The molecular basis of diel and seasonal rhythmicity in the copepod Calanus finmarchicus

The copepod Calanus finmarchicus has an ecological key position in the northern Atlantic pelagic food web and its life is characterized by diel and seasonal rhythmicity. Neither diel nor seasonal rhythmicity of C. finmarchicus are understood with regard to their mechanistic regulation. Endogenous clock systems are central in controlling rhythms in various terrestrial species, but have hardly been investigated in marine organisms. This thesis shows that C. finmarchicus possesses an endogenous circadian clock, that regulated 24h rhythms of gene expression, metabolic activity and vertical migration behavior. The thesis further suggests that clock-based day length measurement and an endogenous annual clock is involved in the regulation of seasonal rhythmicity. The findings on C. finmarchicus’ timing systems are further related to the extreme light conditions in polar environments, discussing potential effects of climate chance on the copepods rhythmicity and biology

Calanus finmarchicus seasonal cycle and diapause in relation to gene expression, physiology, and endogenous clocks: Calanus finmarchicus seasonal rhythmicity

The copepod Calanus finmarchicus plays a crucial role in the north Atlantic food web. Its seasonal life cycle involves reproduction and development in surface waters before overwintering in diapause at depth. Although diapause has been studied for more than a century, the factors responsible for the initiation and termination of it are still unclear. Endogenous clocks have been identified as potent tools for photoperiod measurement and seasonal rhythmicity in many terrestrial species, but knowledge of these remains scarce in the marine realm. Focusing on the dominant CV copepodid stage, we sampled a population of C. finmarchicus from a Scottish sea loch to characterize population dynamics, several physiological parameters, and diel and seasonal expression rhythms of 35 genes representing different metabolic pathways, including the circadian clock machinery. This generated a detailed overview of the seasonal cycle of C. finmarchicus including the most extensive field dataset on circadian clock gene expression in a marine species to date. Gene expression patterns revealed distinct gene clusters upregulated at different phases of the copepod's seasonal cycle. While diel clock cycling was restricted to the active spring/summer phase, many clock genes exhibited the highest expression during diapause. Our results provide new insights into diapause on physiological and genetic levels. We suggest that photoperiod, in interaction with internal and external factors (lipid content, temperature, food availability) and the endogenous clock mechanism, plays an important role in the timing of diapause in C. finmarchicus

Calanus finmarchicus diel and seasonal rhythmicity in relation to endogenous timing under extreme polar photoperiods

Changing environmental conditions cause poleward distribution shifts in many marine organisms including the northern Atlantic key zooplankton species Calanus finmarchicus. The copepod has diel cycles of vertical migration and feeding, a seasonal life cycle with diapause in winter and a functioning circadian clock. Endogenous clock mechanisms control various aspects of rhythmic life and are heavily influenced by environmental light conditions. Here we explore how the extreme seasonal change in photoperiod (day length) in a high Arctic fjord affects circadian clock functioning as well as diel and seasonal cycles in C. finmarchicus. Expression of clock genes was measured in the active life phase at the end of midnight sun, in early diapause when photoperiod was ~12 h, and in late diapause during the polar night. While the clock maintained diel rhythmicity under extremely long photoperiods, it became arrhythmic during diapause. This was probably not due to a lack of light but was related to the physiological state of diapause. Seasonal expression analyses of 35 genes show distinct patterns for each investigated life phase. C. finmarchicus is able to maintain diel clock rhythmicity at photoperiods close to 24 h, and it is discussed how this may be related to the nature of the marine environment. The work also evaluates the potential negative consequences of rigid clock-based seasonal timing in a polar environment exposed to climate change and with high interannual variability

Rhythms and Clocks in Marine Organisms

The regular movements of waves and tides are obvious representations of the oceans’ rhythmicity. But the rhythms of marine life span across ecological niches and timescales, including short (in the range of hours) and long (in the range of days and months) periods. These rhythms regulate the physiology and behavior of individuals, as well as their interactions with each other and with the environment. This review highlights examples of rhythmicity in marine animals and algae that represent important groups of marine life across different habitats. The examples cover ecologically highly relevant species and a growing number of laboratory model systems that are used to disentangle key mechanistic principles. The review introduces fundamental concepts of chronobiology, such as the distinction between rhythmic and endogenous oscillator–driven processes. It also addresses the relevance of studying diverse rhythms and oscillators, as well as their interconnection, for making better predictions of how species will respond to environmental perturbations, including climate change. As the review aims to address scientists from the diverse fields of marine biology, ecology, and molecular chronobiology, all of which have their own scientific terms, we provide definitions of key terms throughout the article

