Scientists have discovered that marine bacteria can work together to break down fucoidan, a complex carbohydrate produced by brown algae and diatoms that plays an important role in the ocean’s carbon cycle.
The study shows that no single bacterial species is capable of completely degrading the molecule. Instead, different bacteria divide the task according to their specialised abilities, collectively making the breakdown process far more efficient.
Fucoidan forms part of the protective outer structures of algae and has a highly complicated chemical architecture, containing numerous sugar linkages and branching patterns. Its resistance to microbial decomposition allows some of it to sink into deeper ocean waters, carrying carbon with it and potentially contributing to long-term carbon storage.
The research was led by Andreas Sichert, formerly a postdoctoral researcher at the Massachusetts Institute of Technology and now at ETH Zurich, and Otto X. Cordero, an associate professor at MIT.
A Complex Molecule Requiring Microbial Teamwork
Researchers enriched a community of fucoidan degrading bacteria using coastal seawater samples and isolated eight bacterial strains.
Together, the strains contained more than 453 genes associated with enzymes capable of acting on different components of fucoidan. Yet none of the individual bacteria could completely break down the molecule on its own.
Using a rapid mass spectrometry technique, the scientists tracked how the bacteria consumed different sugar components of fucoidan. This revealed that the enormous biochemical complexity could be organised into two broad functional roles.
Some bacteria specialised in breaking down the fucose rich backbone of fucoidan, while others focused on removing side branches containing less-common sugars such as xylose and galactose.
When bacteria representing both functions were combined their performance was greater than the sum of their individual activities. The researchers observed a synergistic effect, with communities containing complementary bacterial capabilities showing substantially more efficient degradation.
In some combinations, the bacterial communities approached complete breakdown of the complex polysaccharide.
From Hundreds of Enzymes to Two Functional Roles
The researchers found that this division of labour made the otherwise complicated system surprisingly predictable.
They developed a simplified model that categorised bacterial activity according to two broad traits: the ability to process fucose and the ability to process rarer sugars found in fucoidan’s side chains.
The model was trained using data from small microbial communities containing only one to three bacterial strains. Despite its simplicity, it successfully predicted degradation in communities containing as many as seven strains.
The model also worked across nine structurally different forms of fucoidan produced by different types of algae.
The findings suggest that scientists may not need to identify every enzyme or biochemical interaction within a microbial community to predict its overall function. Instead, identifying a small number of meaningful functional traits may provide a more practical way to understand complex biological systems.
Implications for Ocean Carbon Storage
The study could also help explain why some carbon derived from marine algae remains in the ocean for extended periods.
The researchers propose the concept of “diversity limited degradation”, in which fucoidan can persist when an environment lacks the right combination of complementary bacterial specialists needed to efficiently break it down.
If the necessary microbial partners are absent or present in insufficient numbers, degradation could slow, allowing more carbon rich material to remain intact and potentially sink into deeper waters.
The researchers also found evidence that bacteria with complementary capabilities tend to occur together in natural ocean samples, suggesting that the division of labour observed in laboratory experiments may also operate in marine ecosystems.
Potential Applications in Biotechnology
The findings could have implications beyond understanding ocean ecosystems.
Complex polysaccharides such as those found in brown algae are difficult to process because of their varied chemical structures. Rather than attempting to engineer a single microorganism capable of performing every step, biotechnology researchers could potentially use communities of microbes that naturally specialise in complementary tasks.
Such microbial teams could eventually be explored for processing brown algal biomass and other complex biological materials.
A Broader Framework for Understanding Microbial Communities
Perhaps the most significant finding is the approach used to simplify an apparently overwhelming biological system.
The researchers were able to reduce the activity of hundreds of enzymes to two measurable functional traits and then use those traits to predict how microbial communities would behave.
The approach could potentially be applied to other complex biopolymers whose chemistry remains difficult to fully characterise.
The researchers say the work points to a broader principle in biology: complex ecosystems may become easier to understand when scientists identify the appropriate level of organisation rather than attempting to describe every individual component.
The study also raises an evolutionary question: if fucoidan is abundant and has existed for a long time, why has no single bacterium evolved the ability to digest the entire molecule?
One possibility is that metabolic constraints favour cooperation among specialists. Another is that evolutionary dynamics continually maintain communities of complementary organisms rather than driving all functions into a single species.
The findings highlight how microbial cooperation can influence processes that operate at planetary scale. In the ocean, the breakdown or persistence of carbon-rich biological material is shaped not only by individual organisms but also by how different microbial specialists interact and divide biochemical tasks.