Our team discovered that Prochlorococcus can take up glucose, in a process that changes the transcriptional pattern of several genes involved in glucose metabolization. We have also shown that glcH encodes a very high affinity glucose transporter, and that glucose is taken up by natural Prochlorococcus populations. We demonstrated that the kinetic parameters of glucose uptake show significant diversity in different Prochlorococcus and Synechococcus strains. Here, we tested whether the

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... ional response of glcH to several glucose concentrations and light conditions was also different depending on the studied strain. Methods: Cultures were grown in the light, supplemented with five different glucose concentrations or subjected to darkness, and cells harvested after 24 h of treatment. qRT-PCR was used to determine glcH expression in four Prochlorococcus and two Synechococcus strains. Results: In all studied strains glcH was expressed in the absence of glucose, and it increased upon glucose addition to cultures. The changes differed depending on the strain, both in the magnitude and in the way cells responded to the tested glucose concentrations. Unlike the other strains, Synechococcus BL107 showed the maximum glucose uptake at 5 nM glucose. Darkness induced a strong decrease in glcH expression, especially remarkable in Prochlorococcus MIT9313. Discussion: Our results suggest that marine picocyanobacteria are actively monitoring the availability of glucose, to upregulate glcH expression in order to exploit the presence of sugars in the environment. The diverse responses observed in different strains suggest that the transcriptional regulation of glucose uptake has been adjusted by evolutive selection. Darkness promotes a strong decrease in glcH expression in all studied strains, which fits with previous results on glucose uptake in Prochlorococcus. Overall, this work reinforces the importance of mixotrophy for marine picocyanobacteria. containing divinyl chlorophyll a and chlorophyll b. Archives of Microbiology 157(3):297-300 Delmont TO, Eren AM. 2018. Linking pangenomes and metagenomes: the Prochlorococcus metapangenome. PeerJ 6:e4320 DOI 10.7717/peerj.4320. Der-Vartanian M, Joset-Espardellier F, Astier C. 1981. Contributions of respiratory and photosynthetic pathways during growth of a facultative photoautotrophic cyanobacterium, Aphanocapsa 6714. Plant Physiology 68(4):974-978 DOI 10.1104/pp.68.4.974. Domínguez-Martín M, Díez J, García-Fernández J. 2016. Physiological studies of glutamine synthetases I and III from Synechococcus sp. WH7803 reveal differential regulation. Frontiers in Microbiology 7:969 DOI 10.3389/fmicb.2016.00969. Domínguez-Martín MA, López-Lozano A, Diez J, Gómez-Baena G, Rangel-Zúñiga O, García-Fernández JM. 2014. Physiological regulation of isocitrate dehydrogenase and the role of 2-oxoglutarate in Prochlorococcus sp. strain PCC 9511. PLOS ONE 9(11):e103380 Wolf Y, Hess W. 2003. Genome sequence of the cyanobacterium Prochlorococcus marinus SS120, a nearly minimal oxyphototrophic genome. JM. 2001. In vivo regulation of glutamine synthetase activity in the marine chlorophyll b-containing cyanobacterium Prochlorococcus sp. strain PCC 9511 (Oxyphotobacteria).