Circadian Clock Involvement in Zooplankton Diel Vertical Migration

Biological clocks are a ubiquitous ancient and adaptive mechanism enabling organisms to anticipate environmental cycles and to regulate behavioral and physiological processes accordingly [1]. Although terrestrial circadian clocks are well understood, knowledge of clocks in marine organisms is still very limited [2, 3, 4, 5]. This is particularly true for abundant species displaying large-scale rhythms like diel vertical migration (DVM) that contribute significantly to shaping their respective ecosystems [6]. Here we describe exogenous cycles and endogenous rhythms associated with DVM of the ecologically important and highly abundant planktic copepod Calanus finmarchicus. In the laboratory, C. finmarchicus shows circadian rhythms of DVM, metabolism, and most core circadian clock genes (clock, period1, period2, timeless, cryptochrome2, and clockwork orange). Most of these genes also cycle in animals assessed in the wild, though expression is less rhythmic at depth (50–140 m) relative to shallow-caught animals (0–50 m). Further, peak expressions of clock genes generally occurred at either sunset or sunrise, coinciding with peak migration times. Including one of the first field investigations of clock genes in a marine species [5, 7], this study couples clock gene measurements with laboratory and field data on DVM. While the mechanistic connection remains elusive, our results imply a high degree of causality between clock gene expression and one of the planet’s largest daily migrations of biomass. We thus suggest that circadian clocks increase zooplankton fitness by optimizing the temporal trade-off between feeding and predator avoidance, especially when environmental drivers are weak or absent [8]

Impact of ocean acidification on thermal tolerance and acid–base regulation of Mytilus edulis from the White Sea

Ocean warming and acidification are two important environmental drivers affecting marine organisms. Organisms living at high latitudes might be especially threatened in near future, as current environmental changes are larger and occur faster. Therefore, we investigated the effect of hypercapnia on thermal tolerance and physiological performance of sub-Arctic Mytilus edulis from the White Sea. Mussels were exposed (2 weeks) to 390 µatm (control) and 1,120 µatm CO2 (year 2100) before respiration rate (MO2), anaerobic metabolite (succinate) level, haemolymph acid-base status, and intracellular pH (pHi) were determined during acute warming (10-28°C, 3°C over night). In normocapnic mussels, warming induced MO2 to rise exponentially until it levelled off beyond a breakpoint temperature of 20.5°C. Concurrently, haemolymph PCO2 rose significantly >19°C followed by a decrease in PO2 indicating the pejus temperature (TP, onset of thermal limitation). Succinate started to accumulate at 28°C under normocapnia defining the critical temperature (TC). pHi was maintained during warming until it dropped at 28°C, in line with the concomitant transition to anaerobiosis. At acclimation temperature, CO2 had only a minor impact. During warming, MO2 was stimulated by CO2 resulting in an elevated breakpoint of 25.8°C. Nevertheless, alterations in haemolymph gases (>16°C) and the concomitant changes of pHi and succinate level (25°C) occurred at lower temperature under hypercapnia versus normocapnia indicating a downward shift of both thermal limits TP and TC by CO2. Compared to temperate conspecifics, sub-Arctic mussels showed an enhanced thermal sensitivity, exacerbated further by hypercapnia, indicating their potential vulnerability to environmental changes projected for 2100

Environmental stress responses and experimental handling artifacts of a model organism, the copepod Acartia tonsa (Dana)

