Towards Understanding How Sugars Shape Life in the Oceans

The carbon cycle represents the planet's most extensive biogeochemical process. Each year, more than 100 gigatons (Gt) of carbon are captured through photosynthesis, with terrestrial plants and marine algae each contributing roughly equal amounts. Notably, marine planktonic microalgae, particularly diatoms, play a crucial role, fixing up to 20 Gt of carbon annually. This carbon fixation peaks during phytoplankton blooms, triggered by optimal sunlight and nutrient availability. However, these blooms are transient, often ceasing due to factors like self-shading, nutrient depletion, viral infections, and grazing by protists and invertebrates. Throughout these blooms, substantial amounts of organic material (mainly polysaccharides) enter the seawater, sustaining pools of dissolved and particulate organic matter. Marine bacteria play a vital role in rapidly recycling a significant portion of this organic matter in surface waters.

In our recent study, we set out to uncover the impact of dissolved storage polysaccharides (glycans) on the composition of these abundant bacterial clades during the spring phytoplankton bloom of year 2020 at Helgoland Roads in the North Sea.

Our findings unveiled a compelling narrative: dissolved polysaccharides, particularly the abundant storage polysaccharides, wield a significant influence over the composition of bacterioplankton members during these blooms. As the biomass of algal blooms pulses through the ocean, it carries with it a rich tapestry of distinct polysaccharides. The microbial decomposition of these complex sugars emerges as a process of paramount importance in shaping the microbial landscape.

Intriguingly, our analysis illuminated how the quantities and varieties of dissolved polysaccharides exert a pronounced sway over the composition of bacterioplankton during phytoplankton blooms. The release of algal glycans and the recycling of bacterial glycans, fueled by increased bacterial cell mortality, emerge as pivotal factors reshaping bacterioplankton communities in these dynamic environments. Further exploration revealed the nuanced dynamics of the 2020 spring phytoplankton bloom, characterized by distinct phases dominated by different algal species. Crucially, the composition of bacterioplankton communities mirrored the presence of specific diatom species, unveiling a tightly interwoven relationship.

Through advanced genomic techniques, we reconstructed 251 metagenome-assembled genomes (MAGs) from the diverse array of bacterioplankton populating this bloom. We identified 50 particularly active MAGs from the most abundant clades, including numerous polysaccharide degraders. Saccharide measurements, coupled with bacterial polysaccharide utilization loci (PUL) expression data, pinpointed β-glucans (laminarin) and α-glucans as the primary and actively metabolized dissolved polysaccharide substrates.

Moreover, the expression of carbohydrate-active enzymes belonging to various glycoside hydrolase (GH), polysaccharide lyase (PL), and carbohydrate esterase (CE) families underscored a specialized and efficient resource partitioning among polysaccharide-degrading bacteria.

Additionally, our study showed the role of top-down pressure from viral infections and grazing by protists and invertebrate metazoans in regulating these blooms, adding further depth to our understanding of marine ecosystem dynamics.

In essence, the research casts a revealing spotlight on the complex relationship between phytoplankton blooms and bacterioplankton communities, emphasizing the pivotal role of dissolved storage glycans in shaping these microbial landscapes. These insights into the dynamics of microbial decomposition and the influence of polysaccharide availability provide a deeper understanding of the ecological interactions within marine ecosystems, paving the way for future exploration into life beneath the waves.

By understanding how sugars shape marine communities, we're one step closer to appreciating the delicate balance of our oceans.

Read the full story here: https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-023-01517-x