Mondal Sumana, Biswas Biswajit, Chowdhury Rajojit, Sengupta Rudranil, Mandal Anup, Kotal Hemendra Nath, Giri Chayan Kumar, Ghosh Anjali, Saha Subhajit, Begam Mst Momtaj, Mukherjee Chandan, Das Ipsita, Basak Sandip Kumar, Mitra Ghosh Mahashweta, Ray Krishna
Environmental Biotechnology Group, Department of Botany, West Bengal State University, Kolkata, India.
Department of Microbiology, St. Xavier's College (Autonomous), Kolkata, India.
Front Microbiol. 2024 May 24;15:1324188. doi: 10.3389/fmicb.2024.1324188. eCollection 2024.
Biological nitrogen fixation (BNF), an unparalleled metabolic novelty among living microorganisms on earth, globally contributes ~88-101 Tg N year to natural ecosystems, ~56% sourced from symbiotic BNF while ~22-45% derived from free-living nitrogen fixers (FLNF). The success of symbiotic BNF is largely dependent on its interaction with host-plant, however ubiquitous environmental heterotrophic FLNFs face many limitations in their immediate ecological niches to sustain unhindered BNF. The autotrophic FLNFs like cyanobacteria and oceanic heterotrophic diazotrophs have been well studied about their contrivances acclimated/adapted by these organisms to outwit the environmental constraints for functional diazotrophy. However, FLNF heterotrophs face more adversity in executing BNF under stressful estuarine/marine/aquatic habitats.
In this study a large-scale cultivation-dependent investigation was accomplished with 190 NCBI accessioned and 45 non-accessioned heterotrophic FLNF cultivable bacterial isolates (total 235) from halophilic estuarine intertidal mangrove niches of Indian Sundarbans, a Ramsar site and UNESCO proclaimed World Heritage Site. Assuming ~1% culturability of the microbial community, the respective niches were also studied for representing actual bacterial diversity via cultivation-independent next-generation sequencing of V3-V4 rRNA regions.
Both the studies revealed a higher abundance of culturable Gammaproteobacteria followed by Firmicutes, the majority of 235 FLNFs studied belonging to these two classes. The FLNFs displayed comparable selection potential in media for free nitrogen fixers and iron-oxidizing bacteria, linking diazotrophy with iron oxidation, siderophore production, phosphorus solubilization, phosphorus uptake and accumulation as well as denitrification.
This observation validated the hypothesis that under extreme estuarine mangrove niches, diazotrophs are naturally selected as a specialized multidimensional entity, to expedite BNF and survive. Earlier metagenome data from mangrove niches demonstrated a microbial metabolic coupling among C, N, P, S, and Fe cycling in mangrove sediments, as an adaptive trait, evident with the co-abundant respective functional genes, which corroborates our findings in cultivation mode for multiple interrelated metabolic potential facilitating BNF in a challenging intertidal mangrove environment.
生物固氮是地球上活微生物中一项无与伦比的代谢新奇特性，全球每年为自然生态系统贡献约88 - 101太克氮，其中约56%来自共生生物固氮，约22 - 45%来自自由生活的固氮菌（FLNF）。共生生物固氮的成功很大程度上依赖于其与宿主植物的相互作用，然而无处不在的环境异养型FLNF在其直接生态位中面临许多限制，难以维持不受阻碍的生物固氮。像蓝细菌和海洋异养固氮菌这样的自养型FLNF，它们为克服环境限制以实现功能性固氮所采用的策略已得到充分研究。然而，异养型FLNF在压力较大的河口/海洋/水生生境中进行生物固氮时面临更多逆境。
在本研究中，对来自印度孙德尔本斯（一个拉姆萨尔湿地和联合国教科文组织宣布的世界遗产地）的嗜盐河口潮间带红树林生态位的190个已提交至NCBI的和45个未提交的可培养异养型FLNF细菌分离株（共235个）进行了大规模的基于培养的调查。假设微生物群落的可培养性约为1%，还通过对V3 - V4 rRNA区域进行不依赖培养的下一代测序，研究了各个生态位以代表实际的细菌多样性。
两项研究均表明，可培养的γ-变形菌纲丰度较高，其次是厚壁菌门，所研究的235个FLNF中的大多数属于这两个类别。FLNF在用于自由固氮菌和铁氧化细菌的培养基中显示出相当的选择潜力，将固氮作用与铁氧化、铁载体产生、磷溶解、磷吸收和积累以及反硝化作用联系起来。
这一观察结果验证了以下假设，即在极端的河口红树林生态位中，固氮菌被自然选择为一个专门的多维实体，以加速生物固氮并生存。早期来自红树林生态位的宏基因组数据表明，红树林沉积物中碳、氮、磷、硫和铁循环之间存在微生物代谢耦合，作为一种适应性特征，在各自共丰度的功能基因中很明显，这证实了我们在培养模式下的研究结果，即在具有挑战性的潮间带红树林环境中，多种相互关联的代谢潜力促进了生物固氮。
