Helena van Tol

Helena van Tol

Former graduate student

Benjamin Hall IRB 323


  • Ph.D. Oceanography. University of Washington, Seattle, WA, 2019.
  • M.Sc. Oceanography. University of Washington, Seattle, WA, 2015.
  • B.Sc. Environmental Science (Hon.), Biology minor. Mount Allison University, Sackville, NB, 2011.



Constraint-based metabolic modeling of interaction between Thalassiosira pseudonana and Ruegeria pomeroyi:

For my Ph.D., I created genome-scale metabolic models of the diatom *Thalassiosira pseudonana* CCMP 1335 and the bacterium *Ruegeria pomeroyi* DSS-3 to investigate the dynamics of interaction between these two model marine organisms under different nutrient conditions.

Constraint-based metabolic models can be used to study the flux of metabolites and nutrients in marine microbes, connecting molecular information to the environment. These models have been used to integrate a wealth of different information types and to predict the metabolic behaviour of many different organisms.

In culture, growth of the diatom *Thalassiosira pseudonana* can be supported by vitamin B12-producing bacteria such as *Ruegeria pomeroyi*. In exchange *R. pomeroyi* receives organic carbon and sulfur from *T. pseudonana*. Vitamin B12 is the only organic compound required by most diatoms and it has been demonstrated that *R. pomeroyi* consumes 2,3-dihydroxypropane sulfonate (DHPS) and N-acetyltaurine produced by *T. pseudonana*, although there may be other molecules exchanged between these two organisms.

I used these models to study the dynamics of this interaction and investigate the possible exchange of other molecules.

Effect of Croceibacter atlanticus on cell division in Thalassiosira pseudonana:

For my Master's project I studied interactions between the flavobacterium *Croceibacter atlanticus* and diatoms. *C. atlanticus* was isolated from the pennate chain-forming diatom *Pseudo-nitzschia multiseries*, and was found to inhibit growth in many different diatoms. When *C. atlanticus* was grown with the centric diatom *Thalassiosira pseudonana*, we observed large polyploid cells with multiple plastids. These features indicate that *C. atlanticus* may inhibit cytokinesis in diatoms.

T.pseudonana polyploidy
Epifluorescence microscopy image of T. pseudonana in mono-culture (left) and in co-culture (right) with C. atlanticus. Green = SYBR-stained DNA, red = plastid autofluorescence.