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Welcome to another month’s blog and congratulations to those of you who have finished up your exams for the year! Time now to enjoy a well-earned respite, relax and go on a beach holiday… but do spare a moment to reflect on the incredible life cycle of some of the organisms floating in the ocean with you.

If you happen to be paddling around in the cooler, low nutrient waters of the northeastern Pacific Ocean, you would sharing the water with the cyanobacteria Prochloroccocus, a prokaryotic organism with a density (during the day!) of hundreds of millions per litre of seawater. Recently, an article in the Proceedings of the National Academy of Sciences described their daily cycle of reproduction and death that is of a magnitude uncommon even for prokaryotes. As it turns out, one of the oceans’ most abundant food sources lives and dies like clockwork.

The bacteria, each less than 1 micrometer long, convert sunlight into carbon forms that feed other organisms, supporting food webs all across the Pacific Ocean. Their consistent cycle of growth and death may stabilize marine ecosystems, even under the effects of global warming. These bacteria are found across a huge swathe of the northeastern Pacific, covering thousands of kilometers. However, while you would expect to find some significant variation in the growth and loss of Prochloroccocus across this range, their life cycle pattern is remarkably consistent. This consistent relationship indicates that Prochlorococcus provides reliable food for hungry consumers, with the stable and predictable cyanobacteria population providing a consistent cornerstone to many food-webs.

Viruses and plankton prey upon Prochlorococcus. But in the northern Pacific Ocean in winter, hardly any of the cyanobacteria die during the day. Though previous studies have suggested a tight balance between Prochlorococcus growth and death, this night-time loss presents an interesting biological situation. It is possible that the cyanobacteria’s predators avoid their own enemies by hiding in deeper water during the day and hunting at night. Alternatively, the organisms that hunt or kill Prochlorococcus may be inhibited by light, or may be able to generate their own food from sunlight during the day. Prochlorococcus grows faster in warmer waters, but when the cyanobacteria’s reproduction increased, so did their loss overnight. This response means Prochlorococcus’ cycle may remain stable in waters affected by global warming, which is a cheerful finding given how much data is currently being collected to show how damaging an impact global warming is having on global ocean ecosystems.

Studies like this are interesting for potential Oxbridge students to take note about. While our multi-cellular brethren often get the lion’s share of the spotlight, it is also crucial to take note of studies focusing on prokaryotes. Cambridge especially has a proud heritage of conducting groundbreaking research in this field, so it would be wise to read up on some of these key studies over the summer!

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