The phrase “no man is an island” as put by the 17th-century metaphysical poet John Donne in his prose work ‘Devotions upon Emergent Occasions’ talks about how humans do not do well in isolation and require a community to thrive (Donne, 1987). Similarly, in light of current research, it is becoming increasingly evident that no organism can be considered in isolation. Beyond considering organisms simply within the ecological communities they reside in, it is imperative we also consider the ecological communities they harbor within—their microbiome.

Since the dawn of metagenomic sequencing, research into microbiome community composition has exponentially advanced, shedding light on the complexity of host–symbiont relationships (Huttenhower et al., 2014; Norman et al., 2014; Yoon et al., 2015; Jovel et al., 2016). Yet, much regarding the synchrony of these interactions remains elusive. Rhythmic regulations, including the circadian (Supplementary Box 1) rhythms (∼24-h endogenous biological clocks; Supplementary Box 2) set a metronome for coordinated activity across all domains of life (Edgar et al., 2012), and not only warrant exploration of their critical role in mediating individual cellular activity but also for their role in mediating the rhythmically coordinated activities between host and their symbionts, ultimately facilitating successful adaptive optimization of the holobiont (Supplementary Box 1).

In recent years, the significance of circadian clock regulation has become increasingly apparent. Entrained by external cues, circadian regulation mediates the rhythmic expression of numerous genes under the locally fluctuating environment, including those of innate immunity, metabolism, behavioral responses, cellular division, and global gene expression (Mori et al., 1996; Markson et al., 2013; Thaiss et al., 2016; Parkar et al., 2019). Indeed, the origins of circadian regulation are considered ancient, hypothesized to have first arisen under the strong selecting forces of high ultraviolet (UV-B and UV-C) radiation. Prior to the formation of the Earth’s protective ozone layer, light-vulnerable processes were entrained to the solar night, termed the ‘escape from light’ hypothesis (Pittendrigh, 1993; Nikaido and Johnson, 2000). However, despite these ancient origins, circadian clocks have independently evolved, multiple times across distinct lineages, but with convergent functions (Rosbash, 2009; Takahashi, 2017), and, thus, likely have wide-reaching functional roles in disparate organisms.

Within marine ecosystems, the rhythmic activities and regulation of diverse organisms pertain significantly on a macro-scale (Häfker et al., 2017). The open ocean of the earth’s marine ecosystems comprises vast planktonic communities, important globally for large-scale biogeochemical cycling. These communities comprise a wide variety of organisms from all domains including algae, bacteria, protists, and metazoans. Here, we consider the rhythmically aligned regulation of these communities, entrained to cyclically fluctuating external environmental stimuli. Viruses, though hardly considered living, form an important part of the marine planktonic community and play a huge role in (re)shaping the community structure. Viruses are entirely dependent on their host for proliferation and in the process, often lyse their hosts. The spatiotemporal dynamics of virioplankton communities, like other subsections of the planktonic communities, depend on various direct and indirect biotic and abiotic factors (reviewed in Mojica and Brussaard, 2014). The natural spatiotemporal dynamics of virus–host communities in the marine environment becomes a topic of discussion on the biological rhythms of marine viruses.

Similarly, rhythmic regulation is also significant on a micro-scale, for the nested ecological communities comprising host–symbiont holobionts. Eukaryotic circadian clock mechanisms have been well-studied in diverse multicellular organisms including mammals, arthropods, sponges, and plants (McClung, 2001; Dibner et al., 2010; Jolma et al., 2010; Jindrich et al., 2017; Morioka et al., 2018). Within mammals, a hierarchical system comprises a host of individual circadian clocks that are all centrally aligned by a core circadian clock within the suprachiasmatic nucleus of the hypothalamic region of the brain (Dibner et al., 2010). This allows individual cells of different tissues to independently mediate rhythmic gene expression on diverse periods, appropriate for their cellular function (Dibner et al., 2010). Interestingly, a comparable system has been hypothesized to occur within animal-bacterial holobionts. Here, the central endogenous clock of the host coordinates rhythmic alignment amongst all independent clocks within the microbial symbionts of the microbiome (Lee et al., 2019). Elucidating host and symbiont rhythmic co-regulation could provide further key insights into the establishment of host–symbiont relationships. For instance, whether circadian clock alignment provides an important selection filter in host–symbiont compatibility (temporal mutualism), or whether prokaryotic symbionts first colonize, and later adapt their circadian cycle to match that of their host to stabilize the relationship.

