Diazotrophic Associations

C. Elmerich, Molecular biology and ecology of diazotrophs associated with non-leguminous plants, Bio/Technology 2 967 (1984). 

Figure 4.2 Conceptual model for differing fates of filamentous cyanobacterial diazotrophs (A) compared to diatom-diazotroph associations (B), from Subramaniam e(a/.,2008.

For the purpose of this text on symbioses as they relate to the marine nitrogen cycle, we wiU first emphasize the more common open ocean diatom-diazotrophic associations (DDAs), then summarize the recent advances in our understanding of sponge-nitrifying microbial associations, and provide brief introductions to a few other relevant symbioses. In addition, we recommend, the chapter by O Neil and Capone (Chapter 21) for details on algal-animal symbioses of Coral Reefs as they relate to the nitrogen cycle. Chapter 4 by Carpenter and Capone on Nitrogen Fixation and Chapter 5 by Ward on Nitrification. 

Associative dinitrogen fixation Close interaction between a free-living diazotrophic organism and a higher plant that results in an enhanced rate of dinitrogen fixation. 

Association with nonsymbiotic diazotrophs that can contribnte to the nitrogen nutrition of... 

Perin L, L Martmez-Aguilar, R Castro-Gonzalez, P Estrada-de los Santos, T Cabellos-Avelar, HV Guedes, VM Reis, J Caballero-Mallado (2006) Diazotrophic Burkholderia species associated with field-grown maize and sugarcane. Appl Environ Microbiol 72 3103-3110. 

O Neil, J.M. and Roman, M.R., Grazers and associated organisms of Trichodesmium, in Marine Pelagic Cyanobacteria Trichodesmium and other Diazotrophs, Carpenter, E.J., Ed., Kluwer Academic Press, Netherlands, 1992, 61. 

With regard to other diazotrophic symbioses on reefs, Paerl (1984) reported evidence for nitrogenase activity associated with an intact Procholoroti-Ascidinn association Lissoclinum sp.). However, activity could not be directly associated with the isolated symbiont. Later, Odintsov (1991) provided evidence that the nitrogenase activity associated with encrusting ascidians was in fact not associated with the symbiont. More recently, Khne and Lewin (1999) concluded that, based on the low isotopic ratio of in Lissoclinum sp. associations, at least a portion of... 

Other unique sites of N2 fixation remain to be discovered. Dense populations of Archaea are also found throughout the water column of the worlds oceans (DeLong, 1992 Fuhrman et al., 1992), and some methanogens have the capacity for diazotrophy (Murray and Zinder, 1984). Indeed, diazotrophic archaeal nijH sequences have been obtained from a marine vent community and in deep sea waters away from vents (Mehta et al., 2003, 2005). They also obtained an isolate capable of growth and N2 fixation up to 92°C (Mehta and Baross, 2006). N2 fixing bacteria, including purple sulfur bacteria, are also associated with the intestinal flora of zooplankton (Braun etal., 1999 Proctor, 1997). 

Whereas Trichodesmium is buoyant and resides largely in the upper layers of the water column, endosymbiotic Richelia may be more prone to gravitational settlement to deeper layers in their diatom hosts (Scharek et al, 1999a,b Subramaniam et al, 2008) (Fig. 4.2). Tropical river plumes may be particularly important sites of diazotroph-diatom associations (Voss et al, 2006 Subramaniam et al, 2008). 

As mentioned, diazotrophic diatom associations (DDAs) can, at times, be of quantitative significance. A bloom of the endosymbiont Richelia intracellularis within... 

Thus, diverse results from the field on both the density and activity of Trichodesmium, micro-diazotrophs and DDAs, the relative intensity of N2 fixation is highly variable and heterogeneous. On a local basis, each component may make substantial contributions. A critical evaluation of the relative global importance of each awaits more extensive and robust estimates of the N input by symbiotic associations and the smaller microbial diazotrophs at diverse locations. 

We believe this is remarkable in explaining a substantial fraction of the geochem-icaUy derived estimate. The difference is likely a function ofinput from Trichodesmium blooms (Carpenter and Capone, 1992 Capone et al., 1996), microbial diazotrophs (Montoya et al., 2004) and symbiotic associations (Carpenter et al., 1999). As noted, all three can contribute intense amounts of nitrogen, although the broader inputs over larger time and space scales are not currently well constrained. 

Estimates of N2 fixation rates in the global ocean continue to rise as results emerge from studies with the main N2 fixer in the ocean Trichodesmium, the heterocystous endosymbiont Richelia, as well as more recently discovered N2 fixers including unicellular diazotrophic cyanobacteria and bacterioplankton (Capone et al, 1997 HanseU and Feely 2000 Karl et al, 1997 Lipschultz and Owens, 1996 Montoya et al., 2004 Zehr et al, 1998 and 2001). Trichodesmium is involved in N release directly, through release of amino acids, DON, and NH4 (reviewed in Table 8.2). Trichodesmium is also a source of NH4+ and DON as a result of remineralization by associated bacteria, sloppy feeding and excretion by grazers (SeUner, 1992 Sheridan et al, 2002). 

Reef sponges (Wilkinson and Fay, 1979) and some corals may acquire fixed nitrogen from associated diazotrophic cyanobacteria (Frias-Lopez et al., 2002 Lesser et al., 2004 Rohwer et al., 2001, 2002 Shashar et al., 1994a,b, see also Chapter 4, Capone and Carpenter, this volume). The nutritional significance of these alternative pathways remains to be fuUy evaluated. 

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