Microorganisms battle it out within algal blooms

An unseen war raging among the ocean’s tiniest organisms has significant implications for understanding the ocean’s role in climate change, according to a new study.

USC Dornsife researchers David Needham and Jed Fuhrman sampled water off the coast of Southern California over the course of five months, almost every day shortly after an algal bloom occurred, and found that the cloud of microorganisms is anything but uniform. Instead, the researchers found traces of a constant battle among dozens of species, with the fortunes of war favoring different organisms on a daily basis.

Not only do the tiny organisms, known as phytoplankton, make up the base of the food chain in the ocean, they also are the planet’s main scrubbers of carbon dioxide from the atmosphere.

“We witnessed a daily boom and bust among the phytoplankton species,” said Fuhrman, McCulloch-Crosby Chair in Marine Biology and professor of biological sciences. Fuhrman was senior author of the study, which was published in Nature Microbiology on Feb. 29.

Pulling carbon dioxide from the atmosphere

Scientists concerned with global warming have a vested interest in looking closely at phytoplankton. The microscopic plants, most of which are about as big as a piece of paper is thick, perform roughly half of the world’s carbon fixation — that is, they convert carbon dioxide from the atmosphere into organic compounds that can be used by other organisms.

As creatures that exist on the boundary between sea and sky, they also have an outsized role in carbon fixation — sucking up atmospheric carbon dioxide and locking it away in the ocean.

Jed Fuhrman, McCulloch-Crosby Chair in Marine Biology and professor of biological sciences at USC Dornsife. Photo by Peter Zhaoyu Zhou.

Different phytoplankton manage carbon dioxide to varying degrees, however, making it important for researchers to gain a more nuanced understanding of algal blooms if they hope to quantify the blooms’ role in carbon fixation and carbon sequestration.

Scientists have long wondered about the trigger of algal blooms, which can include “red tides” caused by toxic dinoflagellates that poison marine life like sea lions and can render shellfish in the area unsafe to eat. Those dinoflagellates and other toxic algae were among some of the microorganisms that dominated the bloom periodically.

A distinctive signature

Most previous efforts to study the blooms relied on microscope analysis to classify which species of phytoplankton were in the mix — a problematic strategy, given that many of the organisms tend to look alike, even to a trained eye.

Instead, Needham and Fuhrman analyzed the organisms’ ribosomal RNA, which gives each species a distinctive and quantifiable signature. Specifically, they sequenced the RNA from the parts of the cell called chloroplasts, which perform photosynthesis.

“This could shift how this work is done in the future,” said Needham, a postdoctoral fellow and lead author of the study. “I think a lot of people are going to start taking a closer look at their blooms.”

The samples were collected daily by dipping buckets off the side of the Miss Christi — the ship that sails daily between San Pedro, Calif., and the USC Wrigley Marine Science Center on Catalina Island — at a specific location about halfway between the two ports.

Make room for the bloom

The authors were surprised not only by the sheer diversity of phytoplankton in the bloom they studied — they counted about three dozen different species — but also by the constant and abrupt shifts in which species were dominant within the bloom.

Some of the variability of the species can be attributed to spatial variability; in other words, the researchers were unable to park their boat in the exact same spot every time they took a sample. However, the content of the samples changed too dramatically for that to be the sole cause, Needham and Fuhrman concluded.

In addition, as the phytoplankton varied, so did the species of bacteria and other microorganisms that feed on the organic material produced by them. On one of the sample days, the team was shocked to discover that the dominant species were in a group called the Archaea — single-celled microorganisms once thought to live only in extreme environments like hot springs.

“Until the 1990s, nobody thought Archaea were even present in the sea in appreciable numbers,” Needham said.

Needham and Fuhrman’s findings also have bearing on the causes of algal blooms, which remain shrouded in mystery. Temperature and nutrient content of the ocean have been shown to help trigger the blooms, but they remain unpredictable.

The research was funded by the National Science Foundation (grants 1031743 and 1136818) and the Gordon and Betty Moore Foundation Marine Microbiology Initiative (grant GBMF3779).