Cyanobacterial nitrogen-fixers (diazotrophs) play a key role in biogeochemical cycling of carbon and nitrogen in the ocean. In recent years, the unusual symbiotic diazotrophic cyanobacterium Ateloc ...

/pdf/new-insights-into-the-ecology-of-the-globally-significant-1aaf0wsqoo.pdf

New insights into the ecology of the globally significant uncultured nitrogen-fixing symbiont UCYN-A

. Trichodesmium, a major colonial cyanobacterial nitrogen fixer, forms large blooms in NO3-depleted tropical oceans and enhances CO2 sequestration by the ocean due to its ability to fix dissolved dinitrogen. Thus, its importance in C and N cycles requires better estimates of its distribution at basin to global scales. However, existing algorithms to detect them from satellite have not yet been successful in the South Western Tropical Pacific (SP). Here, a novel algorithm (TRICHOdesmium SATellite) based on radiance anomaly spectra (RAS) observed in SeaWiFS imagery, is used to detect Trichodesmium during the austral summertime in the SP (5° S–25° S 160° E–170° W). Selected pixels are characterized by a restricted range of parameters quantifying RAS spectra (e.g. slope, intercept, curvature). The fraction of valid (non-cloudy) pixels identified as Trichodesmium surface blooms in the region is low (between 0.01 and 0.2 %), but is about 100 times higher than deduced from previous algorithms. At daily scales in the SP, this fraction represents a total ocean surface area varying from 16 to 48 km2 in Winter and from 200 to 1000 km2 in Summer (and at monthly scale, from 500 to 1000 km2 in Winter and from 3100 to 10 890 km2 in Summer with a maximum of 26 432 km2 in January 1999). The daily distribution of Trichodesmium surface accumulations in the SP detected by TRICHOSAT is presented for the period 1998–2010 which demonstrates that the number of selected pixels peaks in November–February each year, consistent with field observations. This approach was validated with in situ observations of Trichodesmium surface accumulations in the Melanesian archipelago around New Caledonia, Vanuatu and Fiji Islands for the same period.

/pdf/an-algorithm-for-detecting-trichodesmium-surface-blooms-in-43eh1v84z3.pdf

An algorithm for detecting Trichodesmium surface blooms in the South Western Tropical Pacific

. Dinitrogen (N2) fixation rates were measured during early spring across the different provinces of Mediterranean Sea surface waters. N2 fixation rates, measured using 15N2 enriched seawater, were lowest in the eastern basin and increased westward with a maximum at the Strait of Gibraltar (0.10 to 2.35 nmol N L−1 d−1, respectively). These rates were 3–7 fold higher than N2 fixation rates measured previously in the Mediterranean Sea during summertime and we estimated that methodological differences alone did not account for the seasonal changes we observed. Higher contribution of N2 fixation to primary production (4–8%) was measured in the western basin compared to the eastern basin (∼2%). Our data indicates that these differences between basins may be attributed to changes in N2-fixing planktonic communities and that heterotrophic diazotrophy may play a significant role in the eastern Mediterranean while autotrophic diazotrophy has a more dominant role in the western basin.

/pdf/springtime-contribution-of-dinitrogen-fixation-to-primary-2kc186mazr.pdf

Springtime contribution of dinitrogen fixation to primary production across the Mediterranean Sea

Dinitrogen (N-2) fixation rates were investigated in the euphotic layer of the Bismarck and Solomon Seas using N-15(2) incubation assays taking into account both the particulate and the dissolved pools. Average depth-integrated particulate N-2 fixation rates were 203 (range 43-399) and 1396 (range 176-3132)mol N m(-2) d(-1) in the Bismarck and Solomon Seas, respectively. In both seas, N-2 fixation measured in the dissolved pool was similar to particulate N-2 fixation, highlighting the potentially substantial underestimation of N-2 fixation in oceanic budgets when only particulate N-2 fixation is considered. Among the diazotroph phylotypes targeted using quantitative polymerase chain reaction amplification of nifH genes, Trichodesmium was the most abundant. Regression analyses suggest that it accounted for the major proportion of N-2 fixation. However, unicellular cyanobacterial and non-cyanobacterial diazotrophs were also occasionally abundant. This study reports high pelagic N-2 fixation rates and confirms that the Western Tropical South Pacific is a hot spot for marine N-2 fixation.

https://hal-amu.archives-ouvertes.fr/hal-01622028/document

High Nitrogen fixation rates in the particulate and dissolved pools in the Western Tropical Pacific (Solomon and Bismarck Seas)

. Dinitrogen fixation (NF) by marine cyanobacteria is an important pathway to
replenish the oceanic bioavailable nitrogen inventory. Light is the key to
modulating NF; however, field studies investigating the light response curve (NF-I
curve) of NF rate and the effect of light on diazotroph-derived nitrogen
(DDN) net release are relatively sparse in the literature, hampering
prediction using models. A dissolution method was applied using
uncontaminated 15 N 2 gas to examine how the light changes may
influence the NF intensity and DDN net release in the oligotrophic ocean.
Experiments were conducted at stations with diazotrophs dominated by
filamentous cyanobacterium Trichodesmium spp. in the western Pacific
and the South China Sea. The effect of light on carbon fixation (CF) was
measured in parallel using the 13 C tracer method specifically for a
station characterized by Trichodesmium bloom. Both NF-I and CF-I
curves showed a Ik (light saturation coefficient) range of 193 to
315  µ E m −2  s −1 , with light saturation at around
400  µ E m −2  s −1 . The proportion of DDN net release
ranged from ∼  6 to ∼  50 %, suggesting an increasing trend as
the light intensity decreased. At the Trichodesmium bloom station,
we found that the CF  ∕  NF ratio was light-dependent and the ratio started to
increase as light was lower than the carbon compensation point of
200  µ E m −2  s −1 . Under low-light stress,
Trichodesmium physiologically preferred to allocate more energy for
CF to alleviate the intensive carbon consumption by respiration; thus, there
is a metabolism tradeoff between CF and NF pathways.
Results showed that short-term (  24 h) light change modulates the
physiological state, which subsequently determined the C  ∕  N metabolism
and DDN net release by Trichodesmium. Reallocation of energy
associated with the variation in light intensity would be helpful for
prediction of the global biogeochemical cycle of N by models involving
Trichodesmium blooms.

/pdf/effect-of-light-on-n-2-fixation-and-net-nitrogen-release-of-3xc0atfsis.pdf

Effect of light on N 2 fixation and net nitrogen release of Trichodesmium in a field study

. Using the 15N2 tracer method and high-sensitivity δ15N analytical systems, we determined N2 fixation rates for ocean samples by dividing them into particulate (>0.7 μm) and filtrate (

/pdf/determination-of-total-n-2-fixation-rates-in-the-ocean-1kmbwg28t2.pdf

Determination of total N 2 fixation rates in the ocean taking into account both the particulate and filtrate fractions

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Determination of total N 2 fixation rates in the ocean taking into account both the particulate and filtrate fractions