Mini‐review: Phytoplankton‐derived polysaccharides in the marine environment and their interactions with heterotrophic bacteria

Abstract

Summary Within the wealth of molecules constituting marine dissolved organic matter, carbohydrates make up the largest coherent and quantifiable fraction. Their main sources are from primary producers, which release large amounts of photosynthetic products – mainly polysaccharides – directly into the surrounding water via passive and active exudation. The organic carbon and other nutrients derived from these photosynthates enrich the ‘phycosphere’ and attract heterotrophic bacteria. The rapid uptake and remineralization of dissolved free monosaccharides by heterotrophic bacteria account for the barely detectable levels of these compounds. By contrast, dissolved combined polysaccharides can reach high concentrations, especially during phytoplankton blooms. Polysaccharides are too large to be taken up directly by heterotrophic bacteria, instead requiring hydrolytic cleavage to smaller oligo‐ or monomers by bacteria with a suitable set of exoenzymes. The release of diverse polysaccharides by various phytoplankton taxa is generally interpreted as the deposition of excess organic material. However, these molecules likely also fulfil distinct, yet not fully understood functions, as inferred from their active modulation in terms of quality and quantity when phytoplankton becomes nutrient limited or is exposed to heterotrophic bacteria. This minireview summarizes current knowledge regarding the exudation and composition of phytoplankton‐derived exopolysaccharides and acquisition of these compounds by heterotrophic bacteria.

Niche-based assembly of bacterial consortia on the diatom Thalassiosira rotula is stable and reproducible

“…Depending on their monomer composition, type and degree of branching and ultrastructure, phytoplankton exopolysaccharides represent highly diverse and often plankton species-specific chemical compounds [13]. These molecules are crucial attractants for heterotrophic bacteria [14]. For bacteria to utilize these exopolysaccharides requires specific carbohydrate-active enzymes [15] encoding the machinery for polysaccharide detection, hydrolysis, and uptake.…”

“…As no single bacterium is adequately specialized to utilize the suite of complex phytoplankton exopolysaccharides, commensal and mutualistic associations of different heterotrophic bacteria with complementary PULs are assumed to co-occur in the phycosphere. This in turn gives rise to the emerging concept of plankton-specific associated consortia [14,16].…”

Potential terrestrial influence on transparent exopolymer particle concentrations in boreal freshwaters

“…The precursors of TEP can be produced by both phytoplankton and bacteria (Passow ), and TEP concentrations are known to correlate well with phytoplankton abundance (Grossart et al ; Berman and Viner‐Mozzini ; Passow et al ). However, most studies have been done in marine systems where phytoplankton production is a main source of organic matter (Mühlenbruch et al ). The ubiquitous occurrence of TEP in lentic systems has also been confirmed (Grossart et al ; Berman and Viner‐Mozzini ; de Vicente et al ) but its relationship with phytoplankton and bacteria is less conclusive.…”

A pipeline for targeted metagenomics of environmental bacteria

“…Closely related flavobacterial species are known to be specialized on these polymers as well [43,45,46]. As the Vis6 clade was detected during phytoplankton blooms in spring times [24], a consumption of substrates secreted by living and released by decaying phytoplankton seems likely [47]. The higher percentage of peptidases compared to CAZymes and the rather small genome of Vis6 is similar to other phytoplankton bloom responders [46,47].…”