טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZeidner Gil
SubjectMolecular Diversity among Photosynthetic Marine
Picoplankton and Associated Bacteriophages
DepartmentDepartment of Biology
Supervisor Professor Oded Beja


Abstract

Photosynthetic microorganisms play a crucial role in the marine environment. In vast areas of the oceans, marine primary productivity is performed by cells smaller than 2-3 µm (picoplankton). A survey of picophytoplankton diversity was carried out by designing general degenerate DNA primers based on well conserved amino acid regions of the D1 protein. D1 encoding gene, psbA, is found in both cyanobacteria and the chloroplast genomes of algae and higher plants and is widely used as a phylogenetic marker. During this study the existence of a new uncultured cyanobacterial-like group was observed. Further work combining bacterial artificial chromosome (BAC) libraries and denaturing gradient electrophoresis (DGGE) data analysis has linked psbA genes attribute this group to marine bacteriophages.

A comparison of cyanobacterial and bacteriophages psbA genes suggests, a strong support for a monophletic group composed of viral environment PCR amplified, DGGE psbA sequences and BAC clones, to the exclusion of any cultured cyanobacterial sequences. Based on statistical analysis we hypothesize that all photosynthetic-gene sequences which cluster in the phage group, belong to mobile genetic element (MGE) assemblages from phages, prophages that affect the  evolution of marine cyanobacterial photosynthesis through possible mechanism of lateral gene transfer.

A quantitative analysis of DNA assemblages (scaffolds) from Sargasso Sea and GOS shotgun sequencing projects was done based on an amino-acids signature  motif for identification of phage-psbA.  This motif was further used to estimate the prevalence of viral genes in the environment and their expression in the marine samples. Surprisingly, the results indicate that viral photosynthetic gene pool composes more than half of the total cyanobacterial psbA pool, and that viral psbA genes expressed in the marine environment.

 We therefore concluded that viral D1 genes are actively expressed in the marine environment and represent functional diversity of viral variants. The phage-encoded proteins may play a direct role on bacterial photosynthesis determining the level of photosynthetic productivity in the oceans (oxygen evolution and carbon fixation) and redefining the possible role of phages in the overall biosphere.