Marine microplankton from a sample off Kona, Hawaii, USA. Credit: David Liittschwager/Wikimedia Commons CC BY-SA 4.0

The oceans teem with photosynthesizing bacteria, tiny-tailed dinoflagellates gobbling other plankton, algae surrounded by intricate glass skeletons. In the 1960s, the ecologist G. Evelyn Hutchinson pointed out something confusing: Why do so many kinds of plankton exist? Mathematically, they shouldn't all be able to survive when they must compete for the same set of nutrients.

One hypothesis for solving 'the paradox of the plankton,' as it's known, comes from terrestrial systems. Many animals exhibit distinct activity cycles, including diurnal rhythms of foraging that minimize conflict for limited food supplies. That led researchers to wonder if plankton diversity might stem from taking up scarce nutrients at different times of day.

For the first time, scientists have replicated genetic evidence for temporal niche partitioning in plankton on opposite sides of the world. Published recently in the Proceedings of the National Academy of Sciences, the study reports that Sargasso Sea plankton take turns expressing genes for consuming a limited resource, phosphorus. The finding supports previous work published in Nature Ecology & Evolution showing temporal niche partitioning in the northern Pacific.

"Taking turns over time is an additional mechanism to support plankton biodiversity and explain the paradox of the plankton," says SFI Complexity Postdoctoral Fellow Daniel Muratore, a first author on both papers.

Learning how plankton consume phosphorus also helps predict how ocean life will respond to prolonged fertilizer use, increased shipping, and climate change.

"As the climate continues changing from additional greenhouse gases in the atmosphere, so too will ocean chemistry. By discovering how ecosystems handle nutrient uptake, we can predict whether they are vulnerable to crashing," Muratore says.

In the middle of the night, moored in the Sargasso Sea hours from Bermuda, Muratore awoke, yawning. They walked to the CTD rosette, a tightly-packed ring of tubes and complicated machinery, and plunged it down into dark water.

At the time, Muratore was a Ph.D. student in the Quantitative Biosciences program at Georgia Tech. For five days in 2019, on a research cruise made possible by grants awarded to Joshua Weitz (University of Maryland) and Steven Wilhelm (University of Tennessee, Knoxville), Muratore and colleagues collected, filtered out, and froze sea-dwelling cells every four hours.

Back home, co-first author Naomi Gilbert (University of Tennessee, Knoxville) extracted and sequenced the cells' RNA. After analyzing 97,829 genes, the co-authors found gene expression patterns suggesting different plankton species absorb phosphorus at different times of day.

Bacteria that primarily rely on dissolved organic matter consumed phosphorus at sunrise. Photosynthesizing plankton with nucleuses gorged during daytime. Cyanobacteria imbibed at dusk.

The results mirror those for nitrogen uptake in the prior Nature Ecology & Evolution study.

"We went to a completely different ocean, did a comparable study, and found the same signature for phosphorus as we did for nitrogen. This suggests that reducing competition by taking turns might be a general feature of maintaining biodiversity in the ocean microbiome," says Muratore.

The simple element phosphorus is a building block for RNA, DNA, cell membranes, and fat stores—crucial for cells to grow and work. Phosphorus offers another powerful benefit: revealing how carbon leaves ocean ecosystems, a major factor in climate change.

Muratore coauthored a paper published in Frontiers in Marine Science in September that compared two eddies (think toilet-bowl swirls) in the Pacific Ocean. The research, led by Shavonna Bent, an MIT-WHOI Joint Program Ph.D. student in Benjamin Van Mooy's lab at Woods Hole Oceanographic Institution, found that one eddy grew rich with phosphorus, and one became deficient. In the eddy with less phosphorus, more carbon-rich material left the ocean surface and sank to the seafloor. That's because, with less phosphorus to go around, plankton absorbed extra carbon into their fats instead. When they died, they took the carbon with them.

"This natural experiment with eddies led to a notable difference in carbon export over just a few days. Learning how carbon leaves the atmosphere helps us understand how Earth compensates for the carbon we emit," says Muratore.

Muratore wants to explore how phosphorus stress might lead to other changes, like viral infections, in microorganisms. At its core, their research reveals the complex interplay of scales in the ocean, from molecular to global. "A small shift to these microscopic individuals, like not having enough phosphorus, results in changes in macroscopic properties ranging from ocean biodiversity to capture of atmospheric carbon," they say.

Clarifying a plankton paradox reveals climate risks

A recent study published in the Proceedings of the National Academy of Sciences provides genetic evidence supporting temporal niche partitioning in plankton, which helps explain the paradox of plankton diversity. The research, conducted by SFI Complexity Postdoctoral Fellow Daniel Muratore and colleagues, shows that Sargasso Sea plankton take turns expressing genes for consuming phosphorus, a key nutrient, at different times of the day. This finding aligns with previous work showing similar patterns in the northern Pacific. The study highlights the importance of nutrient uptake in maintaining biodiversity and predicting ocean life's response to climate change and other environmental stressors.