Studying coral’s biological clock 1/2

© Vincent Hilaire - Fondation Tara Expéditions

- Part 1/2 –

Oren Levy, Associate Professor at the Faculty of Life Sciences, and head of the Laboratory for Molecular Marine Ecology (LMME) at Bar-Ilan University in Israel, boarded the schooner in Palau during the leg organized by the Scientific Center of Monaco (CSM). Occasionally researchers aboard Tara present their work to the crew in an informal group lecture. Oren talked about the research he’s conducting at the LMME, and also at the CSM where he’s spending a sabbatical year. Focus on the mechanisms of a particular kind of clock in this 2-part interview.

 

What is a “biological clock”, and what do we know about it?

Life on Earth has evolved under rhythmic day and night cycles caused by our planet’s rotation. In response to these cyclic changes, most organisms have evolved endogenous clocks allowing them to anticipate daily and seasonal rhythms and to adjust their biochemical, physiological, and behavioural processes accordingly. Among the best conserved properties of these clocks is their ability to be driven by regular changes in light and temperature, and also acute temperature cues. Biological clocks, also called circadian rhythms, are characterised as free running, maintaining a periodicity of ~24-hour circuits under constant stimuli or in the absence of external cues: e.g. at constant light (LL), constant darkness (DD), or constant temperature. Thus, the oscillator will persist autonomously until the clock phases out due to the prolonged absence of external cues.

 

Does every organism have a biological clock?

Pretty much all organisms have a biological clock. The mechanism of the biological clock is similar in all organisms, from the simplest organisms up to human beings, but the components that make it tick can be very different. One of the most important functions regarding the biological clock is called “anticipation”. This means that the organism knows to be or to do right thing at the right time. Not to wait until change occurs but prepare itself for the changes to come.

 

©erottinger-0361
Coral reef studied by scientists embarked aboard Tara – © Eric Rottinger / Fondation Tara Expéditions

 

What are the origins of the circadian clock?

Many hypotheses have been put forward regarding the driving forces that led to the evolution of circadian clocks, but because they are present in all the kingdoms of life they must have evolved very early. Clocks may have arisen primarily to minimize UV damage to DNA by ensuring that replication occurred in the dark. Evidence comes from the universal presence of blue light-sensitive proteins in all organisms, including corals.

 

How does this biological clock work for marine organisms?

Life evolved in the ocean, an ecosystem that is governed by a multitude of environmental cycles. As in terrestrial organisms, it is adaptively important for marine organisms to be able to anticipate future cyclic events. However, species inhabiting coastal environments are challenged with considerably more complex temporal patterns, dominated by tidal and lunar cycles. Consequently, unlike the 24-hour cycles found in terrestrial organisms, intertidal plants and animals show adaptive, free-running 12.4-hour rhythms of behaviour, metabolism, and reproduction that are synchronised to the tidal environment by relevant cues, including turbulence/vibration, moon¬light, salinity, and temperature fluctuations. Such free-running rhythms suggest the presence of an endogenous circatidal clock*.

 

So, for instance, the biological clock helps marine organisms to adjust to the tide.

Yes, the biological clock helps the organism to know when there’s going to be a low tide. So before the low tide occurs, organisms such as crabs are already preparing to go out. Many cycles of behaviours, like spawning or feeding, are related to the change of this geophysical cycle. All biological clocks have the same 3 components. First, you have what we call the input, the information that is coming from the environment. It can be the tide, the light or dark, but also food. These are called “cues”, which synchronise the internal biological clock. The second component is the mechanism responsible for processing the information from the environment. The third component is basically the output: what this clock is responsible for, like coral spawning for example, or crab feeding.

 

So the output can be expressed by many different things.

Yes, the rhythmic output can be gene expression, a hormonal secretion, a change in metabolism or behaviour, etc. In human beings we know that many things are affected by biological clocks: our activities raise body temperature;( ) mating time; our endocrine system; athletic performance – all these are controlled by this kind of mechanism.

 

Coral spawn © the cup of tea - Tara Expeditions Foundation
Most corals reproduce at night on a very short time, just a few days a year – © the cup of tea

 

Why are you studying coral’s biological clock?

I wanted to know how corals time their spawning with such accuracy year after year, always at the same time. What is the mechanism? How are they doing it? Those are the questions I wanted to answer. So during my Postdoctoral phase, I have worked at the Marine Laboratory at Heron Island, University of Queensland, Australia. The first work was done to see if we could find specific photoreceptors, meaning specific proteins sensitive enough to detect changes in low light (like sunset and moonlight) that might synchronise the coral clock to spawn in the right light conditions.

Were you able to isolate the protein that you were looking for?

Yes, we found 2 kinds of proteins called cryptochromes. One of them was more sensitive during full moon nights than in new moon light. So we know from other organisms that these cryptochromes provide the information to the biological clock in many model organismxas. This protein is unique as being common in the biological clock machinery that is found across all evolutionary groups.

 

Noëlie Pansiot

 

Cryptochromes*: are a class of flavoproteins that are sensitive to blue light. They are found in plants and animals.

Eukaryotic*: any organism whose cells have a cell nucleus and other organelles enclosed within membranes.

Circatidal clock*: tides and tidal clocks.