Handling animals during experiments potentially affects the differential expression of genes chosen as biomarkers of sub-lethal stress. RNA sequencing was used to examine whole-transcriptome responses caused by laboratory handling of the calanoid copepod, Acartia tonsa. Salinity shock (S = 35 to S = 5) was used as positive stress control; individuals not exposed to handling or other stressors served as negative stress control. All copepods were grown from eggs to adults without being handled or exposed to any stressors prior the experiment. Survival of nauplii and adults was estimated for up to 10 min of exposure to handling stress and salinity shock. Only adults exhibited decreased survival (44 ± 7% with 10 min of exposure) in response to handling stress and were selected for definitive experiments for RNA sequencing. After 10 min of experimental exposures to handling stress or salinity shock, adults were incubated for 15 min or 24 h at normal culture conditions. A small number of significantly differentially expressed genes (DEGs) were observed 15 min after exposure to handling stress (2 DEGs) or salinity shock (7 DEGs). However, 24 h after exposure, handling stress resulted in 276 DEGs and salinity shock resulted in 573 DEGs, of which 174 DEGs were overlapping between the treatments. Among the DEGs observed 24 h after exposure to handling stress or salinity shock, some commonly-used stress biomarkers appeared at low levels. This suggests that a stress-response was induced at the transcriptional level for these genes between 15 min and 24 h following exposure. Since handling stress clearly affects transcriptional patterns, it is important to consider handling when designing experiments, by either including additional controls or avoiding focus on impacted genes. Not considering handling in gene expression studies can lead to inaccurate conclusions. The present study provides a baseline for studying handling stress in future studies using this model organism and others

Jag och min yxa- En undersökning om vad stridsyxan kan ha haft för betydelse inom SYK i Sydsverige.

Syftet med min uppsats har varit att uppnå bättre kunskap kring de befintliga teorier som finns kring stridsyxan och dess betydelse. Materialet jag har använt har varit relevant och genom att sammanställa befintliga teorier samt skapa en egen tolkning har jag fått ett svar på min problemställning; vilken betydelse hade stridsyxan för SYK (stridsyxekulturen) i Sydsverige? Min kritiska granskning har resulterat i att jag skapat en tolkning som är baserad på att stridsyxan är en identitetsskapare till en grupp människor som har tillverkat, ägt och tagit med sin egen stridsyxa ner i graven. Nya frågeställningar har skapats och vidare diskussioner kan ske utifrån mitt resultat av studierna jag har gjort

Clock genes in a north Atlantic key zooplankter - Expression during overwintering in a high Arctic fjord

The copepod Calanus finmarchicus plays a crucial role in the north Atlantic food web, channelling energy from phytoplankton primary production to higher trophic levels including commercially important fish stocks like herring and cod. The copepod species is spreading northward into the Arctic due to ocean warming. The activity phase of C. finmarchicus in spring/summer is characterized by diel vertical migration, meaning that the animals migrate to surface waters around sunset to feed, and back to deeper layers around sunrise to hide from visual predators. This rhythmic vertical migration behaviour is characteristic for zooplankton communities all around the world. At the end of the activity phase in autumn, C. finmarchicus enters an overwintering mode and inactively dwell in deep waters until next spring when it starts a new generation cycle. Although both rhythms (diel and seasonal) have been studied for more than a century, the exact factors controlling them are still unclear. Molecular techniques have precisely described genetic clockworks in numerous species and there is clear evidence that clock genes are not only involved in the regulation of diel 24h rhythms, but also in the entrainment of the seasonal cycle. We present first records of clock gene expression in Calanus finmarchicus from a high Arctic fjord in Svalbard at 79°N and compare gene activity between specimen in the early and late phase of overwintering. Copepods were sampled from overwintering depth (>220 m) in September 2014 when surface photoperiod was about 10 hours and during polar night in January 2015 when no light was present. Samples were analysed by quantitative real-time PCR (qRT-PCR) using custom designed Taqman® low-density array cards. The results show clear 24h oscillations in most genes for September, whereas gene expression is almost completely arrhythmic during the polar night in January. It furthermore appears that in September most of the investigated clock genes show distinct expressions patterns, which often match pattern previously observed in other (model) species. For example, expression of period (1 & 2) is highest around sunset (per1) or early night (per2) whereas activity of clock sharply increases around sunrise and peaks in the afternoon. Expression of cryptochrome 1 is highest around midnight while expression of cryptochrome 2 shows patterns similar to those of the period genes. The results strongly point towards the existence of a light-entrained genetic clock in Calanus finmarchicus that becomes arrhythmic during the constant darkness of the polar night. Our work presents an example on how the vast mechanistic knowledge about endogenous timekeeping gained from model organisms can be transferred to field studies on non-model species of high ecological relevance