Research into holobiont chronobiology, the dynamic interaction of host–microbe circadian clock regulation, has predominantly focused upon mammalian organisms and provides only tantalizing glimpses at applicable prospects within other holobiont systems. However, considering the global ubiquity and significance of prokaryotes within marine environments, either as symbiotic or free-living forms, and the scarcity of available studies (Sartor et al., 2019), we must extend circadian research to encompass a greater degree of prokaryotic diversity and their crucial functions within holobionts. Here, we outline the currently very limited data from marine holobionts, as well as from more extensively studied terrestrial/mammalian holobionts with the aim that such research can help accelerate that within organisms from marine ecosystems. Indeed, we propose that future chronobiomics (Supplementary Box 1) research should increase its focus upon globally diverse and dominant phyla that comprise basal marine metazoa, such as Porifera, Cnidaria, and Ctenophora. These organisms comprise considerably less complex body plans, making them tractable model systems for the investigation of molecular circadian holobiont regulation. Additionally, since these organisms comprise ancient lineages that were the earliest to diverge from the rest of the animal kingdom (i.e., bilaterians), any shared mechanisms of rhythmic regulation identified may logically be traced back to the last common animal ancestor.

In this review, we first outline the mechanisms of circadian clock regulation within free-living prokaryotes, including non-photosynthetic bacteria, cyanobacteria, and archaea. Secondly, we consider the implication of biological rhythms on marine viruses. We explore the dynamics of host–virus populations and the effect of light in controlling planktonic community dynamics. We also delve into the understudied niche of viral influences upon host biological rhythms by discussing host photosynthesis take-over by the viruses. In the third part, we discuss the significance of circadian rhythm regulation for holobiont health and optimization (using well-studied terrestrial mammalian examples) and, thus, how such chronobiomics research must be considered in the marine environment. We present the currently limited evidence of circadian clocks within marine holobiont systems, demonstrating host–symbiont rhythmically aligned regulatory activity. We aim to provide a comprehensive review of holobiont rhythmic regulation, which promotes the consideration and extension of the universally significant field of chronobiomics research within marine environments.

Current knowledge of prokaryotic circadian regulation is severely lacking. To date, prokaryotic circadian clocks have been formally described within the phylum Cyanobacteria only, of which Synechococcus elongatus serves as the current ‘model’ organism (Johnson et al., 2017). That said, evidence of prokaryotic rhythmicity has been around for some time.

Evidence of rhythmic regulation within prokaryotes (outside of Cyanobacteria) dates back as early as the 1930s. Despite early hypotheses that circadian rhythms would not be adaptive for single-celled organisms, for which generation times are shorter than the diel cycle (“circadian-infradian rule”; Ehret and Wille, 1970), studies did report rhythmic periods for bacteria that, remarkably, surpassed the duration of their cell cycles. Within Escherichia coli cultures, Rogers and Greenbank (1930) demonstrated a growth period indicative of long circadian-like rhythms that were later shown to be periods of 20.5 h (Halberg and Conner, 1961). However, later observations demonstrated this was not fully temperature compensated (Supplementary Box 2) and, thus, does not display a fully robust circadian clock (Sturtevant, 1973). Within Alphaproteobacteria Rhodospirillum rubrum, enzymatic activity of hydrogenase (Hup) uptake, which participates in redox metabolism during photosynthesis and nitrogen fixation, demonstrated circadian rhythmic oscillations of 12–15 h (Van Praag et al., 2000). Moreover, non-photosynthetic bacteria Klebsiella pneumoniae and Klebsiella aerogenes displayed rhythmic growth and activity, respectively (Sturtevant, 1973; Paulose et al., 2016); of which K. aerogenes demonstrated a fully temperature-compensated rhythm. Thus, diverse bacteria from multiple phyla, likely, rhythmically regulate their cellular activities on either a circadian-like or ultradian (Supplementary Box 1) period.

