ASU Research E-Magazine: An Inside Job

Symbiosis occurs when organisms combine efforts and begin to work together almost as one. It may echo the process which gave rise to modern cell structure.

As a profession, scientific research is no different than other pursuits. Science attracts different people for different reasons. Some individuals pursue science as a career because they crave the rigorous challenge of solving difficult problems. Others enjoy the opportunity for exploration and discovery. Most simply like being where the action is.

Mary Beth Saffo enjoys probing the realms of the unknown. A marine biologist by training and associate professor of life sciences at Arizona State University’s West Campus, Saffo likes the challenge of difficult problems. "I was drawn into marine biology because I was intrigued by how little actually is known," she says.

Saffo has done her part to add new knowledge about life in the sea. During the past 10 years, her discoveries have had an impact on fields ranging from urological medicine to the evolution of symbiosis.

The centerpiece of Saffo’s work is a small salt-water creature known commonly as the sea squirt. At full stature, the sea squirt measures only about an inch in length. The tiny filter-feeders live in most of the world’s oceans. They actually are distant relatives of vertebrates which, of course, include humans.

She studies a number of species within the genus Molgula, a subset of the creatures known as sea squirts.While their ocean habitats do vary in certain details, each and every type of molgulid that Saffo has studied to date share a surprising common feature. The creatures entertain a favorite house guest that not only visits, but takes part in the housekeeping.

Saffo met the object of her professional devotion while in graduate school. As part of a class assignment, she reviewed a paper on molgulids which detailed the then-current views on their physical and biochemical makeup.

For example, all molgulids have a structure known as the renal sac. The renal sac usually contains a high amount of generally toxic chemicals—calcium oxalates and uric acid—much like our own kidneys. In fact, the concentration of minerals is so high, that crystals similar to human kidney stones form on a regular basis.

Years ago, marine biologists viewed the renal sac as a type of bladder. They figured it to be a repository for the sea squirt’s waste products. The theory made plenty of sense, except for one major catch. The sea squirt’s renal sac has no outlet. The creature has no way to rid itself of these so-called waste products.

This "bladder without an opening" piqued Saffo’s interest. "Everything about it just seemed wrong," she says.

Saffo began to study the puzzle. She found that the sea squirt’s renal sac usually contains some rather odd organisms. Saffo found seven types of one-celled protozoans that were not readily identifiable. These one-celled creatures infected all of her sea squirt specimens.

But that discovery was not new. Saffo found that a French biologist had noted the organisms more than 100 years ago. However, no one had quite believed his fantastic descriptions of these organisms he called Nephromyces, or "kidney fungus."

In the laboratory, Saffo found that the sea squirt Molgula manhattensis reproduces without the Nephromyces living in its renal sac. The protozoans are not transmitted with the eggs or larvae of their animal host. Each new generation must be re-infected with the organisms. Saffo discovered this fact accidentally while trying to raise the animals in tanks of filtered sea water.

"Without deliberately trying to, I ended up raising symbiont-free animals in the laboratory," she says. "This, incidentally, was the key piece of evidence demonstrating that the symbionts were ‘real’ foreign organisms, not just the figment of the French zoologist’s imagination, and not just cells of the sea squirt."

Using her newly cleansed organism, Saffo tried a few experiments. First, she infected the sea squirt with only one type of the protozoan. She discovered that what she initially thought was just one type of spore was actually seven. Nephromyces, it seems, cycles through seven notably different forms, a life cycle unparalleled anywhere else in the known living world. The zoological garden Saffo uncovered in free-living molgulids turned out to be a single organism.

Most living species tolerate some form of parasite. Saffo found that Nephromyces actually assists the sea squirt by breaking down the crystal-forming chemical soup contained inside the renal sac.

Scientists know that other types of one-celled creatures can perform similar chemical breakdown. However, Nephromyces lack the internal structures that are commonly associated with this kind of chemical activity.

Looking further, Saffo found that Nephromyces harbors other types of tiny bacteria within itself. Conceivably, this bacteria could do the chemical engineering work necessary to bust apart the complex chemicals inside the renal sac.

The result? What had been a scientifically ignored, rather uninteresting organism was suddenly transformed into a creature that consists of a complex web of three species living in symbiosis.

This trio’s ability to grow and break down uric acid and oxalate crystals attracted the attention of urologists. Nephromyces has never been observed outside of the molgulids, with the exception of a life-cycle stage where they swim through the water to infect young sea squirts. In addition, the internal bacteria do not seem to be able live separate from Nephromyces.

Saffo’s work demonstrates only that molgulids can live and reproduce without Nephromyces in the laboratory. "In nature, I’ve never seen any adult, reproductive molgulids without their symbionts. This suggests that molgulids in nature may need their symbionts to reproduce and/or survive adulthood," she adds.

At some point in the past, it seems apparent to Saffo that the ancestors of these organisms lived independently. The symbiotic relationship developed over time. By identifying the unique relationship, Saffo confirmed some popular ideas about eukaryotic cells. Eukaryotic cells are cells which have a nucleus and contain internal structures called organelles. Most cells in the human body are of this type.

One of those ideas is that eukaryotic cells originated when one type of single-celled organism was engulfed by another. Instead of being digested or destroyed, both organisms combined and began to work together as one.

For example, mitochondria are organelles that function as the chemical engines of individual cells. But mitochondria carry their own complement of DNA. Most scientists believe that they once were independent organisms. Saffo’s discoveries add credence to this theory.

Her findings also raise questions about widely held views on symbiosis, specifically, that symbiosis occurs only in tropical, stable environments; that mutualism is unstable or implausible, and that symbiotic organisms quickly give up their independence.

"I think of myself as an integrative biologist," Saffo says. "There’s been this tremendous gulf between cell biologists on the one hand and ecologists on the other. I’m interested in the biology of organisms. You realize pretty quickly that if you want to understand an organism, understand how it evolved, then you really need to understand it at both the cellular and ecological levels."

Saffo approaches her work in a linear, but unusual way: "Every time I ask a different question, I have to learn a new technique and figure out how to apply that technique to my system."

Saffo joined the ASU West faculty after working for years as a research scientist at the University of California, Santa Cruz.

While her brand new ASU West laboratory is equipped with some of the instrumentation she needs to determine the chemical composition of the substances she finds inside sea squirts, much of her work is still done in California. Saffo does field work at the University of California, Davis’ Marine Laboratory in Bodega Bay, just north of San Francisco.

"I was happy to make the switch," she says. "I’ve always wanted to have a regular faculty position. I care a lot about universities and I want to be part of the loop. I’ve missed teaching and I wanted to help build an institution."—John Svetlik