Nutrients go deep, sunlight stays shallow, so how do oceanic algae feed?

An ongoing mystery of the oceans is how marine algae manage to be very productive when the sunshine they need for photosynthesis is only near the surface and the nitrate nutrients they require are in the deep ocean.

A group of scientists working off Hawai'i believes it has a clue—it involves traveling plants and swirling deep ocean currents.

(Image: Microscopic algae may move between the sunlit surface waters where they can conduct photosynthesis, to deeper waters where they can pick up essential nutrients. Image: Kim Fulton-Bennett © 2010 MBARI)

The subject is important for several reasons, among them that the algae are at or near the bottom of the ocean food chain and thus are important to ocean productivity, and also that they lock up a lot of the carbon dioxide in the surface oceans, which has climate change implications.

“It is not understood how biologically mediated (dissolved inorganic carbon) uptake can be supported in the absence of nutrients,” write researchers Kenneth Johnson, Stephen Riser and David Karl, in a paper in Nature.

Johnson is with Monterey Bay Aquarium Research Institute, Riser with the University of Washington School of Oceanography, and Karl with the University of Hawai'i's School of Ocean and Earth Science and Technology.

Here's what a news release on the subject says: “The sea around Hawaii may be clear and blue, but it hides an enduring oceanographic mystery. Surface waters in this and other mid-ocean areas contain almost no nitrate or other plant nutrients. Yet each year, microscopic algae (phytoplankton) flourish in these vast, open-ocean areas. Although miniscule in size, these mid-ocean algae consume about one fifth of all the carbon dioxide taken up by plants and algae worldwide.”

Their theory on the subject is this: The deep ocean nutrients don't make it all the way to the sunlit surface ocean, and the sunshine doesn't reach deep into the ocean. But maybe they're meeting halfway, with some mechanism bringing nutrients nearly to the surface, and algae being able to dip down to the nutrient zone before returning to the photosynthesis zone.

The researchers tossed an Apex profiling float into the ocean north of Hawai'i. This drifting robot stayed at sea for 21 months, rising to the surface and dropping to 1,000 meters deep, then rising again, over and over, measuring what it found in the water as it went.

The drifter found that from January to October each year, there's an increase in oxygen levels in the upper ocean, within 100 meters of the surface. Meanwhile, there is a comparable decrease in nutrient levels—notably nitrates—in the next level down, from 100 to 250 meters down.

Something is making the oxygen and eating the nutrients at comparable rates, even though they're not in the same place. The obvious culprit here is algae, the microscopic ocean plants that breathe in carbon dioxide and exhale oxygen, and which eat nutrients like oceanic nitrates.

The scientists suggest that there are two kinds of movement taking place. First, the nutrients are being periodically carried by natural ocean circulation into the bottom layers of the sunlit zone. Second, it is already known that some algae can move through the water, some by buoyancy regulation mechanisms, and others using tiny tails called flagella to whip their ways forward. So once the nutrients are brought halfway, some of the algae themselves may be making the connection.

Here's how the scientists describe it in their paper: “We suggest that the phytoplankton present in the deep waters must be able to consume the nitrate that is transported vertically in these events. The phytoplankton must then detrain from the upwelled plumes by upward motility, perhaps through buoyancy regulation, before the water returns to depth.”

Their suggestion is not so much that algae are bobbing up and down the water column, but that perhaps dormant algae in the deep ocean rise to near the surface with the nutrients, where they can catch just enough sunlight to “wake up” and rise to the sunlit zone when the nutrient-rich water cycles back into the depths.

The researchers in coming years will be launching more drift floats in other parts of the oceans to get a better handle on the phenomena.

© Jan TenBruggencate 2010