Sufficient literature indicates that the cellular regulation of most, if not all, bacterial lineages are rhythmically influenced, albeit with likely diverse periods, but can these rhythms also influence viruses, that are hardly considered living? This is an area of research that has received little attention to date. Thus, we will explore the intriguing subject of biological rhythms concerning viruses in the marine environment. Typically, viruses are considered highly infectious, disease-causing lethal entities that wreak havoc. Indeed, considering the COVID-19 pandemic and its societal impact (Heymann and Shindo, 2020), this view is not entirely wrong. However, knowledge of marine and other aquatic viruses, along with their roles in ecosystem functioning have barely been valorized and are deserving of a more prominent focus.

In 1989, it was discovered that viral abundance within marine environments may be as high as millions within a cubic milliliter (Bergh et al., 1989). Subsequently, this led to the recognition of viruses as the most abundant biological entity within our oceans; totaling up to 10³⁰ virus-like particles (Wommack and Colwell, 2000; Breitbart, 2012). Viruses form the ocean’s second-largest biomass, second only to the prokaryotes (Suttle, 2005). With the capacity to infect all forms of life, viruses lyse considerable volumes of daily prokaryotic production in the marine environment. Thereby, facilitating the flow of nutrients from higher trophic levels back to the lower levels, in what is known as the ‘viral shunt’ (Fuhrman, 1999; Wommack and Colwell, 2000; Koonin et al., 2006; Suttle, 2007; Miranda et al., 2016). Subsequently, one cannot stress enough the ecological importance of viruses for the functioning of marine ecosystems.

To better understand the influence of biological rhythms, we must consider the spatiotemporal community dynamics of viruses within marine planktonic assemblages. In this section, we will discuss the importance of light as a crucial abiotic factor affecting community dynamics through the viral life cycle, focusing specifically on the intensity, duration, and quality of light in determining viral activity and burst size. Lastly, we will discuss the influence of viruses on their cyanobacterial hosts’ biological rhythm through redirecting photosynthesis to enhance viral replication.

Research from mammalian gut microbial systems provides the backbone to our current understanding of microbiomes. Thus, it is tempting to draw upon mammalian microbiome studies when considering less well-studied species of marine metazoa but this must be done with caution. Mammalian microbiome studies can be extremely useful, considering many microbial symbiont genera are common across diverse host species, and that the mechanisms of both the eukaryote canonical TTFL and peroxiredoxin redox clocks are highly conserved (Jolma et al., 2010; Edgar et al., 2012). However, we must also be careful to consider the disparate influences on microbial communities within divergent host systems. Human gut microbiota is highly diverse and dense, containing up to 1000 horizontally acquired species, the abundances of which go through regular daily fluctuations (Turnbaugh et al., 2009). Thus, the nested ecosystem interactions of mammalian extracellular symbionts, and, hence, influences on their circadian cycling, will be diverse and varied from other host organisms. For instance, host nutrient uptake is known to be one of the most significant influences on human gut community composition and a circadian zeitgeber influencing prokaryotic circadian clocks of the microbiome (Rothschild et al., 2018; Murakami and Tognini, 2019; Perofsky et al., 2019), but whether this is consistent within other organisms remains unknown. Additionally, it was demonstrated that rhythmic cycling of microbial community composition differentiates with host sex (Liang et al., 2015). Ultimately, considering the increased complexity within mammalian study systems, it may prove productive to utilize comparatively less complex, and earlier diverged basal marine metazoa, which can also indicate how conserved the mechanism is.

