Effect of light on N<sub>2</sub> fixation and net nitrogen release of <i>Trichodesmium</i> in a field study
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...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-01-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/1/2018/bg-15-1-2018.pdf |
| Summary: | 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 <sup>15</sup>N<sub>2</sub> 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 <i>Trichodesmium</i> spp. in the western Pacific
and the South China Sea. The effect of light on carbon fixation (CF) was
measured in parallel using the <sup>13</sup>C tracer method specifically for a
station characterized by <i>Trichodesmium </i>bloom. Both NF-I and CF-I
curves showed a <i>I</i><sub><i>k</i></sub> (light saturation coefficient) range of 193 to
315 µE m<sup>−2</sup> s<sup>−1</sup>, with light saturation at around
400 µE m<sup>−2</sup> s<sup>−1</sup>. The proportion of DDN net release
ranged from ∼ 6 to ∼ 50 %, suggesting an increasing trend as
the light intensity decreased. At the <i>Trichodesmium</i> 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 µE m<sup>−2</sup> s<sup>−1</sup>. Under low-light stress,
<i>Trichodesmium </i>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 <i>Trichodesmium</i>. 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
<i>Trichodesmium </i>blooms. |
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| ISSN: | 1726-4170 1726-4189 |