The performance of Antarctic krill Euphausia superba to climate induced environmental changes (started March 2012)
Despite their unique position and 90 years of krill research in the Southern Ocean knowledge of the performance of krill to environmental changes is lacking. In the past krill research was mainly driven by commercial interest and hence focused on krill distribution and abundance and not on the interaction of krill’s life cycle with the highly seasonal environment. Over the past 30 years, krill biomass in the southwest Atlantic sector declined significantly in concert with a decline in sea ice extent and duration, suggesting that this correlation is driven by recruitment rather than predation pressure on adult krill. However, the reason for this linkage is far from clear. For decoding the environmental impact on the life cycle of krill we need to progress beyond correlative studies towards a mechanistic understanding of the system. Therefore, the basic aim of the PhD-study is to investigate how adaptable krill is to climate induced environmental changes, focussing on effects of increasing seawater temperature on adult krill.
Hypothesis: Adult Krill have a narrow temperature range of 0 °C to 4 °C for optimal growth and physiological functions
The major aim is to study the performance of adult krill to increasing seawater temperature – from 0 °C to 7 °C over a period of 6 months – at the krill aquarium of the Australian Antarctic Division (AAD) in the Krill group of Dr. So Kawaguchi. Analyses on an organismic level (respiration rate, feeding activity, moulting frequency and growth) will be conducted during the experiment at the AAD. In addition, the effect of warming on functional and regulatory networks on a molecular and genetic level in krill, subsampled from the experiment, will be analysed in the krill group of Prof. Dr. Bettina Meyer at AWI. As a prerequisite, this group already generated a high-quality normalized cDNA library, obtained from total RNA extracted from different krill tissues sampled in different seasons, which has been subsequently pyrosequenced. Based on these data, the group generated a huge set of specific primers which will allow us to study differential gene expression of genes specifically involved in temperature sensitive pathways. As it was recently shown that important physiological and behavioural functions in krill are driven by an endogenous clock machinery (i.e. circadian clock) on a transcriptional level, another focus will be to study the effect of temperature on the performance of the circadian clock. One highly conserved feature of the circadian clock is that its period remains relatively constant over a physiological range of temperatures, so called temperature compensation. Therefore, we will measure the period of rhythmic clock gene expression of krill subsampled from the above mentioned temperature experiment.
Both data sets provide a comprehensive view of the performance of krill to a warming ocean. The AAD is the only institute worldwide to perform long term experiments with krill and has long standing experience and expertise in keeping and maintaining krill in captivity.
The data yielded from this PhD project are crucial for krill life cycle and ecosystem models and to make predictions about how krill stock and – due to the central position of krill – the ecosystem might change under different climate scenarios.
The molecular basis of diel and seasonal rhythmicity in Calanus finmarchicus and other polar pelagic copepods (started May 2014)
This PhD project will focus on the molecular and genetic basis of daily and seasonal rhythms in calanoid copepods (Crustacea), which play an essential role in many pelagic marine ecosystems, linking primary production to higher trophic levels. At first emphasis will be put on Calanus finmarchicus, a key species of the north Atlantic subpolar biome, which has already been the subject of extensive research and for which enormous genomic resources are available. In higher latitudes, the daily and seasonal changes in environmental conditions (e.g. day length and light intensity) are especially pronounced, which means that the requirements for the stability but also flexibility of internal rhythms are especially high in these regions. Thus, investigations on polar species of calanoid copepods such as C. glacialis (Arctic) and Calanoides acutus (Southern Ocean) in comparison to the subpolar C. finmarchicus are planned.
