. The iron cycle is strongly coupled to other biogeochemical cycles (particularly the sulfur cycle) and thus its response to perturbations has wide significance. The microbial iron cycle in glacial environments is important as glacial runoff is thought to contribute significant concentrations of free iron to the global iron cycle annually. We have used iron-cycling sediment-water microbial microcosms (Winogradsky columns) as a controlled experimental model in which to study the response to perturbations of iron-cycling microbial ecosystems, using sediment from a high altitude glacial meltwater stream. The microbial iron cycle in glacial environments is important as glacial runoff is thought to contribute significant concentrations of free iron to the global iron cycle annually. We analyse ecosystem state by measuring detailed depth profiles of free iron and oxygen. Upon adding organic matter we observe a sudden transition to a reduced state (dominated by Fe²⁺) . We hypothesize that this occurs because higher concentrations of organic matter stimulate the microbial reduction of ferrihydrite (the iron source), by providing increased availability of electron donors for the reduction of iron. Using high-throughput sequencing we also analyse how the microbial community changes with the perturbation. As well as providing insight into present-day biogeochemical cycles, our results may also have implications for ancient environments, such as ferruginous Proterozoic oceans.