The Weird “Moss Balls” That Float By Day And Sink By Night

Why do those adorable velvety green “Japanese moss balls” living in your aquarium float during the day and sink at night?

If you keep a freshwater aquarium or freshwater shrimps, then you may be familiar with those personable green velvety balls that are sometimes referred to as “Japanese moss balls.” These “moss balls” are popular pets in Japan, where they are known as “marimo” (“seaweed balls”). Despite their popular English name, marimo is not a moss at all. Instead, it is a slow-growing filamentous macroalgae species that occurs in a very few cold freshwater lakes in the Northern Hemisphere. In Japan, marimo are important to the indigenous Ainu people, who hold an annual three-day festival in Hokkaido every October to honor them. Marimo are recognized as a national treasure in Japan where they have been a protected species since 1920, and their home, Lake Akan, has been designated a national park.

Normally, this algae grows in clumps on solid surfaces or forms long filaments, but as the direct result of underwater water currents that periodically roll the algae filaments along the sandy lake bed, they entangle and grow into a living sphere that is roughly the size of a golf ball (ref).

Although Carl Linnaeus first described specimens collected from Sweden’s Dannemora Lake in 1753 and gave this species its Latin name, Aegagropila linnaei, the common name — marimo — was provided by Japanese botanist, Takuya Kawakami, at the Sapporo Agricultural College (now Hokkaido University), who discovered them in Japan’s Lake Akan in 1897. Lake Akan is ideal for creating marimo because it has an unusual shallow bowl shape that allows gentle underwater currents to roll the marimo filaments around the sandy surface of the lake bed. Lake Akan is home to an estimated 600 million marimo, and they form dense colonies on the lake floor.

Although marimo are found on dark, cool lake beds, they daily float to the water’s surface (ref) — a mysterious “behavior” that they also exhibit in captivity in aquariums and in graduated cylinders in the lab. But why do they float in the daytime and sink at night?

A newly published paper by Dora Cano-Ramirez, a biology doctoral candidate at the University of Bristol, and her collaborators revealed that the marimos’ secret to periodically floating is a combination of photosynthesis and circadian rhythms. Basically, when marimo photosynthesize, they give off tiny bubbles of oxygen that remain trapped in fine filaments on the surface of the ball. Ms. Cano-Ramirez and her collaborators suspected this was the reason for marimo buoyancy. They tested their hypothesis by exposing marimo to the chemical, DCMU, which stops photosynthesis. Under the influence of DCMU, not only did the surface bubbles stop forming, but the marimo stopped floating, even when exposed to continuous bright lights for 48 hours.

But why do marimo float during the day but not at night? Was light the trigger, or were marimo following their own internal clock? To answer these questions, Ms. Cano-Ramirez and her collaborators “trained” their experimental marimo to synchronize their internal clock to an artificial 12 hour light-12 hour dark cycle. They then transferred the entrained marimo to a continuous dim red light that was only just bright enough to allow their newly synched internal clocks to continue ticking, and left them there for several days. After that treatment regime, Ms. Cano-Ramirez and her collaborators discovered that the entrained marimo weren’t fooled at all: when exposed to bright light at their entrained sunrise, the marimo popped up to the water’s surface more quickly than if they were exposed to bright light during their entrained midday. This confirmed the scientists’ hypothesis that periodic floating — and photosynthesis — is influenced by the marimos’ biological clock. But the circadian clock can be overcome by lighting: Ms. Cano-Ramirez and her collaborators found that marimo remained buoyant when exposed to constant bright light.

Were all the algae within a marimo ball actively photosynthesizing, or just those on the surface? The scientists measured photosynthetic rate on the surface and in the interior of the balls, and found day–night fluctuations were anatomically dependent: the day-time and night-time photosynthesis rate inside the ball was comparable to the exterior of marimo during nighttime. The spherical shape limits marimos’ photosynthetic ability compared to a leaf, and this in turn, limits the size of the balls. (That said, some unusual individuals in Lake Akan can achieve a diameter of 30 centimeters [11.8 inches].)

But what is the advantage to marimo to float in bright light? It’s likely that buoyancy may allow marimo to maximize photosynthesis on all sides of the sphere, and, because light penetration decreases with water depth, floating allows all marimo be exposed to bright light. Although it’s unclear how sinking at night may be beneficial, it’s possible this protects marimo from excessive cold or ice on the water’s surface.

The researchers hope their study helps bring attention to marimo, which are now quite rare (ref).

“Unfortunately, marimo balls are endangered, being currently found in only half the lakes where they were once spotted,” Ms. Cano-Ramirez said in a press release.

“This might be caused by changes in light penetration due to pollution,” Ms. Cano-Ramirez explained. Fertilizer from agricultural fields runs off into rivers and lakes and causes eutrophication, a process where the water becomes contaminated with an excess of nutrients that algae and plants feed on. This in turn, causes algal overgrowth and clouding of the water column, which decreases light penetration.

“By understanding the responses to environmental cues and how the circadian clock controls floating, we hope to contribute to [marimo] conservation and reintroduction in other countries.”