AUTHOR=Alcamán-Arias María E. , Pedrós-Alió Carlos , Tamames Javier , Fernández Camila , Pérez-Pantoja Danilo , Vásquez Mónica , Díez Beatriz 

TITLE=Diurnal Changes in Active Carbon and Nitrogen Pathways Along the Temperature Gradient in Porcelana Hot Spring Microbial Mat

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.02353

DOI=10.3389/fmicb.2018.02353

ISSN=1664-302X

ABSTRACT=<p>Composition, carbon and nitrogen uptake, and gene transcription of microbial mat communities in Porcelana neutral hot spring (Northern Chilean Patagonia) were analyzed using metagenomics, metatranscriptomics and isotopically labeled carbon (H<sup>13</sup>CO<sub>3</sub>) and nitrogen (<sup>15</sup>NH<sub>4</sub>Cl and K<sup>15</sup>NO<sub>3</sub>) assimilation rates. The microbial mat community included 31 phyla, of which only <italic>Cyanobacteria</italic> and <italic>Chloroflexi</italic> were dominant. At 58°C both phyla co-occurred, with similar contributions in relative abundances in metagenomes and total transcriptional activity. At 66°C, filamentous anoxygenic phototrophic <italic>Chloroflexi</italic> were &gt;90% responsible for the total transcriptional activity recovered, while <italic>Cyanobacteria</italic> contributed most metagenomics and metatranscriptomics reads at 48°C. According to such reads, phototrophy was carried out both through oxygenic photosynthesis by <italic>Cyanobacteria</italic> (mostly <italic>Mastigocladus</italic>) and anoxygenic phototrophy due mainly to <italic>Chloroflexi</italic>. Inorganic carbon assimilation through the Calvin–Benson cycle was almost exclusively due to <italic>Mastigocladus</italic>, which was the main primary producer at lower temperatures. Two other CO<sub>2</sub> fixation pathways were active at certain times and temperatures as indicated by transcripts: 3-hydroxypropionate (3-HP) bi-cycle due to <italic>Chloroflexi</italic> and 3-hydroxypropionate-4-hydroxybutyrate (HH) cycle carried out by <italic>Thaumarchaeota</italic>. The active transcription of the genes involved in these C-fixation pathways correlated with high <italic>in situ</italic> determined carbon fixation rates. <italic>In situ</italic> measurements of ammonia assimilation and nitrogen fixation (exclusively attributed to <italic>Cyanobacteria</italic> and mostly to <italic>Mastigocladus</italic> sp.) showed these were the most important nitrogen acquisition pathways at 58 and 48°C. At 66°C ammonia oxidation genes were actively transcribed (mostly due to <italic>Thaumarchaeota</italic>). Reads indicated that denitrification was present as a nitrogen sink at all temperatures and that dissimilatory nitrate reduction to ammonia (DNRA) contributed very little. The combination of metagenomic and metatranscriptomic analysis with <italic>in situ</italic> assimilation rates, allowed the reconstruction of day and night carbon and nitrogen assimilation pathways together with the contribution of keystone microorganisms in this natural hot spring microbial mat.</p>