The cyanobacterium Mastigocladus fulfills the nitrogen demand of a terrestrial hot spring microbial mat

Abstract

Research (CR)2, Santiago, ChileCyanobacteria from Subsection V (Stigonematales) are important components of microbial mats in non-acidic terrestrial hot springs. Despite their diazotrophic nature (N 2 fixers), their impact on the nitrogen cycle in such extreme ecosystems remains unknown. Here, we surveyed the identity and activity of diazotrophic cyanobacteria in the neutral hot spring of Porcelana (Northern Patagonia, Chile) during 2009 and 2011-2013. We used 16S rRNA and the nifH gene to analyze the distribution and diversity of diazotrophic cyanobacteria. Our results demonstrate the dominance of the heterocystous genus Mastigocladus (Stigonematales) along the entire temperature gradient of the hot spring (69-38°C). In situ nitrogenase activity (acetylene reduction), nitrogen fixation rates (cellular uptake of 15 N 2 ) and nifH transcription levels in the microbial mats showed that nitrogen fixation and nifH mRNA expression were light-dependent. Nitrogen fixation activities were detected at temperatures ranging from 58°C to 46°C, with maximum daily rates of 600 nmol C 2 H 4 cm − 2 per day and 94.1 nmol N cm − 2 per day. These activity patterns strongly suggest a heterocystous cyanobacterial origin and reveal a correlation between nitrogenase activity and nifH gene expression during diurnal cycles in thermal microbial mats. N and C fixation in the mats contributed~3 g N m − 2 per year and 27 g C m − 2 per year, suggesting that these vital demands are fully met by the diazotrophic and photoautotrophic capacities of the cyanobacteria in the Porcelana hot spring.

Active Crossfire Between Cyanobacteria and Cyanophages in Phototrophic Mat Communities Within Hot Springs

“…Nucleic acids (DNA and RNA) were extracted as previously described ( Alcamán et al, 2015 ). For RNA, Trizol (Invitrogen, Carlsbad, CA, United States) was added to the mat sample, and homogenized by bead beating, two pulses of 20 s. Quality and quantity of the extracted nucleic acids were checked and kept at -80°C.…”

“…( Steunou et al, 2006 , 2008 ; Bhaya et al, 2007 ; Klatt et al, 2011 ), the filamentous non-heterocystous Oscillatoria spp., the filamentous heterocystous Mastigocladus spp. ( Stewart, 1970 ; Miller et al, 2006 ; Mackenzie et al, 2013 ; Alcamán et al, 2015 ), as well as filamentous anoxygenic phototrophs (FAPs), such as Roseiflexus sp. and Chloroflexus sp.…”

“…These primary producers interact with heterotrophic prokaryotes through element and energy cycling ( Klatt et al, 2013 ). Heterocystous cyanobacteria are a key component in hot springs, since these systems are commonly N-limited due to the rapid assimilation and turnover of inorganic nitrogen forms ( Alcamán et al, 2015 ; Lin et al, 2015 ). Thus, N 2 -fixation by cyanobacteria is identified to be a key biological process in neutral hot spring microbial mats ( Alcamán et al, 2015 ).…”

“…Given the lack of knowledge of viral communities within hot spring phototrophic microbial mats, the present study used the mats of Porcelana hot spring (Northern Patagonia, Chile), as a pH neutral model, to better understand the associated thermophilic viral communities within these mats. This pristine spring is covered by microbial mats that grow along a thermal gradient between 70 and 46°C, dominated by bacterial phototrophs, such as filamentous cyanobacteria from the genus Mastigocladus ( Mackenzie et al, 2013 ; Alcamán et al, 2015 ). This is the dominant and most active cyanobacterial genus in the Porcelana mat environment, carrying out important biological processes such as carbon- and N 2 -fixation ( Alcamán et al, 2015 , 2017 ).…”

“…This pristine spring is covered by microbial mats that grow along a thermal gradient between 70 and 46°C, dominated by bacterial phototrophs, such as filamentous cyanobacteria from the genus Mastigocladus ( Mackenzie et al, 2013 ; Alcamán et al, 2015 ). This is the dominant and most active cyanobacterial genus in the Porcelana mat environment, carrying out important biological processes such as carbon- and N 2 -fixation ( Alcamán et al, 2015 , 2017 ). Thus, this study proposes that the mats in Porcelana hot spring are dominated by viral communities of the Order Caudovirales, which is able to infect Cyanobacteria, preferably Mastigocladus spp.…”