Current evidence demonstrating circadian regulation within microbial symbionts is largely limited to the mammalian gut microbiota, though a few marine systems have been explored. One of the best-studied examples includes luminous Gammaproteobacterium Aliivibrio fischeri (formerly, Vibrio fischeri) housed within the light organ of Hawaiian bobtail squid Euprymna scolopes. Horizontally acquired A. fischeri accumulate each day in the light organ of E. scolopes whereby nightfall, they reach a high enough density to produce bioluminescence through quorum sensing pathways (Miyashiro and Ruby, 2012). This is suggested to confer an advantage to the host by deterring predators through a mechanism known as counterillumination (Jones and Nishiguchi, 2004). Remarkably, each morning, the host then expels (known as “venting”) 90% of these bacteria and the few remaining regrow throughout the day to again build up sufficient densities. This daily cycle forms the basis of their symbiotic relationship, of which host and symbiont demonstrate rhythmically aligned patterns of gene expression (Wier et al., 2010) underlying day/night activities (Heath-Heckman and McFall-Ngai, 2011). For instance, the transcriptional rhythms of E. scolopes’ circadian associated cryptochrome escry1 are upregulated through the temporal cycling of its symbiont A. fischeri (Heath-Heckman et al., 2013). Likewise, the cyclic provision of chitin by the host regulates symbiont metabolism resulting in the cyclic production of bioluminescence, essential to symbiosis (Schwartzman et al., 2015). Despite being one of the best-studied examples, gammaproteobacterial A. fischeri circadian oscillators have not yet been identified. Thus, indicative of the significant challenges in chronobiomics, limiting our current scope on the potentially widespread prevalence of rhythmic regulatory systems within marine ecosystems.

Circadian-like rhythmic activity has also been described within Cnidarian-Symbiodiniaceae holobionts. Diurnal rhythmic activity within corals has been observed since the 1930s and is perhaps unsurprising given that light, considered the major circadian zeitgeber (McClung, 2001), forms the basis of the coral-Symbiodiniaceae relationship through photosynthetic production of metabolites. Within both free-living Symbiodiniaceae and symbiotic Symbiodiniaceae, the rhythmic activity of photosynthetic processes including, oxygen evolution, PSII yield, and concentrations of pigments, is sustained under constant conditions (Sorek and Levy, 2012a; Sorek et al., 2013). This lends support to the hypothesis that an endogenous circadian clock mediates photosynthesis (Sorek et al., 2014). However, molecular evidence for circadian-driven regulation remains elusive, since uncoupling host–symbiont interactions and reciprocal regulation remains a significant challenge, particularly within vertically inherited intracellular endosymbionts (Sorek and Levy, 2012b). That said, molecular clock mechanisms have been demonstrated within several free-living algae, including Lingulodinium polyedrum, the dinoflagellate clock model (Hastings, 2007; Noordally and Millar, 2015). Additionally, considering the significance of ROS within the coral holobiont from Symbiodiniaceae photosynthesis, we predict an important role for the circadian redox clock in the maintenance of ROS homeostasis. Each day, coral tissue exhibits a significant day–night transition in oxidation state during the switch between daytime photosynthesis and evening respiration (Sorek et al., 2013). This could likely provide a dominant zeitgeber entraining peroxiredoxin circadian rhythms (Edgar et al., 2012). Indeed, evidence links the relationship between circadian regulation and several stress-related proteins. For instance, Heatshock protein 90 (HSP90), coupled to the circadian clock, demonstrates rhythmic expression under both constant and diurnal conditions and modulates the circadian period if inhibited (O’Neill et al., 2011; Hindle et al., 2014). Additionally, toxic stress driven by climate change-induced environmental perturbation results in disruption to host circadian genes, as well as an observed high prevalence of symbiont antioxidant genes (Kaniewska et al., 2015). Coral-Symbiodiniaceae symbioses are one of the best well-studied marine holobionts, yet we understand relatively little of their circadian mediated interactions, demonstrating both the current lack of research within this area and the significant challenge it presents. Given the ecological significance and vulnerability under anthropogenic and climate change-induced pressures, coral holobiont circadian activity ought to be a priority for research.

