Heterotrophic bacteria as major nitrogen fixers in the euphotic zone of the Indian Ocean

Abstract

Diazotrophy in the Indian Ocean is poorly understood compared to that in the Atlantic and Pacific Oceans. We first examined the basin-scale community structure of diazotrophs and their nitrogen fixation activity within the euphotic zone during the northeast monsoon period along about 69°E from 17°N to 20°S in the oligotrophic Indian Ocean, where a shallow nitracline (49-59 m) prevailed widely and the sea surface temperature (SST) was above 25°C. Phosphate was detectable at the surface throughout the study area. The dissolved iron concentration and the ratio of iron to nitrate + nitrite at the surface were significantly higher in the Arabian Sea than in the equatorial and southern Indian Ocean. Nitrogen fixation in the Arabian Sea (24.6-47.1 μmolN m À2 d À1 ) was also significantly greater than that in the equatorial and southern Indian Ocean (6.27-16.6 μmolN m À2 d À1 ), indicating that iron could control diazotrophy in the Indian Ocean. Phylogenetic analysis of nifH showed that most diazotrophs belonged to the Proteobacteria and that cyanobacterial diazotrophs were absent in the study area except in the Arabian Sea. Furthermore, nitrogen fixation was not associated with light intensity throughout the study area. These results are consistent with nitrogen fixation in the Indian Ocean, being largely performed by heterotrophic bacteria and not by cyanobacteria. The low cyanobacterial diazotrophy was attributed to the shallow nitracline, which is rarely observed in the Pacific and Atlantic oligotrophic oceans. Because the shallower nitracline favored enhanced upward nitrate flux, the competitive advantage of cyanobacterial diazotrophs over nondiazotrophic phytoplankton was not as significant as it is in other oligotrophic oceans.

N2 Fixation in the Eastern Arabian Sea: Probable Role of Heterotrophic Diazotrophs

“…N 2 fixation is controlled by iron and PO 3− 4 availability (Wu et al, 2000;Capone, 2001;Sañudo-Wilhelmy et al, 2001;Mulholland, 2007). At these sampling locations, N 2 fixation may be regulated by iron input as PO 3− 4 is not the limiting nutrient in the northern Indian Ocean as suggested by P * -values (Shiozaki et al, 2014). Iron limitation is likely, as some part of the western Arabian Sea has been highlighted as a high nutrient low chlorophyll (HNLC) region (Naqvi et al, 2010;Moffett et al, 2015).…”

“…Frontiers in Marine Science | www.frontiersin.orgin the surface water of the eastern Arabian Sea (Shiozaki et al, 2014). Euphotic-depth integrated autotrophic N 2 fixation and carbon uptake rates were highest at NF-a, where N 2 fixation rate was 8.4 ± 2.8 mmol N m −2 d −1 (standard deviation of duplicate samples) and carbon uptake rate was 74.7 ± 17.7 mmol C m −2 d −1 .…”

Basin scale variability of active diazotrophs and nitrogen fixation in the North Pacific, from the tropics to the subarctic Bering Sea

“…While preferential PCR amplification of γ ‐24774A11 [ Turk et al ., ] may have caused the discrepancy between amplicon library and qPCR data, both methods point to a prominent role of this group in the diazotrophic communities of the tropical and subtropical oligotrophic North Pacific. The nifH gene sequence of γ ‐24774A11 has earlier also been retrieved from the subtropical regions of South Pacific, Indian, and Atlantic Oceans [ Moisander et al ., ; Shiozaki et al ., ; Langlois et al ., ]. Although the factors limiting their distribution are not well understood, it seems that they prefer warmer oligotrophic waters, and we confirm this notion by Langlois et al .…”

Nitrogen Fixation by Trichodesmium and unicellular diazotrophs in the northern South China Sea and the Kuroshio in summer

“…ARA = acetylene reduction assay.LocationSurface N 2 fixation rate (nmol N L −1 d −1 )MethodDominant Diazotroph (s)ReferencenSCS and ECS Kuroshio1.14–10.40ARAUnicellular diazotrophsThis studynSCS coastal upwelling0.1–5.6 15 N 2 gas bubbleUnknow 28 SCS (Mekong River plume)0.59–22.77 15 N 2 gas bubble Trichodesmium spp. and DDAs 63 Western North Pacific≈0.3–22ARAUnicellular diazotrophs 64 North Pacific0.360–3.05 15 N 2 gas bubbleHeterotrophic bacteria 65 Northeast Atlantic<0.4ARAUnicellular diazotrophs 66 Eastern North Atlantic0–151.2 15 N 2 gas bubble Trichodesmium and UCYN-A 67 Arabian Sea0.8–225 15 N 2 gas dissolution Trichodesmium bloom 68 Indian Ocean0.18–1.27 15 N 2 gas bubbleHeterotrophic bacteria 69 …”