Tide Associated Rhythms of Brown Algae

Marine habitats are shaped by multiple overlapping cycles including daily (24 h), tidal (12.4 h), semilunar (14.8 d), lunar (29.5 d), and annual, creating complex temporal habitats. To cope with such cyclic environments, many marine species have evolved biological clocks: internal oscillators that align physiology and behavior with predictable environmental changes.

Circadian clocks in eukaryotes seem to fuction through conserved mechanistic principles, however, other clocks, such as circalunar, show independently evolution in several lineages and respond to different mechanistic principles.

Brown algae (Phaeophyceae) are photosynthetic sessile macro-organisms of key ecological importance to coastal ecosystems. Despite their ecological importance, the adaptative mechanisms that allow brown algae to anticipate to environmental cycles remain poorly understood.

Exemplary selection of species with moon-controlled reproductive cycles documented in the scientific literature (Ritter & Tessmar-Raible 2024, EMBOJ).

How do brown algae sense lunar phases ? 

This question has been at the center of our research. Such synchronization requires internal biological clocks that are entrained by environmental cues. As early as the 20th century, studies in Dictyota dichotoma revealed circasemi-lunar reproductive rhythms in this species, showing that gamete release events occur with striking regularity every 14 days, in phase with the lunar cycle. 

Our objective is to unravel the molecular mechanisms underlying brown algal chronobiology, with a particular focus on circasemilunar rhythms. 

The Brown Circalunar Clock

Our main model organism, Dictyota dichotoma, displays robust circasemilunar rhythms  of gamete spawning (~14.8 days) regulated by an endogenous clock. These rhythms persist for months even in abscence of environmental timing cues. Yet, the environmental signals that synchronize these rhythms in nature remain unresolved…

Moonlight, tides, or their interaction? 

We are currently investigation the environmental factors responsible of setting the lunar clock of brown algae by  combining field-work together with lab based experiments. To efficiently monitor gamete spawning rhythms, we developed in-house technologies such as automated phenotyping devices or naturalistic illumination systems. 

Architecture of the brown circalunar clock

Although chronobiological studies in Dictyota date back to the early 20th century, progress in understanding the molecular basis of the brown algal lunar clock has remained limited. Available information shows that light is the primary timing cue for synchronizing the lunar clock of brown algae. For this reason we are currently focusing in characterizing the involvement of photoreceptors as an entry point for deciphering the molecular composition of the brown lunar clock. 

Does reproductive timing serve as a prezygotic barrier
in the Dictyota dichotoma species complex?

Marine broadcast spawners rely on synchronous gamete release for successful fertilization. Brown algae show remarkable temporal precision, with Dictyota dichotoma reproduction under biological clock control. It releases the gametes in a circasemilunar fashion, every ~14 days. Release happens during the fertile season (April to October) in the morning hours after sunrise. The molecular mechanism behind this remains elusive. It has been speculated that differences in gamete release timing serve as prezygotic barriers between brown algae
species. Through population genomic analysis of English Channel populations, we discovered that D. dichotoma is a species complex of 2-3 distinct species. While these species can
hybridize, only few natural hybrids are found. Our current efforts focus in characterizing if reproductive timing serve as a prezygotic barrier in the D. dichotoma species complex. In a second time, this project aims to develop a forward genetics approach to identify loci under selection in Dictyota species showing contrasting reproductive timing. 

Reverse Genetics toolbox

Our understanding in the biology of brown algae is limited, partly due to restricted access to molecular biology tools, which has hindered progress in this field. As part of a emerging model organisms project This project aims to address this gap by fostering new research collaborations between the groups of Andrés Ritter at the Roscoff Marine Station (CNRS – Sorbonne University) and Michele Fabris at the University of Southern Denmark. In this frame, this Blåtand application will support the kick-off of this collaborative research through the short stay of Dr. Aude Le Bail (CNRS research engineer) at the Fabris’ group by April 2025 to investigate foreign DNA delivery and expression in brown algal model species.