Ocean Sciences Department, University of California, Santa Cruz, CA, USA.
ISME J. 2011 Sep;5(9):1484-93. doi: 10.1038/ismej.2011.26. Epub 2011 Mar 31.
Many diatoms that inhabit low-nutrient waters of the open ocean live in close association with cyanobacteria. Some of these associations are believed to be mutualistic, where N(2)-fixing cyanobacterial symbionts provide N for the diatoms. Rates of N(2) fixation by symbiotic cyanobacteria and the N transfer to their diatom partners were measured using a high-resolution nanometer scale secondary ion mass spectrometry approach in natural populations. Cell-specific rates of N(2) fixation (1.15-71.5 fmol N per cell h(-1)) were similar amongst the symbioses and rapid transfer (within 30 min) of fixed N was also measured. Similar growth rates for the diatoms and their symbionts were determined and the symbiotic growth rates were higher than those estimated for free-living cells. The N(2) fixation rates estimated for Richelia and Calothrix symbionts were 171-420 times higher when the cells were symbiotic compared with the rates estimated for the cells living freely. When combined, the latter two results suggest that the diatom partners influence the growth and metabolism of their cyanobacterial symbionts. We estimated that Richelia fix 81-744% more N than needed for their own growth and up to 97.3% of the fixed N is transferred to the diatom partners. This study provides new information on the mechanisms controlling N input into the open ocean by symbiotic microorganisms, which are widespread and important for oceanic primary production. Further, this is the first demonstration of N transfer from an N(2) fixer to a unicellular partner. These symbioses are important models for molecular regulation and nutrient exchange in symbiotic systems.
许多生活在开阔海域贫营养水中的硅藻与蓝细菌密切相关。其中一些共生关系被认为是互利共生的，其中固氮蓝细菌共生体为硅藻提供氮。使用高分辨率纳米尺度二次离子质谱方法在自然种群中测量了共生蓝细菌的固氮速率和向其硅藻伙伴的氮转移。共生体中固氮的细胞特异性速率（1.15-71.5 fmol N 细胞-1 h-1）相似，并且还测量了固定氮的快速转移（在 30 分钟内）。还确定了硅藻与其共生体的相似生长速率，并且共生体的生长速率高于自由生活细胞的估计值。与自由生活的细胞相比，当细胞共生时，估计 Richelia 和 Calothrix 共生体的固氮速率分别高 171-420 倍。当结合这两个结果时，表明硅藻伙伴会影响其蓝细菌共生体的生长和代谢。我们估计 Richelia 固定的氮比其自身生长所需的氮多 81-744%，并且高达 97.3%的固定氮被转移到硅藻伙伴。这项研究提供了有关共生微生物控制开阔海域氮输入的机制的新信息，这些微生物广泛存在且对海洋初级生产力至关重要。此外，这是首次从固氮生物向单细胞伙伴转移氮的证明。这些共生体是共生系统中分子调节和营养交换的重要模型。
