Nitrogen fixation

Event Date: 
Wednesday, October 28, 2015 - 18:15 - 18:30
Institution: 
UTS
Title: 

Heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation

Abstract: 

Australia’s tropical waters represent predicted “hotspots” for nitrogen (N2) fixation based on empirical and modelled data. However, the identity, activity and ecology of N2 fixing bacteria (diazotrophs) within this region are virtually unknown. By coupling DNA and cDNA sequencing of nitrogenase genes (nifH) with size fractionated N2 fixation rate measurements, we elucidated diazotroph dynamics across the shelf region of the Arafura and Timor Seas (ATS) and oceanic Coral Sea during Austral spring and winter. During spring, Trichodesmium dominated ATS assemblages, comprising 60% of nifH DNA sequences, while Candidatus Atelocyanobacterium thalassa (UCYN-A) comprised 42% in the Coral Sea. In contrast, during winter the relative abundance of heterotrophic unicellular diazotrophs (∂-proteobacteria and gamma-24774A11) increased in both regions, concomitant with a marked decline in UCYN-A sequences, whereby this clade effectively disappeared in the Coral Sea. Conservative estimates of N2 fixation rates ranged from < 1 to 91 nmol L-1 d-1, and size fractionation indicated that unicellular organisms dominated N2 fixation during both spring and winter, but average unicellular rates were up to 10-fold higher in winter than spring. Relative abundances of UCYN-A1 and gamma-24774A11 nifH transcripts negatively correlated to silicate and phosphate, suggesting an affinity for oligotrophy. Our results indicate that Australia’s tropical waters are indeed hotspots for N2 fixation, and that regional physicochemical characteristics drive differential contributions of cyanobacterial and heterotrophic phylotypes to N2 fixation.

Event Date: 
Wednesday, November 27, 2013 - 18:00 - 18:15
Institution: 
Hawkesbury Institute for the Environment, UWS
Title: 

Organic phosphorus acquisition may be a functional driver of community structure for ectomycorrhizal fungi in a tri-partite symbiosis

Abstract: 

 
Alnus trees associate with ectomycorrhizal (ECM) fungi and nitrogen-fixing Frankia bacteria, and while their ECM fungal communities are uncommonly host specific and species poor, it is unclear whether the functioning of Alnus ECM fungal symbionts differs from that of other ECM hosts. We used exoenzyme root tip assays and molecular identification to test whether ECM fungi on Alnus rubra differed in their ability to access organic phosphorus and nitrogen as compared with ECM fungi on the non-Frankia host Pseudotsuga menziesii. At the community level, potential acid phosphatase (AP) activity of ECM fungal root tips from A. rubra was significantly higher than those from P. menziesii, while potential leucine aminopeptidase (LA) activity was significantly lower for A. rubra root tips at one of the two sites.  At the individual species level, there was no clear relationship between ECM fungal relative root tip abundance and relative AP or LA enzyme activities on either host. Our results are consistent with the hypothesis that ECM fungal communities associated with Alnus trees have enhanced organic phosphorus acquisition abilities relative to non-Frankia ECM hosts.  This shift, in combination with chemical conditions present in Alnus forest soils, may drive the atypical structure of Alnus ECM fungal communities. 

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