Understanding the symbiosis between UCYN-A and microalga

Symbiotic relationships between unicellular microorganisms are essential components of global ecosystems. Recently, a unique marine symbiosis was discovered between a microalga and an uncultivated unicellular cyanobacterium called UCYN-A. Analyses of UCYN-A revealed extensive genome reduction in this organism, but the retention of an elaborate nitrogen-fixing cluster. UCYN-A lacks the genes for oxygen-evolving photosystem II and carbon fixation, while the microalga has the ability to photosynthetically fix carbon. Studies revealed that a mutual exchange of carbon and nitrogen form the basis of this symbiotic relationship. We are unravelling this critical and fascinating evolutionary aspect of nature. Our objective is to identify the mechanisms (physical, chemical, and biological) that facilitate interactions leading to symbiosis between these two microorganisms. This study will use novel strategies including a state of the art fluorescence kinetic microscope (FKM) developed by Photon Systems Instruments (Czech Republic) for imaging photosynthetic activity in single cells. This work is funded by the Gordon and Betty Moore Foundation.

Examples of unicellular symbioses. (A) Marine diatom with heterocyst-forming filamentous cyanobacterium. (B) Fluorescent in situ hybridization micrograph of UCYN-A (red) and its symbiotic partner, the prymnesiophyte Braarudosphaera bigelowii (green). Blue, DAPI (4′,6-diamidino-2-phenylindole)-stained nucleus. Two types of this cyanobacterium are now known: UCYN-A1 (left) and UCYN-A2 (right). (C) Rhopaloid freshwater diatom showing spheroid body. (Zehr, 2015)


Using the FKM to understand UCYN-A/algal symbiosis

  • UCYN-A cells are involved in a nitrogen fixing symbiosis with a prymnesiophyte alga
  • UCYN-A cells contain the full repertoire of nitrogen fixing genes but lack genes involved in carbon fixation pathways
  • UCYN-A cells have no genes encoding for PSII and phycobilisomes, but contains all genes encoding PSI
  • The algal host has chlorophyll a, chlorophyll b, fucoxanthin
  • The FKM is specifically designed to detect fluorescence signals from UCYN-A and the algal host
The FKM allows us to distinguish between between different cell types based on their fluorescent properties. The system is being optimized to detect UCYN-A cells and their algal hosts in a sea water sample by using different excitation and emission wavlengths. We have used a mixed culture of Synechosystis 6803 mutants (lacking PSII and antenna proteins) with different fluorescent properties to mimic a mixed cell population with distinct fluorescence parameters to optimize our instrument.


The fluorescence kinetic microscope (FKM) was designed by Photon Systems Instruments (PSI).

Mixed Cyanobacterial Cells FKM Raw Data 

Anabaena FKM Raw Data



Modified SEM protocol for imaging of seawater samples