The circadian clock in Calanus finmarchicus – Relation to diel vertical migration

The marine copepod Calanus finmarchicus is an important key species in the Northern Atlantic due to his abundance and his position in the food web. It performs diel vertical migration (DVM), staying in deeper water layers during the day and ascending to the surface in the night. The exact trigger for the DVM is not known yet, but light seems to have an important influence on the position of C. finmarchicus. Some studies suggest an involvement of an endogenous rhythm, which controls the vertical position of C. finmarchicus throughout the day. In this work the DVM and respiration rate of C. finmarchicus were examined under natural simulated light conditions to identify possible circadian rhythms. Therefore, two laboratory experiments were performed with the CV-stage of C. finmarchicus under light/dark (LD) and constant darkness (DD) conditions. The position of C. finmarchicus in the DVM experiment showed a clear diurnal rhythm, with significant differences between day and night. The rhythm persisted in weaker form during constant darkness, indicating that an endogenous circadian clock is involved in the DVM. The results from the respiration experiment supported the assumption, revealing a rhythmicity in the oxygen uptake that also persisted under constant darkness. The light seemed to have in both experiments the role of a Zeitgeber that synchronises the circadian clock. For a final identification of the assumed clock a genetic analysis is necessary. However the experiments showed evidence that the DVM and the metabolic activity of C. finmarchicus are controlled by a circadian clock

Maintenance of Intraspecific Diversity in Response to Species Competition and Nutrient Fluctuations

Intraspecific diversity is a substantial part of biodiversity, yet little is known about its maintenance. Understanding mechanisms of intraspecific diversity shifts provides realistic detail about how phytoplankton communities evolve to new environmental conditions, a process especially important in times of climate change. Here, we aimed to identify factors that maintain genotype diversity and link the observed diversity change to measured phytoplankton morpho-functional traits Vmax and cell size of the species and genotypes. In an experimental setup, the two phytoplankton species Emiliania huxleyi and Chaetoceros affinis, each consisting of nine genotypes, were cultivated separately and together under different fluctuation and nutrient regimes. Their genotype composition was assessed after 49 and 91 days, and Shannon’s diversity index was calculated on the genotype level. We found that a higher intraspecific diversity can be maintained in the presence of a competitor, provided it has a substantial proportion to total biovolume. Both fluctuation and nutrient regime showed species-specific effects and especially structured genotype sorting of C. affinis. While we could relate species sorting with the measured traits, genotype diversity shifts could only be partly explained. The observed context dependency of genotype maintenance suggests that the evolutionary potential could be better understood, if studied in more natural settings including fluctuations and competition

Season affects strength and direction of the interactive impacts of ocean warming and biotic stress in a coastal seaweed ecosystem

The plea for using more “realistic,” community‐level, investigations to assess the ecological impacts of global change has recently intensified. Such experiments are typically more complex, longer, more expensive, and harder to interpret than simple organism‐level benchtop experiments. Are they worth the extra effort? Using outdoor mesocosms, we investigated the effects of ocean warming (OW) and acidification (OA), their combination (OAW), and their natural fluctuations on coastal communities of the western Baltic Sea during all four seasons. These communities are dominated by the perennial and canopy‐forming macrophyte Fucus vesiculosus—an important ecosystem engineer Baltic‐wide. We, additionally, assessed the direct response of organisms to temperature and pH in benchtop experiments, and examined how well organism‐level responses can predict community‐level responses to the dominant driver, OW. OW affected the mesocosm communities substantially stronger than acidification. OW provoked structural and functional shifts in the community that differed in strength and direction among seasons. The organism‐level response to OW matched well the community‐level response of a given species only under warm and cold thermal stress, that is, in summer and winter. In other seasons, shifts in biotic interactions masked the direct OW effects. The combination of direct OW effects and OW‐driven shifts of biotic interactions is likely to jeopardize the future of the habitat‐forming macroalga F. vesiculosus in the Baltic Sea. Furthermore, we conclude that seasonal mesocosm experiments are essential for our understanding of global change impact because they take into account the important fluctuations of abiotic and biotic pressures

Maintenance of Intraspecific Diversity in Response to Species Competition and Nutrient Fluctuations

Intraspecific diversity is a substantial part of biodiversity, yet little is known about its maintenance. Understanding mechanisms of intraspecific diversity shifts provides realistic detail about how phytoplankton communities evolve to new environmental conditions, a process especially important in times of climate change. Here, we aimed to identify factors that maintain genotype diversity and link the observed diversity change to measured phytoplankton morpho-functional traits Vmax and cell size of the species and genotypes. In an experimental setup, the two phytoplankton species Emiliania huxleyi and Chaetoceros affinis, each consisting of nine genotypes, were cultivated separately and together under different fluctuation and nutrient regimes. Their genotype composition was assessed after 49 and 91 days, and Shannon’s diversity index was calculated on the genotype level. We found that a higher intraspecific diversity can be maintained in the presence of a competitor, provided it has a substantial proportion to total biovolume. Both fluctuation and nutrient regime showed species-specific effects and especially structured genotype sorting of C. affinis. While we could relate species sorting with the measured traits, genotype diversity shifts could only be partly explained. The observed context dependency of genotype maintenance suggests that the evolutionary potential could be better understood, if studied in more natural settings including fluctuations and competition

