Multiplication of antenna genes as a major adaptation to low light in a marine prokaryote

Two ecotypes of the prokaryote Prochlorococcus adapted to distinct light ni ches in the ocean have been described recently. These ecotypes are characte rized by their different (divinyl-) chlorophyll (Chl) a to chi b ratios and 16S rRNA gene signatures, as well as by their significantly distinct irrad iance optima for growth and photosynthesis [Moore, L, R., Rocap, G, & Chish olm, S. W. (1998) Nature (London) 393, 464-467]. However, the molecular bas is of their physiological differences remained, so far, unexplained. In thi s paper, we show that the low-light-adapted Prochlorococcus strain SS120 po ssesses a gene family of seven transcribed genes encoding different chi a/b -binding proteins (Pcbs). In contrast, Prochlorococcus sp. MED4, a high-lig ht-adapted ecotype, possesses a single pcb gene. The presence of multiple a ntenna genes in another low-light ecotype (NATL2a), but not in another high -light ecotype (TAK9803-2), is demonstrated. Thus, the multiplication of pc b genes appears as a key factor in the capacity of deep Prochlorococcus pop ulations to survive at extremely low photon fluxes.