A transcriptional cycle suited to daytime N₂ fixation in the unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A)

Abstract

27The symbiosis between a marine alga and a N 2 -fixing cyanobacterium (UCYN-A) is 28 geographically widespread in the oceans and is important in the marine N cycle. UCYN-A is 29 uncultivated, and is an unusual unicellular cyanobacterium because it lacks many metabolic 30 functions, including oxygenic photosynthesis and carbon fixation, which are typical in 31 cyanobacteria. It is now presumed to be an obligate symbiont of haptophytes closely related to 32 Braarudosphaera bigelowii. N 2 -fixing cyanobacteria use different strategies to avoid inhibition of 33 N 2 fixation by the oxygen evolved in photosynthesis. Most unicellular cyanobacteria temporally 34 separate the two incompatible activities by fixing N 2 only at night, but surprisingly UCYN-A 35 appears to fix N 2 during the day. The goal of this study was to determine how the unicellular 36 UCYN-A coordinates N 2 fixation and general metabolism compared to other marine 37 cyanobacteria. We found that UCYN-A has distinct daily cycles of many genes despite the fact 38 that it lacks two of the three circadian clock genes found in most cyanobacteria. We also found 39 that transcription patterns in UCYN-A are most similar to marine cyanobacteria that are capable 40 of aerobic N 2 fixation in the light such as Trichodesmium and heterocyst-forming cyanobacteria, 41 rather than Crocosphaera or Cyanothece species, which are more closely related to unicellular 42 marine cyanobacteria evolutionarily. Our findings suggest that the symbiotic interaction has 43 resulted in a shift of transcriptional regulation to coordinate UCYN-A metabolism with the 44 phototrophic eukaryotic host, thus allowing efficient coupling of N 2 fixation (by the 45 cyanobacterium) to the energy obtained from photosynthesis (by the eukaryotic unicellular alga) 46 in the light. 47 48 49 50 3 51 Importance 52The symbiotic N 2 -fixing cyanobacterium UCYN-A and its eukaryotic algal host, which is 53 closely related to Braarudosphaera bigelowii, have been shown to be globally distributed and 54 important in open ocean N 2 fixation. These unique cyanobacteria have reduced metabolic 55 capabilities, even lacking genes for oxygenic photosynthesis and carbon fixation. Cyanobacteria 56 generally use energy from photosynthesis for nitrogen fixation, but require mechanisms for 57 avoiding inactivation of the oxygen-sensitive nitrogenase enzyme by ambient oxygen (O 2 ) or the 58 O 2 evolved through photosynthesis. This study shows that the symbiosis between the N 2 -fixing 59 cyanobacterium UCYN-A and its eukaryotic algal host has led to adaptation of its daily gene 60 expression pattern in order to enable daytime aerobic N 2 fixation, which is likely more 61 energetically efficient than fixing N 2 at night, as in other unicellular marine cyanobacteria. 62 63 64Nitrogen (N 2 )-fixing microorganisms (diazotrophs), which reduce atmospheric N 2 to 65 biologically available ammonium, are critical components of aquatic and terrestrial ecosystems 66 because they supply fixed inorganic N (1). Cyanobacteria are part...

Combined pigment and metatranscriptomic analysis reveals highly synchronized diel patterns of phenotypic light response across domains in the open oligotrophic ocean

“…Transcripts for most pathways in this diazotroph only showed a weak relationship or no significant positive correlation with the pathways in the other phytoplankton populations. However, this is in agreement with other culture [68] and field-based observations [15,69]. The process of N 2 fixation in this unicellular cyanobacteria seems to require a tightly orchestrated diel cycle distinct from other diazotrophs and nondiazotrophic microorganisms [15,69].…”

Processes and Microorganisms Involved in the Marine Nitrogen Cycle: Knowledge and Gaps

“…Several mechanisms have been considered, such as its nitrogenase is confined to specialized cells called diazocytes (Bergman and Carpenter, 1991), the photoreduction of O 2 to H 2 O in the photosystem I (the Mehler reaction) (Milligan et al, 2007), the uncoupling of CO 2 and N 2 fixation in cyanophycin granules (Finzi-Hart et al, 2009), or the down-regulation of photosynthesis during the period of maximum nitrogenase activity at midday (Berman-Frank et al, 2001b), which suggests that light may also be a determinant factor for BNF. Interestingly, the symbiosis between UCYN-A (which lacks genes for oxygenic photosynthesis and C fixation, and its algal host has led to enable daytime BNF (Muñoz-Marín et al, 2018), although nifH expression of UCYN-A has also been observed at night (Moisander et al, 2014).…”

Hopanoid lipids may facilitate aerobic nitrogen fixation in the ocean

“…Furthermore, in Crocosphaera and Cyanothece , the transcription of the shc gene peaks right before the nitrogenase-encoding gene ( nifH ) starts increasing its expression level (data collected from ref. 20), and simultaneous expression of both markers has also been detected in UCYN-A (21). These patterns are further supported by previous observations of hopanoid production in the cyanobacterium Crocosphaera watsonii WH8501 in the context of N 2 fixation (22, 23).…”

“…Asterisk (*), experimentally tested in refs. 20 and 21 (n.t., not tested). Enzymes participating in hopanoid pathways: squalene−hopene cyclase (SHC), hopanoid biosynthesis-associated radical SAM protein (HpnH), hopanoid-associated phosphorylase (HpnG), hopanoid biosynthesis-associated glycosyltransferase protein (HpnI), hopanoid biosynthesis-associated protein (HpnK), hopanoid biosynthesis-associated radical SAM protein (HpnJ), aminotransferase (HpnO), hopanoid 2-methyltransferase (HpnP), and hopanoid C3 methylase (HpnR); 3-methylhopanoid production has never been found in marine cyanobacteria (28); hpnO was absent in all of the screened strains.…”