Photosynthetic organisms — also known as primary producers — form the base of the food chain, making most life on Earth possible. Using energy from the sun, primary producers fix, or convert, carbon from the air into organic, or carbon-based, matter. But primary producers also release carbon through a process called autotrophic respiration, which is somewhat akin to breathing. The rate of carbon gain after accounting for loss through respiration is called net primary production.

“Net primary production measures the amount of energy photosynthetic organisms capture and make available to support nearly all other life in an ecosystem,” said first author Yulong Zhang, a research scientist in the lab of Wenhong Li at Duke University’s Nicholas School of the Environment. “As the foundation of food webs, net primary production determines ecosystem health, provides food and fibers for humans, mitigates anthropogenic carbon emissions and helps to stabilize Earth’s climate.”

Previous research on net primary production has typically focused on either land or ocean ecosystems, leaving gaps in our understanding of net primary production across Earth and the potential implications for climate mitigation.

For this study, the team explored annual trends and variability in global net primary production, with a focus on the interplay between land and ocean ecosystems.

“If you’re looking at planetary health, you want to look at both terrestrial and marine domains for an integrated view of net primary production. The pioneering studies that first combined terrestrial and marine primary production have not been substantially updated in over two decades,” said co-author Nicolas Cassar, Lee Hill Snowdon Bass Chair at the Nicholas School who jointly oversaw the research with Zhang.

Satellite Insights

Observations from satellites offer continuous perspective on photosynthesis by plants and marine algae called phytoplankton. Specifically, specialized satellite instruments measure surface greenness, which represents the abundance of a green pigment called chlorophyll produced by photosynthetic life. Computer models then estimate net primary production by combining greenness data with other environmental data, such as temperature, light and nutrient variability.

The authors of the new study used six different satellite-based datasets on net primary production — three for land and three for oceans — for the years from 2003 to 2021. Using statistical methods, they analyzed annual changes in net primary production for land and, separately, for the ocean.

They found a significant increase in terrestrial net primary production, at a rate of 0.2 billion metric tons of carbon per year between 2003 and 2021. The trend was widespread from temperate to boreal, or high-latitude, areas, with a notable exception in the tropics of South America.

By contrast, the team identified an overall decline in marine net primary production, of about 0.1 billion metric tons of carbon per year for the same time period. Strong declines mainly occurred in tropical and subtropical oceans, particularly in the Pacific Ocean.

All told, trends on land dominated those of oceans: Global net primary production increased significantly between 2003 and 2021, at a rate of 0.1 billion metric tons of carbon per year.

Environmental Drivers

To understand the potential environmental factors at play, the team analyzed variables such as light availability, air and sea-surface temperature, precipitation and mixed layer depth — a measure that reflects the extent of mixing in the ocean’s top layer by wind, waves and surface currents.

“The shift toward greater primary production on land mainly stemmed from plants in higher latitudes, where warming has extended growing seasons and created more favorable temperatures, and in temperate regions that experienced local wetting in some areas, forest expansion and cropland intensification,” said Wenhong Li, a professor of earth and climate sciences at the Nicholas School and a co-author on the study.

Warming temperatures appeared to have an opposite effect in some ocean areas.

“Rising sea surface temperatures likely reduced primary production by phytoplankton in tropical and subtropical regions,” Cassar added. “Warmer waters can layer atop cooler waters and interfere with the mixing of nutrients essential to algal survival.”

Although land drove the overall increase in global primary production, the ocean primarily influenced year-to-year variability, especially during strong climate events such as El Niño and La Niña, the authors found.

“We observed that ocean primary production responds much more strongly to El Niño and La Niña than land primary production,” said co-author Shineng Hu, an assistant professor of climate dynamics at the Nicholas School. “A series of La Niña events was partly responsible for a trend reversal in ocean primary production that we identified after 2015. This finding highlights the ocean’s greater sensitivity to future climate variability.”

Broad Implications

The study points to the important role of terrestrial ecosystems in offsetting declines in net primary production among marine phytoplankton, according to the authors.

But they added that declines in net primary production in tropical and subtropical oceans, coupled with stagnation on land in the tropics, can weaken the foundation of tropical food webs, with cascading effects on biodiversity, fisheries and local economies. Over time, these disruptions could also compromise the ability of tropical regions to function as effective carbon sinks, potentially intensifying the impacts of climate warming.

“Whether the decline in ocean primary production will continue — and how long and to what extent increases on land can make up for those losses — remains a key unanswered question with major implications for gauging the health of all living things, and for guiding climate change mitigation,” Zhang said. “Long-term, coordinated monitoring of both land and ocean ecosystems as integrated components of Earth is essential.”

Funding: Y.Z., W.L., and G.S. were partially supported by the Duke University-USDA Forest Service collaboration (23-JV-11330180-119). N.C. was supported by the National Science Foundation (OCE-2123198). J.M. was supported by the Oak Ridge National Laboratory. J.X. is supported by the National Science Foundation (Macrosystem Biology) and NEON-Enabled Science Program (DEB-2017870).

But a new Dartmouth-led study shows that hominins began feasting on these carbohydrate-rich foods before they had the ideal teeth to do so. The study provides the first evidence from the human fossil record of behavioral drive, wherein behaviors beneficial for survival emerge before the physical adaptations that make it easier, the researchers report in Science.

