Phosphate deprivation restricts bacterial degradation of the marine polysaccharide fucoidan

Overview

This study investigates the relationship between phosphate availability and bacterial degradation of fucoidan, a marine polysaccharide synthesized by brown algae and diatoms. The research examines whether phosphate limitation could suppress fucoidan degradation and thereby enhance carbon sequestration in oceanic systems. Although marine bacteria possess fucoidanase genes, the mechanisms controlling their enzymatic activity remain incompletely understood. The study employs an experimental system combining a fucoidan-producing microalga with a fucoidan-degrading bacterium to test phosphate-dependent constraints on polysaccharide mineralization.

Methods and approach

An experimental system was assembled containing Glossomastix sp. PLY432, a microalga capable of fucoidan synthesis, and a Verrucomicrobiaceae bacterium strain 227, selected for its fucoidan-degrading capacity. The system was manipulated across varying phosphate concentrations to assess differential impacts on carbon fixation and polysaccharide degradation. Fucoidan degradation rates were measured under phosphate-replete and phosphate-deprivate conditions. The structural simplicity hypothesis was tested by comparing fucoidan degradation with that of laminarin, an alternative marine polysaccharide with less complex architecture, to determine whether phosphate sensitivity extends broadly across polysaccharide substrates or is substrate-specific.

Results

Carbon dioxide fixation into fucoidan by the producing microalga occurred independently of phosphate concentration. In contrast, bacterial degradation of fucoidan by the Verrucomicrobiaceae strain exhibited substantial inhibition under phosphate deprivation. Laminarin degradation demonstrated reduced sensitivity to phosphate limitation relative to fucoidan degradation, suggesting structural complexity influences the phosphate-dependency of polysaccharide mineralization. Phosphate deprivation simultaneously promoted carbon fixation into fucoidan while suppressing its subsequent degradation, creating conditions favorable for carbon accumulation in the fucoidan pool.

Implications

The phosphate-dependent regulation of fucoidan degradation identifies a potential biochemical control point for modulating carbon cycling in marine environments. The differential impact of phosphate limitation on fucoidan versus laminarin degradation suggests that polysaccharide structure influences nutrient requirements for bacterial enzymatic activity. These findings indicate that nutrient limitation, particularly phosphate scarcity, could constrain the microbial mineralization of complex polysaccharides, thereby extending the residence time of carbon in the dissolved organic matter pool.