Whales and penguins may be magnets for tourists visiting the Southern Hemisphere, but phytoplankton should be the real draw—these waterborne plants anchor marine food chains. However, phytoplankton have an Achilles’ heel: They rely on iron, a nutrient that’s downright scarce in many parts of the world’s oceans.
Now, researchers working in Antarctica have investigated how an unlikely catalyst—feces from tiny marine organisms—helps phytoplankton take up iron. The team demonstrated that the unicellular plants more readily take up dissolved iron in the presence of salp feces than they do in the presence of krill feces. As climate change erodes krill habitats in the Southern Ocean and salp flourish in their stead, phytoplankton populations might be primed for a boom, the team has suggested.
Phytoplankton are found in the top 100 or so meters of the water column, where they absorb sunlight and photosynthesize. It’s a good thing these tiny plants are there—researchers believe that the majority of oxygen in Earth’s atmosphere is produced by phytoplankton. Besides pumping out prodigious quantities of this life-sustaining element, phytoplankton are also an important source of food for marine organisms: Animals like krill eat phytoplankton, and krill in turn are devoured by larger species, all the way up to animals like whales and penguins.
Phytoplankton anchor the food chain, said Sebastian Böckmann, an ecologist at the University of Bremen and the Alfred Wegener Institute in Germany. “They’re the basis of the ecosystem.”
But in many parts of the world’s oceans, the growth of phytoplankton is limited by the availability of nutrients, specifically iron. (Iron-containing proteins play an important role in photosynthesis.) “This requirement for iron is at the very basis of almost all phototrophic life,” Böckmann told Eos.
Iron can be delivered to the upper reaches of the oceans in several ways. It can hitch a ride within dust particles that blow off landmasses. It can be carried upward from reserves of deeper, more nutrient-rich water. And it can trickle out of melting icebergs whose parents—glaciers—initially scooped up iron-containing sediments as they scraped over a landscape. Despite these multiple delivery mechanisms, however, iron remains a limiting nutrient in many marine ecosystems, particularly those far from land.
In 2018, Böckmann and his collaborators traveled to Antarctica aboard the R/V Polarstern to study how phytoplankton take up iron. In particular, they investigated how the presence of feces from tiny marine organisms affects the plants’ ability to absorb dissolved iron. “We wanted to see how the biology of the phytoplankton community reacts to whatever is being released from the fecal pellets,” said Böckmann.
Comparing the Feces
The researchers set up several experiments. To begin, they collected seawater near Elephant Island, filtered it to remove the phytoplankton, and then added either fecal pellets from salp or fecal pellets from krill. After 48 hours, they measured the amount of iron within the water. There was more than 3 times as much iron in the bottles that received the salp fecal pellets, the team found.
That’s a pronounced difference, Böckmann and his collaborators noted, but iron in the water column doesn’t benefit phytoplankton—or any other life form—unless it’s bioavailable. The real question, therefore, is how much of that iron can be taken up by phytoplankton and put to use, the team concluded.
Deborah K. Steinberg, a biological oceanographer at the Virginia Institute of Marine Science in Gloucester Point not involved in the research, concurred. Other studies have examined how the fecal pellets of zooplankton release iron, she said, but this team looked at bioavailability, too. “They took it a step further to look at the uptake,” said Steinberg.
The researchers started by adding a community of phytoplankton to each bottle of seawater from the prior experiment, along with a small quantity of the radioactive iron isotope 55Fe. They then let the plants munch on their surroundings for 24 hours.
Next, Böckmann and his colleagues filtered the seawater to isolate the phytoplankton. They then used a scintillation counter, which measures flashes of light created by radioactive decay, to estimate the amount of 55Fe within the plants’ cells. After accounting for nonradioactive iron isotopes (56Fe and 57Fe) also being taken up at presumably the same rate, the researchers estimated the total amount of iron processed by the phytoplankton. They found that the presence of salp fecal pellets boosted iron uptake in phytoplankton by nearly a factor of 5 compared with krill fecal pellets.
Using these results, the team drew two conclusions. First, compared with krill fecal pellets, salp fecal pellets release more iron. Second, dissolved iron becomes more bioavailable in the presence of salp fecal pellets than in the presence of krill fecal pellets. There are logical explanations for both of these findings, Böckmann and his colleagues proposed.
First, salp fecal pellets release more iron because they’re more fragile than krill fecal pellets and tend to fragment more easily, previous research has shown. To explain the second finding, Böckmann and his collaborators suggested that ligands—ions or molecules that readily bind to other atoms—released by the salp fecal pellets render dissolved iron more bioavailable to phytoplankton. That’s plausible, the team reasoned, because krill and salp differ significantly in their digestive mechanisms.
The Carbon Question
These results, published today in Current Biology, may have real implications for phytoplankton populations, the researchers proposed. That’s because sea surface temperatures are rising worldwide because of climate change, and krill are moving to higher latitudes in search of cooler waters. As krill habitats contract, salp populations are flourishing in their place. That population shift translates into more bioavailable iron, which could mean booming phytoplankton communities.
But what that ultimately means from a carbon sequestration standpoint is an open question, the team conceded. If phytoplankton populations grow unchecked, they’ll eventually die and sink to the seafloor, locking up carbon. However, if animals like salp and krill (and by extension, species farther up the food chain) dine on the bounty, animal populations could boom. Animals pump carbon dioxide into the atmosphere via respiration, but they also send carbon to the seafloor in their excrement, Böckmann told Eos. “It’s a highly complex interplay between different organisms and elements.”
—Katherine Kornei (@KatherineKornei), Science Writer