最近的研究表明，生物固氮在支持开阔海洋中贫营养区域的新生产中起关键作用。大型形成群落的藻青菌过去长期被认为是最重要的固氮者，但来自太平洋的新数据表明，小型单细胞藻青菌和浮游细菌是主要固氮者。这些以前被忽视的生物的种群大小可以有所不同，但当它们“人丁兴旺”时，它们对总生物固氮的贡献达一半以上。所以，我们可能需要对海洋氮循环及其与碳循环的相互作用进行一次重大的重新评估，将这些虽然小、但却很有影响的生物考虑进去。
High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific Ocean
The availability of nitrogen is important in regulating biological productivity in marine environments. Deepwater nitrate has long been considered the major source of new nitrogen supporting primary production in oligotrophic regions of the open ocean, but recent studies have showed that biological N2 fixation has a critical role in supporting oceanic new production1-7. Large colonial cyanobacteria in the genus Trichodesmium and the heterocystous endosymbiont Richelia have traditionally been considered the dominant marine N2 fixers, but unicellular diazotrophic cyanobacteria and bacterioplankton have recently been found in the picoplankton and nanoplankton community of the North Pacific central gyre, and a variety of molecular and isotopic evidence suggests that these unicells could make a major contribution to the oceanic N budget8. Here we report rates of N2 fixation by these small, previously overlooked diazotrophs that, although spatially variable, can equal or exceed the rate of N2 fixation reported for larger, more obvious organisms. Direct measurements of 15N2 fixation by small diazotrophs in various parts of the Pacific Ocean, including the waters off Hawaii where the unicellular diazotrophs were first characterized, show that N2 fixation by unicellular diazotrophs can support a significant fraction of total new production in oligotrophic waters.
Figure 1 Volumetric rate of N2 fixation by unicellular organisms in the North Pacific Ocean and near Australia. The area of each circle is proportional to the N2 fixation rate as shown (in nmol N l-1 h-1). Stations where no rate measurements were made are marked with diamonds. a, N2 fixation rates measured at ALOHA and during cruise Cook-25 (Jun - Jul 2002). Insets show occurrence of nifH (bands) as a function of depth in metres at selected stations. At all depths, clone libraries constructed by amplification with degenerate nifH primers were dominated by group A cyanobacterial sequences. b, Rates measured during cruise EW9912 (October to November 1999). Charts prepared using the Generic Mapping Tools20,21.
Figure 2 Time course of N2 fixation in incubations under simulated in situ conditions. Each symbol represents the integrated rate of N2 fixation between the start (arrow) and the endpoint of an incubation. a, Experiments performed in July 2000 (circles) and August 2001 (squares) with water from 25 m at ALOHA and an experiment performed in February 2002 (diamonds) with water collected just outside Kaneohe bay. Results are means s.d. for three replicates. b, Experiments at two stations in the Arafura Sea using water from the subsurface pigment maximum (50–60 m). Results are means s.d. for two replicates.
Figure 3 Summary of rate measurements as a function of depth at stations along a transect in the eastern subtropical Pacific from Hawaii to San Diego (cruise Cook-25). At station 15 (31° 32.21' N, 141° 15.66' W), N2 fixation was measured in water samples from five different depths through the mixed layer (open circles). At other stations, N2 fixation was measured in water collected either from the subsurface pigment maximum or from about 10 m depth (filled circles).