There is extensive literature about the diel vertical migration (DVM) of C. finmarchicus, which is characterized by an ascent to the surface for feeding around sunset and a descent back to daytime depth around sunrise. Also there are numerous studies about the seasonal changes in community structure, vertical distribution and the associated diapause, a state that individuals of the fifth copepodid stage (CV) of C. finmarchicus and other calanoid copepods enter in winter and that is characterized by behavioral and metabolic inactivity in deeper water layers. However, although these biological cycles haven been extensively studied, the genetic evidence for the existence of an endogenous clock in this or any other copepod species and for its involvement in the control of biological rhythmicity is still missing. Furthermore, little is known about the way in which circadian clocks are entrained by external cues and – by sensing photoperiod – enable adaptions to seasonal changes. Most copepods show clear seasonal patterns of activity and community structure, but little is known about how and whether these rhythms are connected to seasonal changes in gene expression. Also, there are pronounced differences in the variability and entrainment capacity of circadian cycles among and within copepod species. Both factors seem to increase at higher latitudes where the seasonal changes in photoperiod are especially pronounced. Therefore, the PhD project aims to address the following three core topics:
Detection of an endogenous circadian clock in the calanoid copepod C. finmarchicus:
This shall be achieved by measuring changes in the expression of potential clock genes over a 24 hour time course. Measurements will be performed on C. finmarchicus collected from their natural environment as well as on C. finmarchicus kept under a controlled light/dark-cycle or under constant darkness in the laboratory. Showing the persistence of rhythmic clock gene expression in nature and under constant laboratory conditions would clearly point towards the existence of an endogenous circadian clock in C. finmarchicus. Investigating diurnal cycles of transcription in the wild, together with the information about clock gene expression, can give some indication about the role of the circadian clock in the control of daily behavioural and physiological patterns such as DVM.
Photoperiodic entrainment of the circadian clock in C. finmarchicus and changes in the seasonal transcriptome:
Entrainment-effects of seasonal changes in photoperiod on the circadian clock could be crucial in the regulation of important annual biological rhythms in the phenology of C. finmarchicus, such as diapause. To this end, a comparison of clock gene expression of C. finmarchicus sampled over 24 hour cycles before, during and after diapause, and seasonal transcriptomic changes related to diapause may grant insights into the genetic basis of this annual rhythmicity as well as into mechanisms of photoperiodic time measurement.
Latitude-dependent variability and entrainment-capacity of circadian cycles in calanoid copepods:
Seasonal changes in photoperiod are especially pronounced at higher latitudes and there is evidence that species account for this by showing stronger flexibility and entrainment-capacity of their circadian clocks. To investigate this, results obtained from the subpolar C. finmarchicus will be compared to two truly polar calanoid copepod species, C. glacialis (Arctic) and Calanoides acutus (Southern Ocean). Therefore, 24 hour sampling will be performed at different seasons for C. glacialis and C. acutus and differences in the expression pattern over the day and over the seasons will be analyzed. This shall reveal possible latitude-dependent adaptations used by species living under strongly fluctuating photoperiods.
The molecular mechanisms underlying biological rhythms and clocks in Antarctic krill Euphausia superba at the daily and annual scale (started September 2015)
Within the overall aim of the HVI PolarTime, my PhD project will focus on the effect of photoperiod as the main Zeitgeber for the endogenous clock of the Antarctic krill Euphausia superba.
The presence of an endogenous circadian clock in the Antarctic krill has now been established, and its major molecular components have been identified. The circadian clock regulates daily patterns of metabolic activity, physiology and behavior, which are of main importance for the adaptation of the organism to its environment. Like for other species, also in the Antarctic krill photoperiod has been indicated as the main Zeitgeber for the functioning of the circadian clock.
Living in a polar environment, the Antarctic krill is exposed throughout the year to a significant photoperiodic variation, whit the light-dark cycle being dominated by darkness during winter and by light during summer. How does the circadian clock of the Antarctic krill cope with this extreme situation? Does the clock turn off when the photoperiodic cue is weakening, or does it continue to work? And if it works, how is it possible? And if it does not work, how do the circadian patterns regulated by the clock react, do they keep on with their oscillation or do they turn off together with the clock?
Apart of the circadian patterns of behavior and metabolic activity which are under the influence of the circadian clock, the Antarctic krill is also characterized by seasonal patterns of body growth, feeding and metabolic activity, respiration and lipid utilization, which are of extreme importance for the maintenance of a favorable energy budget throughout the year. Growing evidence indicates that these patterns may be driven by endogenous factors, and that photoperiod may play a major role in their entrainment as well, but how exactly this process does work, it is still an open question. Which are the molecular mechanisms underlying seasonal timekeeping? From other species, we know that the circadian clock can play an important role in the regulation of the seasonal rhythms by sensing the changes in photoperiod and by measuring day length: will it be the case also for the Antarctic krill?
The seasonal patterns which are shown at the metabolic, physiological and behavioral level all play their role in the seasonal cycle of maturation, reproduction and regression, which dominates the life cycle of the Antarctic krill in the Southern Ocean. Most interestingly, this seasonal cycle closely resembles the seasonal pattern of food availability, which is characterized by high food in spring-summer and low food in autumn-winter. Food availability is therefore another key factor that we must take into account to understand the regulation of seasonal timekeeping in krill. How do photoperiod and food availability interact to control maturation and reproduction? Is the maturation process influenced in a deeper way by changes in light or in food conditions? Can the maturation process be completed successfully if one of these two components is not present in the right amount at the right moment? What may happen if, under a not so unlikely climate change scenario, the natural patterns of food availability in the Southern Ocean will be altered?
I will address these questions by analyzing gene expression of clock and clock-related genes, together with other functionally important genes, in experimental krill exposed to different photoperiodic and feeding conditions.
Seasonal, physiological and genetic functions in Antarctic krill, Euphausia superba, at different latitudes in the Southern Ocean (started March 2015)
Antarctic krill, Euphausia superba, a key species in the Southern Ocean, is able to adapt to extreme seasonal fluctuations in light availability, primary productivity and sea ice extent, synchronizing its seasonal life cycle to the local light regime and food supply. This includes the growth and reproductive cycle as well as seasonal physiological patterns in metabolic activity and lipid turnover. It has been proposed that these adaptational seasonal rhythms are controlled by an endogenous timing system in krill and that photoperiod seems to act as a main Zeitgeber, synchronizing the internal clock with the natural year. However, the signaling cascade that links the photoperiod cue to the target response is unknown yet and molecular mechanisms of the seasonal timing system remain far from being understood. Moreover, the circumpolar distribution of Antarctic krill extends a remarkably broad latitudinal range from ~51°S to ~70°S meaning that the seasonal photoperiodic regime as well as the accompanied seasonal food pattern experienced by krill may differ significantly with latitude.
This PhD project aims to study latitude dependent gene expression patterns during the phenology of krill focussing on physiological functions such as lipid dynamics, metabolic activity and maturation. In addition, the project aims to investigate the putative role of the krill’s circadian clock in photoperiodic time measurement. The project is divided into three sub-projects: (1) the analysis of seasonal gene expression patterns from different latitudinal regions, (2) studying differential gene expression during long-term photoperiodic controlled lab experiments of simulated latitudinal regions and (3) determining transcript levels of canonical clock genes in correlation with key physiological target genes around the diurnal cycle under different photoperiodic conditions throughout the season.
An RNAseq approach will be used to examine the transcriptomic profile of summer and winter field samples of three different latitudinal regions: South Georgia (54°S), Bransfield Strait (63°S) and Lazarev Sea (66°-70°S). Based on these results selected genes will be used to develop a TaqMan low density array card representing up to 300 seasonally important target genes. Subsequently, these cards will be used to test the impact of photoperiod on physiological functions of Antarctic krill within lab experiments under controlled light conditions (54°S, 63°S and 66°S) and maximum food supply over a period of 1 year. The role of the endogenous clock of Antarctic krill in seasonal time keeping will be analysed in field samples of 24-h sampling periods in different seasons. Single gene assays and a clock orientated TaqMan card will be used to look for rhythmic gene expression patterns of internal clock genes in correlation to oscillations of metabolic key enzymes.