Periodic Reporting for period 1 - Cocco-Next (Physiological adaptations to ecological niches in coccolithophore haplodiplontic life cycle)
Période du rapport: 2022-11-14 au 2024-11-13
Marine microalgae, also called phytoplankton, are crucial contributors to global carbon cycle and oceanic ecology. Indeed, they perform photosynthesis, the light-driven capture on CO2 into biomass, which is a carbon sink and a founding reaction of marine food webs. More specifically, coccolithophores are an important group of unicellular algae because they are very abundant and produce an “exoskeleton” made of calcium carbonate.
In contrast with most eukaryotes (e.g. animals and land plants), coccolithophores thrive through a haplo-diplontic life cycle: both haploids and diploids (equivalent to gametes and egg cells, respectively) can undergo asexual divisions (mitosis). Moreover, within a given species, haploid and diploid cells generally form distinct extra-cellular calcareous plates. In surveys of phytoplankton spatiotemporal distribution, abundance of coccolithophore’s haploid and diploid cells are generally contrasted. It has thus been proposed that haploids and diploids display different physiological features to colonize distinct ecological niches of the ocean. The aim of this project is to challenge this hypothesis.
Phytoplankton species have been relatively rarely studied in laboratory cultures because their growth is generally slow and requires highly specific conditions. I focus on the calcifying coccolithophore Calcidiscus leptoporus as a new alternative model specis and combine growth tests and measures of photosynthetic performance in strains from various locations around the globe. To gain insight at the molecular level, I also investigate the genome. Studying the life cycle of coccolithophore addresses both a fundamental topic and global ecological questions; indeed, human activities have multiple drastic effects on the ocean, and how this impacts phytoplankton, the first link in ecosystems food networks, is a major issue for biodiversity, society and economy.
In contrast with most eukaryotes (e.g. animals and land plants), coccolithophores thrive through a haplo-diplontic life cycle: both haploids and diploids (equivalent to gametes and egg cells, respectively) can undergo asexual divisions (mitosis). Moreover, within a given species, haploid and diploid cells generally form distinct extra-cellular calcareous plates. In surveys of phytoplankton spatiotemporal distribution, abundance of coccolithophore’s haploid and diploid cells are generally contrasted. It has thus been proposed that haploids and diploids display different physiological features to colonize distinct ecological niches of the ocean. The aim of this project is to challenge this hypothesis.
Phytoplankton species have been relatively rarely studied in laboratory cultures because their growth is generally slow and requires highly specific conditions. I focus on the calcifying coccolithophore Calcidiscus leptoporus as a new alternative model specis and combine growth tests and measures of photosynthetic performance in strains from various locations around the globe. To gain insight at the molecular level, I also investigate the genome. Studying the life cycle of coccolithophore addresses both a fundamental topic and global ecological questions; indeed, human activities have multiple drastic effects on the ocean, and how this impacts phytoplankton, the first link in ecosystems food networks, is a major issue for biodiversity, society and economy.
Several strains of Calcidiscus were cultured, representing haploid and diploid cells from distinct sampling locations around the world. In replete media, the growth rate was similar between haploid and diploid strains, although the haploid strains reached a higher cell concentration at the end of the exponential phase. The differences in photosynthetic performance (e.g. electron transfer rate) was rather strain-specific than attributable to ploidy.
Another angle to study the niche of haploids and diploids was to induce transitions from one state to the other. I placed the strains in multiple distinct growth media and changed some of the physical constraints, such as temperature and light intensity. However, both haploids and diploids were sensitive to the stressful conditions, e.g. excess micro-nutrients. Under less constrained conditions, cells were observed under a microscope but I did not detect any meiosis (division of diploid into haploid cells) nor syngamy (fecundation) between haploid strains. This suggests that life cycle transitions are triggered by a more complex stimulus, or even a combination of stimuli.
Finally, I sequenced the DNA of C. leptoporus and produced the first genome for this species. The sequencing also encompassed the bacteria that are co-cultured with this coccolithophore. From these genomes, I predicted their capacity to synthetize vitamins or their reliance on other members of this micro-community. I then challenged this metabolic interplay by studying how the supplementation of vitamins in the growth media alters the growth of C. leptoporus and the composition of its bacterial community.
Another angle to study the niche of haploids and diploids was to induce transitions from one state to the other. I placed the strains in multiple distinct growth media and changed some of the physical constraints, such as temperature and light intensity. However, both haploids and diploids were sensitive to the stressful conditions, e.g. excess micro-nutrients. Under less constrained conditions, cells were observed under a microscope but I did not detect any meiosis (division of diploid into haploid cells) nor syngamy (fecundation) between haploid strains. This suggests that life cycle transitions are triggered by a more complex stimulus, or even a combination of stimuli.
Finally, I sequenced the DNA of C. leptoporus and produced the first genome for this species. The sequencing also encompassed the bacteria that are co-cultured with this coccolithophore. From these genomes, I predicted their capacity to synthetize vitamins or their reliance on other members of this micro-community. I then challenged this metabolic interplay by studying how the supplementation of vitamins in the growth media alters the growth of C. leptoporus and the composition of its bacterial community.
The metagenome that we will publish soon includes the first genome for C. leptoporus, which is only the 10th genome from haptophytes, thus greatly enriching the dataset for this important group of phytoplankton. Besides, the long-read based bacterial genomes also provide new resolution of the bacterial capacities to synthetize vitamins, especially for new bacterial species. This will enable us and others to highlight the ecological relations, e.g. symbiosis or predation, at the base of food webs. This is critical to understand the spatiotemporal niches of the distinct phytoplankton species, which are major contributors to primary productivity, in particular the still unpredictable episodes of bloom and collapse of coccolithophores.
Our project had the very ambitious goal of investigating the triggers of the sexual cycles of coccolithophores; we have not been able to evidence them, although we thoroughly observed diploid and haploid cells in many distinct growth conditions. We can rule out some of the simplistic environmental cues (e.g. low nitrogen or light level or temperature), but we need more high-throughput methods to investigate the potential impact of multiple cues (e.g. low N, high temperature and high light).
Relating to the host institute, I have implemented several new approaches in the laboratory: photosynthesis measurements, molecular biology and sequencing. These new tools are being used beyond our group and are parts of several grant applications submitted by other group leaders which are not primarily biologists. These are key contributions to the ambition of our laboratory to provide inter-disciplinary approaches in marine research.
Our project had the very ambitious goal of investigating the triggers of the sexual cycles of coccolithophores; we have not been able to evidence them, although we thoroughly observed diploid and haploid cells in many distinct growth conditions. We can rule out some of the simplistic environmental cues (e.g. low nitrogen or light level or temperature), but we need more high-throughput methods to investigate the potential impact of multiple cues (e.g. low N, high temperature and high light).
Relating to the host institute, I have implemented several new approaches in the laboratory: photosynthesis measurements, molecular biology and sequencing. These new tools are being used beyond our group and are parts of several grant applications submitted by other group leaders which are not primarily biologists. These are key contributions to the ambition of our laboratory to provide inter-disciplinary approaches in marine research.