This energy source is also used by some organisms as a signal to adapt to the environment and orientate themselves, and there are even bacteria that use light to grow and survive.
Sunlight plays a very important role in sustaining life in the ocean. It first penetrates the water column, heats it, generates currents, and finally, is absorbed by phytoplankton, which uses this source of energy captured by pigments such as chlorophylls to synthesise organic matter from water and inorganic nutrients.
This explains the presence of phytoplankton and primary production only in the photic layer, i.e. the illuminated zone of the ocean, which reaches down to a depth of 100 to 200 metres. Beyond that, organisms do not receive enough light for photosynthesis, although sufficient light reaches depths of up to 1,000 metres for vision to remain the most used orientation system.
A source of energy also for heterotrophs
In addition to phytoplankton, sunlight is also useful for those organisms that maintain symbiotic relationships with photosynthetic organisms living within them. This is the case of some jellyfish, corals or anemones that live in illuminated areas of the ocean where they survive thanks to algae called zooxanthellae that provide food to them. Sunlight is also vital for some heterotrophic organisms that incorporate the chloroplasts of the ingested algae and keep them functional to obtain energy. This process is called kleptoplasty and occurs in ciliates, dinoflagellates and even in some worms or sea slugs.
But sunlight not only acts as the engine of photosynthesis, it is also used by some organisms as a signal to adapt to the environment and orientate themselves. It also affects the circadian rhythms of marine species, which depend on the day-night changes caused by the Earth's rotation. As a result of these cycles, some organisms are more or less diurnal, while others vary their position in the water column according to the moment of the day.
Rhodopsins, widespread pigments with global functions
Regarding other functions of light in the ocean, a couple of decades ago it was discovered that some heterotrophic bacteria in the plankton use light to grow and survive. Using rhodopsins, membrane proteins associated with carotenoid pigments, these bacteria use sunlight to create proton gradients and generate energy in the form of ATP.
According to a study by the Institute of Marine Sciences (ICM) recently published in The ISME Journal, a large number of marine heterotrophic flagellate species that feed mainly on bacteria also have rhodopsins. Because most of the species belonging to this group have not yet been cultured in the laboratory, to reach this conclusion the experts had to use single cell genomic analyses, a field in which the Institute is now a pioneer thanks to various methodologies developed in the framework of the European SINGEK project.
Thanks to these methodologies, rhodopsins have been found in heterotrophic flagellate species that are globally distributed in the oceans, suggesting that they may be of great ecological importance for these eukaryotic microorganisms.
As for the functionality of rhodopsins, ICM researchers suggest that the most plausible hypothesis is that the proton gradient generated by these proteins participates in the acidification of the digestive vacuoles of heterotrophic flagellates, which is essential for the digestion of bacteria ingested by these small predators.
Much remains to be discovered about the currently unknown role of sunlight on oceanic bacteriovory. Nevertheless, this study shows that this energy source may play an important role in organisms that were expected to be indifferent to light, such as heterotrophic flagellates. This would explain, among other things, the presence of some of these species only in the photic layer, which has intrigued experts since their discovery.
