Coban Oksana, Rasigraf Olivia, de Jong Anniek E E, Spott Oliver, Bebout Brad M
Exobiology Branch, NASA Ames Research Center, Moffett Field, CA, USA
Hydrology and Quantitative Water Management Group, Wageningen University and Research, Wageningen, The Netherlands.
Appl Environ Microbiol. 2021 Apr 15;87(8). doi: 10.1128/AEM.03118-20. Epub 2021 Feb 12.
Microbial mats, due to stratification of the redox zones, have a potential to include a complete N cycle, however an attempt to evaluate a complete N cycle in these ecosystems has not been yet made. In this study, occurrence and rates of major N cycle processes were evaluated in intact microbial mats from Elkhorn Slough, Monterey Bay, CA, USA, and Baja California Sur, Mexico under oxic and anoxic conditions using N-labeling techniques. All of the major N transformation pathways, with the exception of anammox, were detected in both microbial mats. Nitrification rates were found to be low at both sites for both seasons investigated. The highest rates of ammonium assimilation were measured in Elkhorn Slough mats in April and corresponded to high ammonium concentration in the overlying water. Baja mats featured higher ammonification than ammonium assimilation rates and this, along with their higher affinity for nitrate compared to ammonium and low dissimilatory nitrate reduction to ammonium rates, characterized their differences from Elkhorn Slough mats. Nitrogen fixation rates in Elkhorn Slough microbial mats were found to be low implying that other processes such as recycling and assimilation from water are main sources of N for these mats at the times sampled. Denitrification in all of the mats was incomplete with nitrous oxide as end product and not dinitrogen. Our findings highlight N cycling features not previously quantified in microbial mats and indicate a need of further investigations in these microbial ecosystems. Nitrogen is essential for life. The nitrogen cycle on Earth is mediated by microbial activity and has had a profound impact on both the atmosphere and the biosphere throughout geologic time. Microbial mats, present in many modern environments, have been regarded as living records of the organisms, genes, and phylogenies of microbes, as they are one of the most ancient ecosystems on Earth. While rates of major nitrogen metabolic pathways have been evaluated in a number of ecosystems, it remains elusive in microbial mats. In particular it is unclear what factors affect nitrogen cycling in these ecosystems and how morphological differences between mats impact nitrogen transformations. In this study we investigate nitrogen cycling in two microbial mats having morphological differences. Our findings provide insight for further understanding of biogeochemistry and microbial ecology of microbial mats.
由于氧化还原区的分层，微生物席有可能包含完整的氮循环，然而尚未有人尝试评估这些生态系统中的完整氮循环。在本研究中，利用氮标记技术，在美国加利福尼亚州蒙特雷湾的埃尔克霍恩湿地以及墨西哥南下加利福尼亚州完整的微生物席中，评估了在有氧和缺氧条件下主要氮循环过程的发生情况和速率。在这两种微生物席中均检测到了除厌氧氨氧化之外的所有主要氮转化途径。在所调查的两个季节中，两个地点的硝化速率均较低。4月在埃尔克霍恩湿地的微生物席中测得的铵同化率最高，且与上覆水中的高铵浓度相对应。南下加利福尼亚州的微生物席的氨化作用高于铵同化作用，并且与埃尔克霍恩湿地的微生物席相比，其对硝酸盐的亲和力更高，异化硝酸盐还原为铵的速率较低，这些特点体现了它们之间的差异。在埃尔克霍恩湿地的微生物席中发现固氮速率较低，这意味着在采样时，诸如从水中回收和同化等其他过程是这些微生物席的主要氮源。所有微生物席中的反硝化作用均不完全，最终产物是一氧化二氮而非氮气。我们的研究结果突出了微生物席中以前未被量化的氮循环特征，并表明需要对这些微生物生态系统进行进一步研究。氮是生命所必需的。地球上的氮循环由微生物活动介导，在整个地质时期对大气和生物圈都产生了深远影响。微生物席存在于许多现代环境中，由于它们是地球上最古老的生态系统之一，因此被视为微生物的生物体、基因和系统发育的活记录。虽然已经在许多生态系统中评估了主要氮代谢途径的速率，但在微生物席中仍然难以捉摸。特别是尚不清楚哪些因素影响这些生态系统中的氮循环，以及微生物席之间的形态差异如何影响氮转化。在本研究中，我们调查了两种具有形态差异的微生物席中的氮循环。我们的研究结果为进一步理解微生物席的生物地球化学和微生物生态学提供了见解。
