Physiological characteristics of natural cell death across diversity in Prochlorococcus

The death of marine microbes plays a major role in nutrient cycling at sea. Natural cell death, for example as a result of UV damage or nutrient starvation, might be common in the oceans yet is not widely studied. Here, we investigate natural cell death caused by nutrient starvation in Prochlorococcus, a globally abundant marine cyanobacterium. Prochlorococcus is commonly divided into two ecotypes: the high-light-adapted (HL) strains, that grow at the ocean surface where nutrients are scarce, and low-light-adapted (LL) strains which inhabit deeper waters where nutrients are more available. In addition, the oceanic regions where Prochlorococcus live also differ in the availability of nutrients. We are testing the hypothesis that different Prochlorococcus strains respond differently to P and N starvation, depending on their gene content and their origin (where they were isolated) rather than their phylogeny. Preliminary experiments reveal that three strains, MED4 and MIT9312 (HL) and MIT9313 (LL), show distinct death patterns between N and P starvation. N starvation results in a sharper and faster decline compared to P starvation, and both MED4 and MIT9312 seem to survive longer under P-starvation compared to MIT9313. We are currently expanding this analysis using a culture collection of >20 strains of Prochlorococcus (and its close relative, Synechococcus), and characterizing the effect of nutrient starvation on cell biochemical structure. We anticipate that this study will identify conserved and strain-specific mortality patterns, possibly reflecting niche-adaptation.