Zeev Edo Bar, Yogev Tali, Man-Aharonovich Dikla, Kress Nurit, Herut Barak, Béjà Oded, Berman-Frank Ilana
Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat Gan, Israel.
ISME J. 2008 Sep;2(9):911-23. doi: 10.1038/ismej.2008.56. Epub 2008 Jun 26.
Biological nitrogen fixation has been suggested as an important source of nitrogen for the ultra-oligotrophic waters of the Levantine Basin of the Mediterranean Sea. In this study, we identify and characterize the spatial and temporal distribution of the N-fixing (diazotrophic) cyanobacterium Richelia intracellularis. R. intracellularis is usually found as an endosymbiont within diatoms such as Rhizosolenia spp and Hemiaulus spp. and is an important diazotroph in marine tropical oceans. In this study, two stations off the Mediterranean coast of Israel were sampled monthly during 2005-2007. R. intracellularis was identified by microscopy and by reverse transcribed-PCR which confirmed a 98.8% identity with known nifH sequences of R. intracellularis from around the world. The diatom-diazotroph associations were found throughout the year peaking during autumn (October-November) at both stations. Abundance of R. intracellularis ranged from 10 to 55 heterocysts l(-1) and correlated positively with the dissolved Si(OH)(4)/(NO(3)+NO(2)) ratio in surface waters. Although the rates of nitrogen fixation were very low, averaging approximately 1.1 nmol N l(-1) day(-1) for the R. intracellularis size fraction (>10 microm) from surface waters, they correlated positively with heterocyst counts during thermal stratification. The lack of large-scale diatom-diazotroph blooms and the low rates of nitrogen fixation by these diazotrophs may result from the P-starved conditions affecting the Levantine basin.
生物固氮被认为是地中海黎凡特盆地超寡营养水域氮的重要来源。在本研究中，我们鉴定并描述了固氮（固氮营养型）蓝藻内共生黎氏藻的时空分布特征。内共生黎氏藻通常作为硅藻（如根管藻属和半管藻属）内的一种内共生体被发现，并且是热带海洋中一种重要的固氮生物。在本研究中，2005年至2007年期间每月对以色列地中海沿岸的两个站点进行采样。通过显微镜检查和逆转录PCR鉴定出内共生黎氏藻，该方法证实其与来自世界各地的已知内共生黎氏藻nifH序列有98.8%的同源性。硅藻 - 固氮生物的共生关系全年都有发现，在两个站点秋季（10月至11月）达到峰值。内共生黎氏藻的丰度范围为10至55个异形胞/升，并与表层水中溶解的Si(OH)(4)/(NO(3)+NO(2))比值呈正相关。尽管固氮速率非常低，表层水中内共生黎氏藻大小级分（>10微米）的平均固氮速率约为1.1纳摩尔氮/升·天，但在热分层期间它们与异形胞数量呈正相关。这些固氮生物缺乏大规模的硅藻 - 固氮生物水华以及固氮速率低可能是由于影响黎凡特盆地的磷饥饿条件所致。
