Großkopf Tobias, Laroche Julie
Research Division 2: Marine Biogeochemistry, Helmholtz-Centre for Ocean Research Kiel GEOMAR, Kiel, Germany.
Front Microbiol. 2012 Jul 20;3:236. doi: 10.3389/fmicb.2012.00236. eCollection 2012.
The recent detection of heterotrophic nitrogen (N(2)) fixation in deep waters of the southern Californian and Peruvian OMZ questions our current understanding of marine N(2) fixation as a process confined to oligotrophic surface waters of the oceans. In experiments with Crocosphaera watsonii WH8501, a marine unicellular diazotrophic (N(2) fixing) cyanobacterium, we demonstrated that the presence of high nitrate concentrations (up to 800 μM) had no inhibitory effect on growth and N(2) fixation over a period of 2 weeks. In contrast, the environmental oxygen concentration significantly influenced rates of N(2) fixation and respiration, as well as carbon and nitrogen cellular content of C. watsonii over a 24-h period. Cells grown under lowered oxygen atmosphere (5%) had a higher nitrogenase activity and respired less carbon during the dark cycle than under normal oxygen atmosphere (20%). Respiratory oxygen drawdown during the dark period could be fully explained (104%) by energetic needs due to basal metabolism and N(2) fixation at low oxygen, while at normal oxygen these two processes could only account for 40% of the measured respiration rate. Our results revealed that under normal oxygen concentration most of the energetic costs during N(2) fixation (∼60%) are not derived from the process of N(2) fixation per se but rather from the indirect costs incurred for the removal of intracellular oxygen or by the reversal of oxidative damage (e.g., nitrogenase de novo synthesis). Theoretical calculations suggest a slight energetic advantage of N(2) fixation relative to assimilatory nitrate uptake, when oxygen supply is in balance with the oxygen requirement for cellular respiration (i.e., energy generation for basal metabolism and N(2) fixation). Taken together our results imply the existence of a niche for diazotrophic organisms inside oxygen minimum zones, which are predicted to further expand in the future ocean.
近期在加利福尼亚南部和秘鲁海洋缺氧区（OMZ）的深水中检测到异养固氮作用，这对我们目前将海洋固氮作用视为局限于海洋贫营养表层水的过程的理解提出了质疑。在对海洋单细胞固氮蓝细菌沃森球石藻（Crocosphaera watsonii）WH8501进行的实验中，我们证明，在长达2周的时间里，高硝酸盐浓度（高达800μM）的存在对其生长和固氮作用没有抑制作用。相反，在24小时内，环境氧浓度显著影响了沃森球石藻的固氮速率和呼吸速率，以及其细胞内的碳氮含量。在低氧环境（5%）下生长的细胞，其固氮酶活性更高，在黑暗周期中呼吸消耗的碳比在正常氧环境（20%）下更少。黑暗时期的呼吸氧消耗，在低氧条件下可完全由基础代谢和固氮作用的能量需求来解释（104%），而在正常氧条件下，这两个过程仅占所测呼吸速率的40%。我们的结果表明，在正常氧浓度下，固氮过程中大部分的能量消耗（约60%）并非源于固氮作用本身，而是源于去除细胞内氧气或逆转氧化损伤（如固氮酶重新合成）所产生的间接成本。理论计算表明，当氧气供应与细胞呼吸的氧气需求（即基础代谢和固氮作用的能量产生）达到平衡时，固氮作用相对于硝酸盐同化吸收具有轻微的能量优势。综合来看，我们的结果意味着在海洋缺氧区内存在一个适合固氮生物生存的生态位，预计在未来海洋中该区域还会进一步扩大。