The study authors analyzed fossilized hominin teeth for carbon and oxygen isotopes left behind from eating plants known as graminoids, which includes grasses and sedges. They found that ancient humans gravitated toward consuming these plants far earlier than their teeth evolved to chew them efficiently. It was not until 700,000 years later that evolution finally caught up in the form of longer molars like those that let modern humans easily chew tough plant fibers.

The findings suggest that the success of early humans stemmed from their ability to adapt to new environments despite their physical limitations, says Luke Fannin, a postdoctoral researcher at Dartmouth and lead author of the study.

“We can definitively say that hominins were quite flexible when it came to behavior and this was their advantage,” Fannin says. “As anthropologists, we talk about behavioral and morphological change as evolving in lockstep. But we found that behavior could be a force of evolution in its own right, with major repercussions for the morphological and dietary trajectory of hominins.”

Nathaniel Dominy, the Charles Hansen Professor of Anthropology at Dartmouth and senior author of the study, says isotope analysis overcomes the enduring challenge of identifying the factors that caused the emergence of new behaviors — behavior doesn’t fossilize.

“Anthropologists often assume behaviors on the basis of morphological traits, but these traits can take a long time — a half-million years or more–to appear in the fossil record,” Dominy says.

“But these chemical signatures are an unmistakable remnant of grass-eating that is independent of morphology,” he says. “They show a significant lag between this novel feeding behavior and the need for longer molar teeth to meet the physical challenge of chewing and digesting tough plant tissues.”

The team analyzed the teeth of various hominin species, beginning with the distant human relative Australopithecus afarensis, to track how the consumption of different parts of graminoids progressed over millennia. For comparison, they also analyzed the fossilized teeth of two extinct primate species that lived around the same time — giant terrestrial baboon-like monkeys called theropiths and small leaf-eating monkeys called colobines.

All three species veered away from fruits, flowers, and insects toward grasses and sedges between 3.4 million to 4.8 million years ago, the researchers report. This was despite lacking the teeth and digestive systems optimal for eating these tougher plants.

Hominins and the two primates exhibited similar plant diets until 2.3 million years ago when carbon and oxygen isotopes in hominin teeth changed abruptly, the study found. This plummet in both isotope ratios suggests that the human ancestor at the time, Homo rudolfensis, cut back on grasses and consumed more oxygen-depleted water.

The researchers lay out three possible explanations for this spike, including that these hominins drank far more water than other primates and savanna animals, or that they suddenly adopted a hippopotamus-like lifestyle of being submerged in water all day and eating at night.

The explanation most consistent with what’s known about early-human behavior, they report, is that later hominins gained regular access to underground plant organs known as tubers, bulbs, and corms. Oxygen-depleted water also is found in these bulging appendages that many graminoids use for storing large amounts of carbohydrates safely away from plant-eating animals.

The transition from grasses to these high-energy plant tissues would make sense for a species growing in population and physical size, Fannin says. These underground caches were plentiful, less risky than hunting, and provided more nutrients for early humans’ expanding brains. Having already adopted stone tools, ancient humans could dig up tubers, bulbs, and corms while facing little competition from other animals.

“We propose that this shift to underground foods was a signal moment in our evolution,” Fannin says. “It created a glut of carbs that were perennial — our ancestors could access them at any time of year to feed themselves and other people.”

Measurements of hominin teeth showed that while they became consistently smaller — shrinking about 5% every 1,000 years — molars grew longer, the researchers report. Hominins’ dietary shift toward graminoids outpaced that physical change for most of their history.

But the study found that the ratio flipped about 2 million years ago with Homo habilis and Homo ergaster, whose teeth exhibited a spurt of change in shape and size more suited to eating cooked tissues, such as roasted tubers.

Graminoids are ubiquitous across many ecosystems. Wherever they were, hominins would have been able to maximize the nutrients derived from these plants as their teeth became more efficient at breaking them down, Dominy says.

“One of the burning questions in anthropology is what did hominins do differently that other primates didn’t do? This work shows that the ability to exploit grass tissues may be our secret sauce,” Dominy says.

“Even now, our global economy turns on a few species of grass–rice, wheat, corn, and barley,” he says. “Our ancestors did something completely unexpected that changed the game for the history of species on Earth.”

HTLV-1 is a cancer-causing retrovirus known to lead to adult T-cell leukemia/lymphoma (ATL), an aggressive and often fatal disease. Although most infected individuals remain asymptomatic for life, a fraction eventually develops leukemia or other inflammatory conditions. The virus achieves long-term persistence by entering a “latent” state, during which its genetic material hides inside the host’s genome with minimal activity — evading immune detection.

In this study, the research team, led by Professor Yorifumi Satou from the Joint Research Center for Human Retrovirus, Kumamoto University, identified a specific region within the HTLV-1 genome that functions as a viral silencer. This sequence recruits host transcription factors, particularly the RUNX1 complex, which suppresses the virus’s gene expression. When this silencer region was removed or mutated, the virus became more active, leading to greater immune recognition and clearance in lab models.

Remarkably, when the HTLV-1 silencer was artificially inserted into HIV-1 — the virus that causes AIDS — the HIV virus adopted a more latent-like state, with reduced replication and cell killing. This suggests that the silencer mechanism could potentially be harnessed to design better therapies for HIV as well.

“This is the first time we’ve uncovered a built-in mechanism that allows a human leukemia virus to regulate its own invisibility,” said Professor Satou. “It’s a clever evolutionary tactic, and now that we understand it, we might be able to turn the tables in treatment.”

The findings offer hope not only for understanding and treating HTLV-1, especially in endemic regions like southwestern Japan, but also for broader retroviral infections.