Nitrogen cycle

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatica

High plasticity of nitrogen fixation and denitrification of common coral reef substrates in response to nitrate availability

Nils Rädecker

Nitrogen cycling in coral reefs may be affected by nutrient availability, but knowledge about concentration-dependent thresholds that modulate dinitrogen fixation and denitrification is missing. We determined the effects of different nitrate concentrations (ambient, 1, 5, 10 μM nitrate addition) on both processes under two light scenarios (i.e., light and dark) using a combined acetylene assay for two common benthic reef substrates, i.e., turf algae and coral rubble. For both substrates, dinitrogen fixation rates peaked at 5 μM nitrate addition in light, whereas denitrification was highest at 10 μM nitrate addition in the dark. At 10 μm nitrate addition in the dark, a near-complete collapse of dinitrogen fixation concurrent with a 76-fold increase in denitrification observed for coral rubble, suggesting potential threshold responses linked to the nutritional state of the community. We conclude that dynamic nitrogen cycling activity may help stabilise nitrogen availability in microbial communities associated with coral reef substrates.

2021

Highly Variable and Non-complex Diazotroph Communities in Corals From Ambient and High CO2 Environments

Nils Rädecker

The ecological success of corals depends on their association with microalgae and a diverse bacterial assemblage. Ocean acidification (OA), among other stressors, threatens to impair host-microbial metabolic interactions that underlie coral holobiont functioning. Volcanic CO2 seeps offer a unique opportunity to study the effects of OA in natural reef settings and provide insight into the long-term adaptations under a low pH environment. Here we compared nitrogen-fixing bacteria (diazotrophs) associated with four coral species (Pocillopora damicornis, Galaxea fascicularis, Acropora secale, and Porites rus) collected from CO2 seeps at Tutum Bay (Papua New Guinea) with those from a nearby ambient CO2 site using nifH amplicon sequencing to characterize the effects of seawater pH on bacterial communities and nitrogen cycling. Diazotroph communities were of generally low diversity across all coral species and for both sampling sites. Out of a total of 25 identified diazotroph taxa, 14 were associated with P. damicornis, of which 9 were shared across coral species. None of the diazotroph taxa, however, were consistently found across all coral species or across all samples within a species pointing to a high degree of diazotroph community variability. Rather, the majority of sampled colonies were dominated by one or two diazotroph taxa of high relative abundance. Pocillopora damicornis and Galaxea fascicularis that were sampled in both environments showed contrasting community assemblages between sites. In P. damicornis, Gammaproteobacteria and Cyanobacteria were prevalent under ambient pCO(2), while a single member of the family Rhodobacteraceae was present at high relative abundance at the high pCO(2) site. Conversely, in G. fascicularis diazotroph communities were indifferent between both sites. Diazotroph community changes in response to OA seem thus variable within as well as between host species, potentially arguing for haphazard diazotroph community assembly. This warrants further research into the underlying factors structuring diazotroph community assemblages and their functional role in the coral holobiont.

FRONTIERS MEDIA SA2021

Highly Variable and Non-complex Diazotroph Communities in Corals From Ambient and High CO2 Environments

Nils Rädecker

FRONTIERS MEDIA SA2021

Cultivation of green microalgae Chlorella vulgaris using an effluent of the anaerobic digestion process as nitrogen source

High plasticity of nitrogen fixation and denitrification of common coral reef substrates in response to nitrate availability

Highly Variable and Non-complex Diazotroph Communities in Corals From Ambient and High CO2 Environments

High plasticity of nitrogen fixation and denitrification of common coral reef substrates in response to nitrate availability

Highly Variable and Non-complex Diazotroph Communities in Corals From Ambient and High CO2 Environments

Cultivation of green microalgae Chlorella vulgaris using an effluent of the anaerobic digestion process as nitrogen source