Anammox from head-water streams to ocean sediments - different strategies for different systems

The vast majority of what we know about the anammox reaction, and the putative bacteria that drive this process, has been gathered in either marine or estuarine sediments and some even dismissed its significance in freshwater sediments before any real exploration of this domain had begun. Further, the evidence to date suggests that the nitrite required to sustain anammox in marine and estuarine sediments most likely comes from anaerobic nitrate reduction, with this claim being substantiated by correlational, isotopic and ¹⁵N process evidence. Here we begin by presenting our case for a kinship between anammox and denitrification in marine sediments and contrast that with our recent work in permeable river beds, where anammox is largely overlooked. The little what we know about anammox in rivers comes from impermeable, clay riverbeds where diffusional oxygen penetration is restricted to a few mm – as it is in the well-studied, soft marine sediments. Here we present a contrast and describe how anammox makes a significant contribution to in situ N₂ production in permeable (chalk-gravel, sand), oxic riverbeds. We also highlight the markedly greater diversity of the anammox community in permeable, compared to the impermeable riverbeds – where the latter mirrors the low diversity often described in impermeable, marine sediments. Together, the marine and freshwater data suggest different ‘kinships’ between anammox and the organisms that supply it with nitrite depending on the environment.