Co‐occurrence of denitrification and nitrogen fixation in a meromictic lake, Lake Cadagno (Switzerland)

Abstract

The nitrogen cycling of Lake Cadagno was investigated by using a combination of biogeochemical and molecular ecological techniques. In the upper oxic freshwater zone inorganic nitrogen concentrations were low (up to approximately 3.4 microM nitrate at the base of the oxic zone), while in the lower anoxic zone there were high concentrations of ammonium (up to 40 microM). Between these zones, a narrow zone was characterized by no measurable inorganic nitrogen, but high microbial biomass (up to 4 x 10(7) cells ml(-1)). Incubation experiments with (15)N-nitrite revealed nitrogen loss occurring in the chemocline through denitrification (approximately 3 nM N h(-1)). At the same depth, incubations experiments with (15)N(2)- and (13)C(DIC)-labelled bicarbonate, indicated substantial N(2) fixation (31.7-42.1 pM h(-1)) and inorganic carbon assimilation (40-85 nM h(-1)). Catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH) and sequencing of 16S rRNA genes showed that the microbial community at the chemocline was dominated by the phototrophic green sulfur bacterium Chlorobium clathratiforme. Phylogenetic analyses of the nifH genes expressed as mRNA revealed a high diversity of N(2) fixers, with the highest expression levels right at the chemocline. The majority of N(2) fixers were related to Chlorobium tepidum/C. phaeobacteroides. By using Halogen In Situ Hybridization-Secondary Ion Mass Spectroscopy (HISH-SIMS), we could for the first time directly link Chlorobium to N(2) fixation in the environment. Moreover, our results show that N(2) fixation could partly compensate for the N loss and that both processes occur at the same locale at the same time as suggested for the ancient Ocean.

An integrative study of a meromictic lake ecosystem in Antarctica

“…In an Alpine freshwater meromictic lake (Lake Cadagno), a Chlorobium species was reported to have become dominant (by 2006), after replacing purple sulfur bacteria and other strains of GSB that had previously been abundant (before 2000) (Halm et al, 2009;Gregersen et al, 2009). However, the available data (Coolen et al, 2006) suggest that the same species has been dominant, if not unique, in the Ace Lake oxycline since its inception.…”

Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

“…3). Evidence for co-occurrence of denitrification and N 2 fixation has previously only been documented for an anoxic lake (Halm et al, 2009) and for cyanobacterial aggregates in the Baltic Sea (Klawonn et al, 2015). Recent investigations from Baltic Sea sediments on N 2 fixation and diazotrophic abundance in sediments show, however, that a very close spatial link between N loss and N 2 fixation might exist (Bertics et al, 2013).…”

Active nitrogen-fixing heterotrophic bacteria at and below the chemocline of the central Baltic Sea

“…Similarly, nifH genes and transcripts related to heterotrophs were reported from the Arabian Sea OMZ (Jayakumar et al, 2012). Interestingly, a recent study documented the co-occurrence of N 2 fixation and denitrification in a meromictic lake (Halm et al, 2009). Combined, these findings suggest that two central N transformation processes, thus far thought to be segregated, can co-occur in OMZs, challenging the conventional view of N cycling in the oceans and unveiling the intriguing possibility of NH 4 þ -rich suboxic and anoxic waters as environments for heterotrophic N 2 fixation.…”

“…Similarly, N 2 fixation has been measured off Peru and in a meromictic lake at measurable NH 4 þ levels (Halm et al, 2009;Fernandez et al, 2011). In the laboratory, NH 4 þ downregulates or shuts down nitrogenase expression (for example, Kanemoto and Ludden, 1984;Klugkist and Haaker, 1984).…”