Potential nitrification activity reflects ammonia oxidizing bacteria but not archaea activity across a soil-sediment gradient

Terrestrial-marine ecosystems have important ecologically relevant roles influencing the retention and mobility of nitrogen entering coastal ecosystems. The sharp physico-chemical gradients represent an ideal environment to elucidate the relative contributions of ammonia oxidizing archaea (AOA) and bacteria (AOB) to the nitrification process. Here we examined the activity of ammonia oxidisers (AO) across two coastal bays soil-sediment gradients to explore the functional shift from AOA to AOB, and determine if transcriptional activity within the environment (in situ) was emulated in laboratory potential nitrification activity incubations. To do this gene and transcripts abundance and diversity were measured along with potential nitrification activity (PNA) and recovery nitrification potential (RNP) from a series of soil, interface and sediment sites. We compared the composition of amoA transcript community structure in situ vs. PNA/RNP to see if the active AOA and AOB were similar in the environment and in the laboratory experiments. AOA was dominant at gene and transcript level in soil and interphase sites, but active transcripts in situ did not match those within the PNA/RNP assays. AOB was dominant at gene and transcript level in sediments and here transcripts in situ and within the PNA/RNP were similar. A high correlation between AOB transcripts and PNA in sediments was observed but a negative correlation for AOA in soils was seen. Our data indicates that while the PNA/RNP may be a good proxy for AOB activity in these sediments, it was not for AOA dominated soil due to unfavorable incubation conditions.