Closely related sea anemones can also provide tractable cnidarian holobiont study systems since, unlike coral, they exhibit both symbiotic and aposymbiotic morphs. In Aiptasia diaphana, Sorek et al. (2018) demonstrated symbiotic morphs display 24-h circadian rhythms of gene expression, whilst aposymbiotic morphs displayed only 12-h circatidal rhythms. Notably, when aposymbiotic morphs were repopulated with algae, they returned to a 24-h behavioral rhythm. This dramatic rhythmic shift observed between symbiotic and aposymbiotic hosts indicates a strong influence from an endogenous symbiont circadian clock on photosynthesis and host rhythmicity through the transfer of metabolites (Sorek et al., 2018). Moreover, within the sea anemone Nematostella vectensis, significant shifts in the abundance of bacterial community members were observed under constant conditions (light–light LL or dark–dark DD). Under DD, bacterial community diversity increased, as well as upregulation of innate immunity genes, thus, indicative of a symbiont-driven circadian clock (Leach et al., 2019). Outside of Cnidaria, endogenous circadian regulation of Symbiodiniaceae symbionts has also been identified within the giant clam Tridacna crocea, where it was suggested that endogenous symbiont and/or host circadian rhythms, stimulated by light, mediate T. crocea daily trace element incorporation (Warter et al., 2018).

Many marine invertebrate organisms exhibit pelago-benthic life cycles that encompass a free-swimming larval stage, often particularly dependent on environmental cues (reviewed by Hodin et al., 2017). For instance, larvae of the coral reef sponge Amphimedon queenslandica inhabit a shallow-water reef flat where they are highly dependent on regularly oscillating environmental cues for the onset of development and timing of larval settlement to the substrate; occurring 4–6 h post-release, in the late afternoon each day (Degnan and Degnan, 2010; Figure 2). When maintained under conditions of constant environmental light, larvae do not settle, but when re-exposed to darkness, again exhibit settlement (Say and Degnan, 2019). Thus, environmentally entrained clocks can have a profound influence on behavior. As with other larval metazoa, A. queenslandica larvae house a vertically inherited microbiome dominated by proteobacteria (Fieth et al., 2016; Gauthier et al., 2016), which may also be rhythmically regulated. For instance, bacterial production of LPS endotoxin, a key component within the outer membrane of nearly all gram-negative bacteria (Baltrons and García, 1999; Alexander and Rietschel, 2001), may increase under rhythmic disturbance (Deaver et al., 2018). Significant upregulation of the export permease protein (LptF) and an LPS synthesis gene (kdsB) was observed within circadian disrupted gut microbiota (Deaver et al., 2018). As well as the direct toxic effects from LPS, increased production of such compounds may interfere with signaling pathways that sustain crucial holobiont molecular and behavioral activities. Indeed, LPS can interfere with chemosensory G-protein coupled receptors (GPCRs) (Baltrons and García, 1999; Alexander and Rietschel, 2001), which facilitate signaling cascades for an array of diverse host responses. The pelago-benthic life cycle, observed in many marine invertebrates, often relies on GPCR activation to induce settlement of larvae to the substrate (Schneider and Leitz, 1994; Strader et al., 2018; Say and Degnan, 2019), and, thus, may potentially be inhibited through GPCR circadian induced pathway disturbances. The successful development and establishment of free-swimming larvae is a crucial stage in the dispersal of many significant marine holobionts, functional as ecosystem engineers (e.g., coral, sponges, ascidians, bryozoans). Subsequently it is particularly important we understand the rhythmically aligned regulation of these organisms that could likely be disrupted through climate change induced environmental perturbation.

