Mechanisms underlying regime shifts to picophytoplankton dominance in coastal water bodies

• Applicant: Dr. Maximilian Berthold
• Subject Area: Microbial Ecology and Applied Microbiology ; Oceanography
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426659886

• Final Report Year: 2024

Final Report Abstract

Picocyanobacteria, including Cyanobium, are major contributors to global primary production. These picocyanobacteria occur from fresh water to fully marine areas, from oligo- to eutrophic water bodies, and across light gradients. Recently, picocyanobacteria started to emerge in more aquatic ecosystems potentially as follow up of human restoration measures. The wide distribution and diversity of picocyanobacteria can be explained by genomic capacities to cope with variable environmental factors. The goals of this study were to describe the ecological potentials of coastal Cyanobium strains and further explain niche formation and phytoplankton size class abundances in such coastal water bodies. I combined physiomic with genomic studies to identify if picocyanobacteria can increase their abundance taking increased water temperature, and increased nutrient thresholds into account. I used five phylogenetically related Cyanobium strains (sub-cluster 5.2) with described genomes, and a model cyanobacteria Synechocystis PCC6803. I developed a high-throughput well approach to test strain-specific growth responses to salinity (six levels), temperature (four levels), photoperiod (four levels), nitrogen (four levels), phosphorus (five levels) and irradiance (three levels), to analyze and describe this vast diversity. Growth rates were determined in vivo using absorbance at OD680nm and OD720nm. Among others, cell counts, maximum quantum yield of Photosystem II, pigment compositions and sinking rates were analyzed across the growth trajectory. Growth rates were used to describe an optimum niche space using generalized additive mixed models. Strains showed distinct biomass and growth rate peaks at different salinities, temperatures, photoperiods, nutrient concentrations and light levels. Furthermore, pigment composition changed at each light level, which affected the fluorescence yield, but was also co-dependent on optimum salinity, temperature, photoperiod, or nitrogen concentrations. Strains usually reached highest growth rates under increased temperatures and longer photoperiods, thus indicating a potential to increase their abundance in the future ocean. I concluded that across an environmental gradient, each strain shows a growth optimum for a set combination of variables, thus explaining observed diversity within ecosystems. These findings make controlling such strains in the environment particularly difficult, as their meta-population genomic capacity supports their resilience against perturbations.

Publications

• Restoration, conservation and phytoplankton hysteresis. Conservation Physiology, 9(1).
  Berthold, Maximilian & Campbell, Douglas A.
  
• Code repository for calibrating FRRf patterns against oxygen evolution
  Berthold, M. et al.
  
• Code repository for calibrating, tidying, analyzing and fitting well plate data
  Berthold, M. et al.
  
• Code repository for high-throughput analyses of phytoplankton sinking
  Berthold, M. et al.
  
• Code repository for machine learning work on phytoplankton blooms
  Berthold, M. et al.
  
• Cyanobium Genome sequencing and assembly. NCBI
  Berthold, M.; Omar, N. & D. A. Campbell