Is There Photoferrotrophy In The Anoxic Hypolimnia Of Boreal Shield Lakes?

English Abstract:

Photoferrotrophy, where Fe²⁺ is used as an electron donor for carbon fixation in the presence of light, plays an important role in Fe cycling in lakes and in the Archaean ocean. Photoferrrotrophy in ancient ferruginous oceans may pre-date the evolution of cyanobacteria and oxygenic photosynthesis, playing a role in supplying Fe³⁺ for the globally distributed banded iron formations. Lake 227 at the Experimental Lakes Area has been the site of a long term and controversial nutrient addition experiment spanning more than 45 years. Following the start of nutrient additions in 1969, the enitre hypolimnion became completely anoxic with O₂ penetration only at lake turnover. Fe²⁺, NH₄⁺ and DIC concentrations are very high and both SO₄^2- and S^2- are low. Light levels in the hypolimnion are minimal; are they sufficient for these photoautotrophs? Stable carbon and nitrogen isotope analyses reveal that particulate organic matter (POM) in the hypolimnion curiously does not match surface POM or lake sediments. Differences between stable isotopes of NH^­­₄⁺ and DIC compared to POM are consistent with fractionation due to biological uptake of both N and C. Stable isotopic analyses of Fe show fractionation similar to laboratory cultures of ferrotrophs. Measures of DOC quality indicate unusually high levels of biopolymers that are likely associated with active biological production and inorganic colloids are high. Sequence analysis of 16S rRNA genes indicates that half of the most abundant bacterial taxa are putatively involved in Fe cycling. Results from Lake 227 are compared with those from other lakes with anoxic hypolimnia but no nutrient additions. Is Lake 227 a modern analogue for the Archaean Ocean? Is Lake 227 “one of a kind” or is photoferrotrophy important in other boreal shield lakes?