Over the last couple decades, a raft of studies have explored the relationship between marine bacteria and phytoplankton, the microorganisms at the lowest rung of the food chain in the open ocean. The role of bacteria in the carbon cycle has been particularly well-studied. During the carbon cycle, atmospheric carbon dioxide dissolves in the upper reaches of the ocean and is fixed as organic carbon through photosynthesis.
Scientists used to dismiss the importance of microbes in this process, assuming lowly bacteria were too sparse to play an important role. Accurate estimates of total cell numbers in the ocean, which became available in the 1970s and 1980s, turned this assumption on its head. Soon afterwards researchers discovered that half of the organic carbon fixed by phytoplankton is cycled through bacteria.
Recent studies on bacteria-phytoplankton interactions have focused on mutualistic interactions, such as how microorganisms promote growth for each other. But not all interactions between bacteria and phytoplankton are friendly. Other studies have shown that mutualism can break down into antagonistic exchanges where one partner promotes the death of the other. Mutualistic and antagonistic exchanges between phytoplankton and bacteria may contribute significantly to how algal blooms develop and crash, influencing the flow of carbon and energy in marine ecosystems.
Less well understood is how bacteria-phytoplankton interactions might contribute to the marine iron cycle. Iron fuels the ocean: It’s a critical micronutrient. The scant amount of iron in the ocean can limit phytoplankton growth, and fertilizing the ocean with iron can spur massive phytoplankton blooms. Heterotrophic bacteria in the ocean also need iron, especially to convert carbon from phytoplankton into energy. The iron that bacteria and phytoplankton use when growing happily together, or when one of the partners dies, hasn't been well documented.
A study published in this week’s mSystems takes a step toward filling that knowledge gap. In the new research, researchers from Scripps Institute of Oceanography in La Jolla, California, used a variety of -omics tools to study the mechanisms used by Roseobacter — bacteria that frequently occur with algal blooms — to take in and use iron. The researchers charted the genetic pathway used to take in heme, an iron-rich molecule abundant in phytoplankton. They identified heme uptake systems in nearly half of the 153 Roseobacter genomes they analyzed, suggesting that many bacterial types rely on heme for their iron needs. If bacteria are consuming heme from dying phytoplankton, this process might play a role in recycling iron in the ocean.
Not all marine bacterial species are equipped to use heme -most probably aren’t. When the scientists searched available public databases of metagenomes and metatranscriptomes, they found that heme uptake pathways weren't abundant – suggesting that the most highly abundant bacteria in the ocean probably don’t use heme. But study leader Shane Hogle, now a postdoctoral researcher at MIT, says those databases mainly include free-floating marine bacteria and exclude those that live in close association with phytoplankton, like Roseobacter.
Hogle says that it’s easy to overlook the important ecological roles of microbes that are mostly rare. “Some people say that because it’s not there in the metagenomes then it must not be important,” he says. But his findings say otherwise. “A lot can be said for these hidden little guys, which are often discounted in a big way.”
Hogle and his collaborators genetically engineered a Roseobacter strain to inactivate the heme uptake mechanism. They tested the altered strains against wild-type bacteria and found that the wild-type bacteria were better at gobbling up iron from algal cells that had lysed, or died and fallen apart. The experiment suggests that bacteria that use heme might have a competitive advantage when an algal bloom dies off.
Previous studies have looked at the symbiosis of microbes and phytoplankton, but the new findings from Hogle and his collaborators explore the opposite perspective: What happens when the phytoplankton die?
“If they’re not eaten by bacteria, they just sink to the bottom of the ocean,” says Hogle. That would lead to a flux of iron out of the upper layer of the ocean. “When Roseobacter bacteria extract heme from phytoplankton, maybe that’s reducing iron export from the surface layers of the ocean.”
Bacteria that use heme may remain in the background most of the time, but become critical when algal cells die. Understanding how iron cycles through the ocean could bolster scientists’ knowledge of bacteria-phytoplankton interactions, but, perhaps more importantly, it could improve models of how ocean chemistry will change under climate change scenarios.