Alexandra Z. Worden

Worden dock1

After obtaining a B.A. in History from Wellesley College you eventually went on to complete your Ph.D. in Ecology at the University of Georgia. Describe your path from History major to Ecology Ph.D. and now Senior Scientist and Lead Investigator at the Monterey Bay Aquarium Research Institute. How have mentors played an important role in your career path?

When I started college I was interested in science and the intersection between environment, societal stability and political regimes. So history made a lot of sense – I concentrated in African history with an inspiring professor at Wellesley, Lidwien Kapteijns. However, the majority of my classes were at MIT in science – and yes mentors played a huge role. In fall of my first year I took Fundamentals of Ecology with Penny Chisholm and loved it. Later I worked in her lab which focuses on marine cyanobacteria. I also had exceptional professors in chemistry as well as geology and atmospheric sciences. For these latter two topics the classes were small and highly interactive. 

Being a history major gave me the freedom to take science classes from many different departments, whereas if I had been a science major I would have been locked into a full department curriculum without time for much else. At the time it felt risky to not major in a science knowing I wanted to apply to science graduate programs – but I really wanted to take courses from multiple scientific disciplines.

After college I planned to spend a year working with horses out west – but I never managed to tear myself away from the labs I was working as a technician in – so after two years I decided I might as well get on with my Ph.D.! I planned to do my Ph.D. in biological oceanography – but followed the guidance of one of my early mentors who, as I debated whether to go to Scripps Institution of Oceanography or to Ecology at Georgia, said something like ‘oceanography needs more ecology …and you’ve already had exposure to oceanography’. I am deeply grateful for this advice – it gave me exposure to ideas and approaches from terrestrial and freshwater ecology that I could then bring back to my thinking about the ocean (my Ph.D. after all was on ecology of marine cyanobacteria).

After my postdoctoral fellowship at Scripps I started as an Assistant Professor at University of Miami. Soon thereafter I was recruited to MBARI by another incredibly inspiring individual and scientist – Marcia McNutt, who is now head of the National Academy of Sciences. This move was exciting because my lab has been able to push on technologies that allow us to investigate microbial activities and interactions at the organismal level – microbe to microbe – which in my mind is the only way we will ever be able to develop mechanistic and predictive understanding of the marine biosphere.


Your research focuses on regulation of photoautotrophic microbes and carbon cycling in marine systems. What are picoeukaryotes and how has your lab used genomics to characterize these organisms? What role do they play in carbon cycling?

Picoeukaryotes are single celled (microbial) eukaryotes that have a diameter less than 2 micrometers – it is simply a size classification. These tiny cells package all the complexity typical of eukaryotes (Mitochondrion, Golgi, Nucleus) and many of them are photosynthetic – so they also have a Chloroplast. Photosynthetic picoeukaryotes can perform a significant portion of primary production (synthesis of organic material from photosynthetic fixation of CO2) in different regions of the surface ocean. They are also extremely diverse. What we now need to understand is who dominates primary production at different times (there are lots of other types of algae in addition to picoeukaryotes), as well as what triggers shifts in the key taxa and their respective fates.

To this end, we performed the first study in which the genomes of two marine microbial eukaryotes (thought to be the same species) were sequenced and compared. This highlighted massive differences in their gene content. It also resulted in the discovery of highly repetitive introns that have since been observed in fungi as well – and the discovery of riboswitches on a series of novel genes, whereas the riboswitches previously identified in eukaryotes were on known thiamin biosynthesis genes. However the genome project didn’t provide the insights into niche differentiation that we were looking for when we undertook the analysis. The majority of proteins encoding by genes present in one species, but not the other, have unknown functions. This is something we are working to change. It seems quite likely that some of these proteins will give us secrets to life in the sea, since most of those with known functions come from industrial, medical or agricultural model taxa. We have also sequenced genome fragments and organellular genomes from picoeukaryotes that can’t be cultured by developing an approach we call targeted metagenomics. For this we take our cell sorter out to sea, flow sort cells of interest, and then sequence them. This data allows us to study the evolution of uncultivated microbial eukaryotes and the aspects of their biology that can be deduced from genome analysis.


What methods do you use to sample organisms from their environments (i.e. in the sea, sea floor, and sea surface?

We do everything from collecting 12 liters of water in bottles that are mounted on a rosette (hooked to a winch and deployed over the side of the ship), where we can ‘fire’ bottles at multiple depths in the ocean, to using a remotely operated vehicle to experiment on sediments at 3900 m below the sea surface.


What approaches has your lab developed to study the predator-prey relationships and accurately model food webs?

We focus on intact cells, because a microbe’s ecological role is not accurately represented by a single gene or a single metabolic pathway. Moreover, the way an organism responds to environmental change depends on the cell in its entirety. To understand ecology and interactions you need to understand cellular biology as well as behavior. This is why we developed approaches in the early stages of metagenomics for flow sorting and sequencing individual microbial eukaryotes as a cohesive cellular unit – at least then we could look at the collection of pathways present in an uncultured eukaryote and know they belonged together.

We pursue actual interactions in the field by using stable isotope probing and other methods for following the movement of carbon into and through different trophic levels and individual cells. This information is essential to improving ecosystem models and here we are just starting to pair with modelers. At the start of my career I thought my lab would also pursue the modeling – but there is still too much to learn about the basic biology of microbial eukaryotes, their responses to environmental perturbations (natural and anthropogenic) and interactions and compound exchanges within ocean microbiomes.

In addition to field research, we work in the lab with cultured algae in order to study their responses to predicted future conditions (lower surface ocean nutrients, higher atmospheric CO2, warming etc.). Our goal is to characterize how the communities responsible for biological fixation of CO2 via photosynthesis will change (half of global primary production happens in the ocean, so it is an important term!). To this end, we have been refining high-tech photo-bioreactors in which we can adjust all of these parameters and track them in real time. We have also developed systems for studying interactions of predatory mixotrophs (microbial eukaryotes that perform photosynthesis and consume other microbial cells, making them an uncertain mix of producer and consumer) with tractable and traceable prey. Developing these types of systems is incredibly challenging, but it is worth it because it positions us to really track interactions, rates and fates of diverse microbes. It also lets us identify gene and protein sets that could serve as indicators of activities in the field and growth controlling factors acting on natural populations.


If you had to change careers today, what would you choose and why?

I would be interested in a leadership position at an NGO or Think Tank – possibly even in infectious diseases/microbiome science at a biotech, if the company had enough of not-for-profit attitude. I love bringing together groups of people from different disciplines and developing new approaches to scientific problems. At an NGO I could do this in a different arena likely by linking with groups in developing nations. I am also interested in STEM, changing the ‘voice of science’ to one that is more inclusive and representative of the general population, and mentoring individuals who are also interested in promoting diversity in science. So I don’t have a title for a specific, different career – but those are areas I would be interested in contributing to.


What is something interesting about yourself that others may not know?

Everyone in my family (my partner and two young children) was born at least 3000 miles from each other, on three different continents. I love soccer and have an affinity for Ashtanga Yoga (actually all four of us play soccer and do yoga). On a different note, I funded the last 6 months of my Ph.D., after my NASA fellowship had ended, by baking tortes and pies for a cafe in Cambridge, Mass. Some of my family thought it looked like I might finally have landed a promising career option (as a baker!).


Do you have a favorite book that has influenced your career as an ecologist?

Silent Spring (Rachel Carson), given to me my mother when I was in Junior High or so.



More in this category: « Jonathan Zehr