Brevetoxin biosynthesis in Karenia brevis in relation to light, growth phase, and across the cell cycle

Karenia brevis synthesizes picograms of brevetoxins per cell and regularly produces extensive blooms with cell concentrations exceeding 1 million cells L^-1. Biological population growth requires repeated cellular biosynthesis and cell cycling, however, it is not yet known how molecular biosynthesis of brevetoxins might be tied to cellular biosynthesis and the cell cycle. Here, using laboratory cultures of K. brevis, we report two complementary experiments exploring how brevetoxin biosynthesis is linked to cellular biosynthesis (proxied through cell biovolume, growth rates, DNA, and chlorophyll-a). The first experiment assessed brevetoxin quotas, cell physiology, and the cell cycle over 27 h. The second assessed brevetoxin quotas and RuBisCO transcript expression (rbcL) in response to growth phase and light limitation/supersaturation. Multiple lines of evidence revealed relatively synchronous (∼55-60% of the population) mitotic cell division which occurred within a 4.5 h period before 'dawn'. Following division, cell biovolume increased throughout the day, reaching maximum at 'dusk'. Different from biovolume, brevetoxin quotas slightly increased late in the daytime coinciding with entry into S-phase, and reached a maximum during the night before division. Brevetoxin quotas returned to G1 phase levels following cytokinesis. In the second experiment, only growth phase, not light level, affected cellular brevetoxin quotas, showing that decreased growth rates do not necessitate increased brevetoxin quotas. Coherent with the diel cell cycle experiment, it is suggested that brevetoxin quotas are regulated by processes also tied to cell cycle progression. Interestingly, a positive relationship between brevetoxin quotas and rbcL expression hinted at a link between the pyrenoid and brevetoxin production.