In the ocean, plankton consists mainly of single-celled organisms that interact in complex ways throughout their lives. In addition to eating each other, they can also live together, a process known as symbiosis (sensu lato). For example, a plankton host may harbor intracellular microalgae in symbiosis, giving it direct access to energy from photosynthesis using sunlight.
But there's another form of interaction: some plankton “steal” parts of microalgae, in particular chloroplasts, the photosynthetic organelles that produce energy in the form of sugars and lipids. This phenomenon between predation and symbiosis is called kleptoplastidy. These interactions enable the host plankton to produce its own energy or “fuel” from light.
Knowledge of the mechanisms underlying this phenomenon is still limited, particularly as regards how the interaction takes place at subcellular level and what happens to the stolen chloroplasts over time.
Advanced 3D imaging and artificial intelligence put plankton interaction under the microscope
In the study published in the journal Current Biology, scientists dissected an interaction between a type of plankton called dinoflagellate and a microalga called Phaeocystis. By combining advanced imaging techniques, they were able to determine the cellular organization of the interaction, and more specifically the three-dimensional morphology of the microalgae's flown organelles in the host cell. After “ingesting” the microalgae, like an undigested prey, its nucleus enlarges, its chloroplasts increase in volume and become more efficient for photosynthesis. Its mitochondria then form a vast, complex network interacting with the chloroplast.
After several weeks, the algal nucleus disappears, but the chloroplast and mitochondria are preserved and remain functional for months, raising many questions about the autonomy of these organelles and host control. The “thief” plankton is thus a chimera composed of two nuclei, two mitochondria and chloroplasts.
Figure: 3D electron microscopy reconstruction of the marine microalga Phaeocystis and the dinoflagellate (Ross Sea Dinoflagellate). The dinoflagellate can steal microalgal organelles such as the chloroplast (green) and mitochondria (red), whose morphology and physiology are modified.
Using subcellular imaging, this study led by Ananya Kedige Rao, PhD student of University Grenoble Alpes (UGA), highlights a complex strategy in marine plankton. This work improves our knowledge of interactions and dependencies between planktonic cells in marine ecosystems.
In addition to their ecological importance, these cellular interactions between predation and symbiosis are at the origin of major innovations in the course of evolution, such as the acquisition of the chloroplast in plants and various microalgal lineages.
Collaborations:
Several international collaborations have contributed to the challenges of imaging and associated analysis: