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Abstract
The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligotrophic oceans, the foundation of marine food webs, and an important component of global biogeochemical cycles. The prominence of Prochlorococcus across the environmental gradients of the open ocean is attributed to its extensive genetic diversity and flexible chlorophyll physiology, enabling light capture over a wide range of intensities. What remains unknown is the balance between temporal dynamics of genetic diversity and chlorophyll physiology in the ability of Prochlorococcus to respond to a variety of short (approximately one day) and longer (months to year) changes in the environment. Previous field research established depth-dependent Prochlorococcus single cell chlorophyll distributions in the North Pacific Subtropical Gyre. Here, we examined whether the shifts in chlorophyll distributions correspond to changes in Prochlorococcus genetic diversity (i.e. ecotype-level community structure) or photoacclimation of stable communities over short time intervals. We report that community structure was relatively stable despite abrupt shifts in Prochlorococcus chlorophyll physiology, due to unexpected physiological plasticity of high-light adapted Prochlorococcus ecotypes. Through comparison with seasonal-scale changes, our data suggest that variability on daily scales triggers shifts in Prochlorococcus photoacclimation, while seasonal-scale dynamics trigger shifts in community structure. Together, these data highlight the importance of incorporating the process of photoacclimation and chlorophyll dynamics into interpretations of phytoplankton population dynamics from chlorophyll data as well as the importance of daily-scale variability to Prochlorococcus ecology.
Highlights
The picocyanobacterium Prochlorococcus is the numerically dominant photosynthetic microorganism in the Earth’s subtropical ocean gyres and by extension the most abundant photosynthetic cell on Earth (Biller et al, 2015)
The abundance and global distribution of Prochlorococcus is attributed to its affinity for warm oligotrophic waters, genetic diversity that spans at least 12 distinct lineages, and physiological plasticity that permits the tuning of chlorophyll concentrations to light availability across the euphotic zone (Moore et al, 1998; Moore and Chisholm, 1999; Hess et al, 2001; Coleman and Chisholm, 2007; Biller et al, 2015)
We found that the chlorophyll physiology of Prochlorococcus populations could not be explained solely by ecotype composition
Summary
The picocyanobacterium Prochlorococcus is the numerically dominant photosynthetic microorganism in the Earth’s subtropical ocean gyres and by extension the most abundant photosynthetic cell on Earth (Biller et al, 2015). The 12 Prochlorococcus ecotypes have been separated broadly into high light (HL) ecotypes and low light (LL) ecotypes based on their 16S-23S intergenic transcribed (ITS) region sequence similarity (Rocap et al, 2002, 2003) and laboratorybased photophysiological characteristics (Moore et al, 1998; Moore and Chisholm, 1999). In culture, while both HL and LL ecotypes can photoacclimate over a wide range of light intensities, HL ecotypes have lower chlorophyll b/a2 ratios and are inhibited at very low light intensities where LL ecotypes thrive (Moore and Chisholm, 1999). Shifts in ecotype community structure could impact the contributions of the Prochlorococcus “federation” to biogeochemical cycling (Biller et al, 2015)
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