Wintertime ecology in the Chesapeake Bay
Currently I am assessing the quantitative role of heterotrophic activity among mixotrophs across seasons in Chesapeake Bay (with emphasis on the important, but woefully undersampled winter). The Chesapeake Bay exhibits high variation in environmental conditions and biological processes to provide a valuable stage for studying in situ mixotrophic processes seasonally and subsequently put in context their responses to longer-term trends associated with environmental and climate change. In the winter, a chlorophyll maximum develops in a low light environment, at which time dinoflagellates dominate the phytoplankton community. Mixotrophy is thought to be an essential strategy for these dinoflagellates to dominate in the winter. In collaboration with Drs. Greg Silsbe and Sairah Malkin at UMCES-HPL, I am investigating these dinoflagellates through dilution and bacterial grazing experiments during bi-monthly sampling of the main-stem Chesapeake Bay.
Developing new tools to detect the mixotrophic “middle-ground”
In nature, a long held paradigm classifies organisms that make their own food as producers (ex. Plants) and those that obtain their energy externally as consumers (ex. Animals). But organisms have adapted to challenge this model with a metabolic lifestyle called mixotrophy that mixes plant and animal nutrition. While terrestrial mixotrophy is rare, Venus flytraps provide a perfect example of how a single organism can obtain their energy and nutrition from various sources. Venus flytraps produce their own energy from the sun through photosynthesis like typical plants, but they also consume insects for supplemental nutrition. This allows an organism to be particularly competitive when one of those sources of energy is limited. As it turns out, it is increasingly recognized that this seemingly rare feeding strategy is in fact common within many of the smallest microscopic organisms in the ocean and has been long recognized. But what scientists don’t have a good handle on is how these ‘mixotrophic’ organisms affect food-web structure and function and thus integration into food web models is difficult. This is largely due to the inability to correctly attribute this lifestyle to organisms in their natural samples. Currently, the standard for detecting mixotrophy is either observing the presence of certain types of machinery: chloroplasts for photosynthesis or a food vacuole as indication of feeding.
My current work aims to develop novel tools to better understand how energy is obtained in microbial communities, which contain mixotrophy by studying how stable isotopes of nitrogen change due to various metabolic lifestyles. Nitrogen can be found in various forms of energy an organism relies on and can be found in different isotopic forms. Stable isotopes have varying amounts of neutrons and the ratio of isotopes of different weights can be used as indicators of biotic and abiotic processes. Stable isotope analysis of nitrogen and carbon has just recently been used to establish the sources of energy in mixotrophs in culture.
The unique signatures of the nitrogen isotopes may provide a distinction between mixotrophs and those living as plants or animals (as mixotrophic plankton may have distinguishable nitrogen signatures because consumers will be enriched with the heavier forms of nitrogen). I’ll be using a mixotrophic dinoflagellate, Heterocapsa rotundata, which is commonly found in the Chesapeake Bay as my model organisms to test these hypotheses.
A look into the natural history of ciliates
As a PhD student, I investigated the community dynamics of marine ciliates on the New England Shelf. Ciliates are traditionally difficult to quantify due to difficulties in collecting, culturing, and observing these often-delicate cells. To understand ciliate community dynamics, I utilized data obtained from a custom-built, automated, submerged imaging flow cytometer, Imaging FlowCytobot (IFCB) (developed by Drs. Rob Olson and Heidi Sosik, of the Woods Hole Oceanographic Institution) deployed at the Martha’s Vineyard Coastal Observatory (MVCO) (See the time series of all plankton here!).
This instrument obtains not only high-resolution images of chlorophyll-containing ciliates, but hourly measurements of cell abundance, cell size, and subsequently cell biovolume and biomass. To study and understand a more complete ciliate community beyond those exhibiting herbivory, I developed an updated Imaging FlowCytobot (IFCB-S) modified with automated staining capabilities. Not only were some cell types detected that were not previously, but the comparison of fluorescence properties between staining and non-staining offered insight into the seasonal feeding habits of protist micrograzers. I also found that with IFCB-S, cell abundances were consistently similar to or higher than counts from manual light microscopy indicating that capturing cell abundances with a live application may be more accurate than traditional sampling and preservation.
Molecules and morphology: integrative taxonomic approaches
A large part of my interests involve the development of innovative, integrative tools to study aquatic protists. During my thesis, I combined morphological information from IFCB with genetic techniques to understand temporal changes of community structure. While morphology is a standard method for identifying many ciliates, DNA sequencing has provided new insight into diversity, predator-prey interactions and discrepancies between morphologically defined species and genotypes. I used high-throughput sequencing (HTS) to explore the genetic seasonal community change of tintinnid populations, including both chlorophyll-containing and non-chlorophyll-containing taxa. With their distinct lorica characteristics, tintinnids allowed a direct comparison between IFCB and HTS detection in this study.
I found many species and genera of tintinnids for which morphotype and genotype displayed high congruency. In comparing how well temporal aspects of genotypes and morphotypes correspond, we found that HTS was critical to detect and identify tintinnid genera not efficiently captured with the IFCB.
I extended this work to explored seasonal patterns of subclades within the mixotrophic Mesodinium rubrum/major species complex. The M. rubrum/major subclades have historically been associated with various cell sizes. Despite numerous observations of genetic diversity and size structure plasticity, the response of these two characteristics to seasonal environmental variation has not been rigorously characterized. Integrating morphology from IFCB and sequencing allowed for detection of M. rubrum/major subclades associated with seasonal temperature niches, but not always unique size distributions. These results suggest the interplay of genetic and physiological factors regulating size structure/temperature relationships for the M. rubrum/major species complex.