Trait measurements and long-term experiment on maintenance of intraspecific diversity using genotypes of phytoplankton species Chaetoceros affinis and Emiliania huxleyi

Measurements of cell density, nutrient concentration and genotype composition in a long term experiment (91 days) with the marine phytoplankton species Chaetoceros affinis and Emiliania huxleyi, each consisting of nine genotypes. Cultivation of species was done separately in mono-cultures and together in mix-cultures at three different nutrient regimes (10N:1P, 20N:1P, and 30N:1P) with increasing nitrate concentration in a semi-continuous batch cycle system. Transfer of part of the cells into bottles with new nutrients every 7 days at fixed batch cycle length and after 7,4, and 10 days in a recurring fashion at variable batch cycle length. With the information about the genotype abundance we assessed how intraspecific diversity is maintained in response to species competition and nutrient fluctuations. Individual trait measurements for growth, nutrient uptake, and cell volume of the genotypes at seven nitrate levels in a 4-day experiment allowed us to connect traits to the genotype sorting of the long term experiment

Trait measurements of the phytoplankton species Chaetoceros affinis and Emiliania huxleyi

Trait measurements for growth, nutrient uptake, and cell volume for nine genotypes each of the phytoplankton species Chaetoceros affinis and Emiliania huxleyi individually during 4 days of cultivation at seven nitrate levels. Nitrate levels for genotypes of C. affinis were 2.5, 5, 7.5, 12.5, 20, 30, and 45 μmol L−1 with 2 μmol L−1 phosphate and 2.5, 5, 7.5, 10, 15, 20, and 25 μmol L−1 with 1.5 μmol L−1 phosphate for genotypes of E. huxleyi

Long term experiment using a semi-continuous batch cycle system with the phytoplankton species Chaetoceros affinis and Emiliania huxleyi

Long term experiment using a semi-continuous batch cycle system with the phytoplankton species Chaetoceros affinis and Emiliania huxleyi each consisting of 9 genotypes (http://roscoff-culture-collection.org/; Emiliania huxleyi: GC22: RCC6383, C42: RCC6376, C41: RCC6375, C47: RCC4546, C96: RCC6381, C91: RCC6380, C35: RCC6374, C30: RCC6371, C48: RCC6377, Chaetoceros affinis: B63: RCC6347, B67: RCC6348, B68: RCC6349, B74: RCC6350). Cultivation of species separately in mono-cultures and together in mix-cultures at three different nutrient regimes (10N:1P, 20N:1P, and 30N:1P) with increasing nitrate concentration over 91 days. Nutrient concentrations were 8.60 ± 0.62, 19.08 ± 0.32, and 29.36 ± 0.47 µmol L−1 nitrate and 0.93 ± 0.09 phosphate. Silicate concentrations were aligned to reflect a 4:1 N:Si ratio. Transfer of part of the cells into bottles with new nutrients every 7 days at fixed batch cycle length and after 7,4, and 10 days in a recurring fashion at variable batch cycle length. Measurements of cell density and cell volume at the end of every batch cycle and cell density or fluorescence daily during the third batch cycle. Nitrate, phosphate, and silicate measurements in mix-cultures at variable batch cycle length at the end of the first (7 days), second (4 days), and third (10 days) batch cycle. Daily nutrient measurements in mix-cultures from day 4 to day 7 at batch cycle seven

Long term experiment with the phytoplankton species Chaetoceros affinis and Emiliania huxleyi: genotype abundance

Genotype abundance present in the cultures of the long term experiment after batch cycles 7 and 13, identified using microsatellites

Experiments on coexisting phytoplankton species Chaetoceros affinis and Emiliania huxleyi in response to nutrients

Measurements of cell size, cell density, nutrient concentration and genotype composition in a long-term experiment (182 days) with the marine phytoplankton species Chaetoceros affinis and Emiliania huxleyi, each consisting of nine genotypes. The species were cultivated together at three different nutrient regimes (10 N, 20 N, 30 N) with increasing nitrate supply in a semi-continuous batch cycle system. The genotype composition of both species was assessed after 49, 91, and 182 days using microsatellites. In a short-term experiment cell size and density of nine Chaetoceros affinis genotypes separately were measured after 7 days growth at seven nitrate levels (2.5, 5, 7.5, 12.5, 20, 30, and 45 μmol L−1 N)