Abstract
Crocosphaera watsonii WH 8501 is a marine unicellular cyanobacterium that fixes nitrogen primarily during the dark phase of a light-dark (LD) cycle. Circadian clocks modulate gene transcription and cellular activity in many, if not all, cyanobacteria. A model for circadian control has been proposed in cyanobacteria, called the oscilloid model, which is based on topological changes of nucleoid DNA which in turn regulates gene transcription. In this study, the marine unicellular diazotrophic cyanobacteria C. watsonii WH 8501 and Cyanothece sp. ATCC 51142 were found to have daily fluctuations in DNA staining using Hoechst 33342 and SYBR I Green fluorescent dyes. Up to 20-fold decreases in DNA fluorescence of Hoechst-stained cells were observed during the dark phase when cultures were grown with a 12:12 LD cycle or under continuous light (LL). The variation in DNA staining was consistent with changes in DNA topology proposed in the oscilloid model. The abundance of nifH transcripts in C. watsonii WH 8501 was rhythmic under LD and LL cycles, consistent with a circadian rhythm. Cycles of DNA fluorescence and photosynthetic efficiency were disrupted when cultures were shifted into an early dark phase; however, nifH transcripts predictably increased in abundance following the premature transition from light to darkness. Thus, nifH gene expression in C. watsonii WH 8501 appears to be influenced by both circadian and environmental factors.
Highlights
Cyanobacteria are oxygenic phototrophic prokaryotes that are ubiquitous in the biosphere (Whitton and Potts, 2000)
N2 fixation was believed to be primarily due to filamentous nonheterocystous species (Trichodesmium) (Capone et al, 1997) until the relatively recent discovery of N2-fixing unicellular cyanobacteria in the open ocean (Zehr et al, 1998; 2001)
Studies using epifluorescence microscopy showed that S. elongatus PCC 7942 had diel changes in DNA fluorescence that appear to be linked to DNA topological changes that are regulated by the circadian clock (Mori et al, 1996; Smith and Williams, 2006)
Summary
Cyanobacteria are oxygenic phototrophic prokaryotes that are ubiquitous in the biosphere (Whitton and Potts, 2000). They play important roles in marine environments, and can be responsible for up to 50% of the primary production in surface waters (Chisholm et al, 1988). N2 fixation was believed to be primarily due to filamentous nonheterocystous species (Trichodesmium) (Capone et al, 1997) until the relatively recent discovery of N2-fixing unicellular cyanobacteria in the open ocean (Zehr et al, 1998; 2001). Unicellular cyanobacteria, including Crocosphaera watsonii WH 8501, have been shown to be quantitatively important in N2 fixation in tropical and subtropical oceans (Zehr et al, 2001; Falcón et al, 2004; Mazard et al, 2004; Montoya et al, 2004; Campbell et al, 2005; Church et al, 2005; Foster et al, 2007)
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