Why marine bacteria chase a seaweed sugar they cannot even eat

A sweet trail you cannot digest

Think about walking toward the smell of roast dinner, only to find you cannot eat it. In the sea, many marine bacteria run this odd chase every single day.

Brown seaweeds, or macroalgae, shed lots of carbon-rich debris into the sea. In fact, a tough sugar called fucoidan makes up to 50% of the dissolved organic matter they release each day. This complex sugar takes a long time to rot. Because of that, teams wondered if passing microbes notice it at all.

Following the sugar trail

To test this, a team tracked 14 kinds of marine bacteria in microfluidic assays. These tiny devices hold narrow fluid channels. They let the team set up steady sugar trails using fucoidan from two brown seaweed species.

Every single one of the 14 strains swam straight toward the fucoidan. This happened when the sugar was dense, at 0.1 to 1 mg/mL. This directed swimming toward a cue is called chemotaxis. It only triggered at high levels found right next to a plant surface. When levels were low, the microbes ignored it. Some even swam away, showing mild chemorepulsion.

Curiously, the bugs could not use fucoidan to grow. It did not stop them from eating other food, but gave them no fuel on its own. The sugar acts like an airport radio beacon. It guides an airplane to land, even though the radio signal cannot power the plane.

What we still do not know

This work is an un-peer-reviewed preprint. It used 14 strains inside tiny plastic lab channels rather than open ocean waves. We do not yet know the exact tools microbes use to sense the sugar. We also do not know why faint traces push some bugs away. Still, this slow-rotting sugar seems to help cells locate rich seaweed surfaces where real meals wait.