The bilaterally dependent feedback of host–symbiont rhythmic regulation is an intriguing area of research, presenting many significant challenges. For instance, does each individual within the partnership contribute its own endogenous clock, or do they function cohesively, acting and reacting to rhythmic cues of the other? Regarding the canonical molecular clock within prokaryotic organisms, we observe a loss of features (Schmelling et al., 2017) and less robust rhythms within all non-cyanobacterial species, regardless of whether they are free-living or symbiotic. Thus, microbial symbiont clock features may feasibly have become redundant and led to a shift in reliance upon their hosts for entrainment cues, and even the expression of their clock-controlled genes via host oscillators. Indeed, this may be to a comparatively greater extent within endosymbionts. Evidence indicates free-living ancestral mitochondria and chloroplasts once had their own endogenous clocks before becoming fully obligate within the eukaryotic cell. Likewise, within free-living Symbiodiniaceae, we observe longer-lasting, robust rhythms under constant conditions than within its symbiotic counterpart (Sorek et al., 2014). Robust clocks enable organisms to sustain rhythmic regulation for longer periods, despite variable environmental entrainment cues. Thus, for organisms housed within the relative stability of the microbiome, such a robust clock, that is likely more energetically costly, may not have been selected for. Conversely, considering the redox clock as a conserved and ancient mechanism, that likely predates the canonical clock (Edgar et al., 2012), we hypothesize individuals within the holobiont to each comprise their own endogenous redox clocks, that have optimally aligned through common/shared environmental stimuli. However, it is the interplay between the redox and canonical holobiont clocks that will be an important and challenging aspect of future chronobiomics research to address.

We must also consider the diversity and dominance of environmental cues that a holobiont may use for entrainment. An individual holobiont may each be exposed to a unique complexity of diverse environmental stimuli, specific to its local environment (Figure 1). Thus, as observed within the mantle of the giant clam Tridacna squamous, multiple rhythms set on different periods (e.g., circadian, circatidal) may regulate specific activities (Hiong et al., 2018), such that the efficiency of each is optimized through oscillations under with the relevant environmental stimulus. Furthermore, the relative dominance of specific cues may vary for different members of the holobiont, in accordance with the often-disparate conditions experienced by the microbiome compared with its host. For instance, within human gut microbiota, symbiont rhythms are entrained most dominantly by host nutrient intake, whilst for the host, environmental light provides the dominant zeitgeber (Bass and Takahashi, 2010; Oosterman et al., 2015). In sponges, the ceasing of pumping activity can lead to internal anoxic conditions, which bacterial symbionts of the microbiome are hypothesized to mitigate (Hoffmann et al., 2008; Lavy et al., 2016). Circadian clock timing enables organisms to anticipate daily fluctuating environmental conditions and respond appropriately, thus, maximizing the efficiency of the holobiont; by maintaining an optimum balance in the energy requirements demanded from activities, such as ROS homeostasis, growth, or reproduction, and to help ward off viral infections at a cellular level.

It is widely observed that organisms exhibit thresholds of disturbance, beyond which they are unable to sustain homeostatic cellular functioning and, thus, exhibit, often irreversible, health decline. This can most commonly be observed under climate change-induced disturbances, such as coral holobionts during exposure to degree heating weeks (DHW), whereby different coral holobionts exhibit varying degrees of resistance to the number of DHWs (Sully et al., 2019). Here, we emphasize the importance of understanding circadian clock molecular mechanisms to elucidate the relationship between specific environmental cues and regulatory mechanisms that nudge organisms past their tipping points beyond which they can cope. For instance, in vitro experiments have demonstrated the varied abilities of different organisms to sustain circadian regulation under conditions of prolonged circadian disturbance. We hypothesize that varying degrees of circadian clock robustness equate with longevity until disturbance thresholds are surpassed. That is, in the absence of appropriate environmental zeitgebers, a more robust clock may sustain rhythmic regulation of cellular activities for a longer duration, and, thus, be less likely to exhibit environmental disturbance-induced stress.

