Putative electron flows in UCYN-A. The horizontal straight black
lines represent the boundaries of the thylakoid membrane, with the cytosol above and the thylakoid lumen below. Q, quinone; Fd, ferredoxin; FNR, ferredoxin–NADP1 reductase; Pc, plastocyanin; PSI, photosystem I; PSII, photosystem II; NDH-1, NADPH dehydrogenase type 1; NDH-2,NADPH dehydrogenase type 2; c553, cytochrome c-553. From Tripp et al., (2010), Figure 3.
Photosystem I activity in UCYN-A
Candidatus Atelocyanobacterium thalassa (UCYN-A), is a very unique nitrogen-fixing marine cyanobacterium. When our research group assembled and closed the full UCYN-A genome in 2010, we discovered that it is genetically incapable of oxygenic photosynthesis, since it lacks the entire photosystem II (PSII) apparatus, despite having photosystem I (PSI). This lack of oxygen evolution capability explains why they can fix nitrogen during the day. We now know that UCYN-A lives in close association with a single-celled photosynthetic Haptophyte alga, Braarudosphaera bigelowii. The only similar known symbiosis is between a freshwater diatom and its spheroid bodies, which are also of unicellular cyanobacterial origin, yet lack all photosystems. Thus, the fact that UCYN-A has retained its PSI implies that it is still used.
We have been trying to understand the function of UCYN-As PSI, and are studying the diel expression of some of the PSI genes using reverse transcription quantitative PCR (RT-qPCR). Our first order questions are: 1) whether UCYN-A expresses PSI genes, and 2) if PSI activity is linked to nitrogen fixation.
We have also been investigating the whole genome expression of UCYN-A during the diel cycle in the environment (as this associaiton is not in culture), using a custom high density microarray.
This research is partially supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme awarded to Mari Munoz-Marin.