We have only just begun to touch the surface of understanding rhythmic regulations and the extent to which they can mediate cellular, organismal, or ecosystem-level functioning. Yet, circadian-driven regulatory systems, through both canonical molecular and redox clocks, have evolutionary ancient origins, of which the environmental zeitgebers entraining such rhythms are ubiquitous, although nuanced, for all organisms. Thus, we cannot stress enough the importance of elucidating the mechanisms by which organisms perceive entrainment cues and utilize them to optimize rhythmically coordinated regulation within their local environment.

The trophic interactions within large-scale planktonic systems, including the viral shunt, are extremely important for global biogeochemical cycling on a humungous scale, with numerous downstream effects for energy transfer within locally significant ecosystems, such as tropical coral reefs or deep-sea habitats. Dominant cues such as environmental light entrain the rhythms for free-living prokaryotes and viruses within large-scale planktonic communities. However, the molecular mechanisms by which these organisms coordinate such rhythms are less clear. To date, molecular evidence describing prokaryote circadian regulation is limited to Cyanobacteria and a few proteobacterial species. Though within the last decade, research has demonstrated that a circadian redox clock is ubiquitous across all domains, as well as less robust, simplified forms of the canonical molecular circadian clocks are also plentiful. Future research should expand our understanding of such mechanisms outside of Cyanobacteria to other important bacterial and archaeal phyla, which make up a significant part of these macro-scale ecosystems.

In parallel, rhythmically coordinated regulation of cellular activity is equally significant within micro-scale nested ecosystems of marine holobionts. Holobionts, which comprise a host and complex cohort of prokaryotic communities, interacting either competitively or beneficially with one another, are considered within an ecological context. Each member within this nested ecosystem will regulate activities on an independent endogenous clock. However, remarkably, the interdependent relationship between a host and its symbionts is also rhythmically coordinated, through both redox and canonical circadian clocks. This association is complex, particularly as there are likely numerous entrainment cues, of which the relative importance of each will differ for host and symbiont. Untangling the links between host and symbiont-driven circadian regulation will require extensive research. Currently, the best models for this are from mammals and a few terrestrial organisms. Whilst studies from these models are insightful, they are not necessarily representative and we, therefore, encourage further research on marine metazoan, particularly from earlier diverged lineages such as Cnidaria, Porifera, and Ctenophora. This will facilitate greater evolutionary insight into the origins of such mechanisms since they provide the earliest sister group to all other bilaterians. Additionally, these organisms are very important parts of reef ecosystems in marine habitats currently facing climate change-induced stressors.

It is important to recognize the rhythmically interconnected nature of both micro-and macro-scale ecosystems. Indeed, many of the above-mentioned reef organisms (e.g., coral, sponges) help couple benthic and pelagic zones themselves through their filter-feeding, meroplanktonic life cycle, provision of larval settlement cues, and providing breeding grounds for a remarkably high number of species. Within each ecosystem, every organism is regulated by its endogenous rhythms, but crucially, those rhythms must also be aligned with both the abiotic cues and other clock-controlled biotic cues from within their local environment.

With this review, we have introduced the relatively new field of marine chronobiomics within both macro-scale planktonic ecosystems and micro-scale holobiont nested ecosystems. We highlighted the interplay of biological rhythms across the living domains, as well as the not-so-living viruses, both within and between micro-and macro-scale ecosystems. Our review provides a baseline in which we emphasize the currently large gaps in knowledge and active research, to help guide future exploration and recognition of chronobiomics within the marine domain.

OH introduced the subject for this review. HS and OH drew Figures 1, 2, respectively, contributed equally toward the conception of this review, and provided essential and critical feedback throughout the writing process. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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