Name: David Garcia Prieto
Date: 08/06/2026
Time (EST/EDT): 10:00 am
Location: Horn Point Laboratory - AREL Lecture Hall
Remote Access: email: mees@umd.edu

Committee Chair: Dr. Clara A. Fuchsman
Committee Members: Dr. Sairah Y. Malkin, Dr. Tsvetan R. Bachvaroff, Dr. Jacob
A. Cram, Dr. James J. Pierson
Dean’s Representative: Dr. Mihai Pop

Title: Viral, microbial, and zooplankton community dynamics across marine
biogeochemical gradients revealed through metagenomics and eDNA

Abstract: Marine ecosystems are changing due to climate-driven deoxygenation
and increased water column stratification which reduces nutrient inputs to
surface waters. Yet, the responses of microbial and metazoan communities to
these changes remain poorly resolved. Next-generation DNA sequencing
technologies are rapidly evolving allowing us to explore ecosystem composition
from viruses and bacteria to metazoans. In chapter 1, using metagenomics, we
found that SAR11, the most abundant heterotrophic bacteria in the ocean, and
their infecting viruses showed increased abundance of the phosphorus
acquisition gene, pstS, in P deficient euphotic zone waters. We then linked
SAR11 viruses with pstS to deep euphotic zone SAR11 ecotypes. In chapter 2, we
used environmental DNA (eDNA) from the water to analyze the metazoan
communities above, within and below the anoxic zones of two stations in the
Eastern Tropical South Pacific (ETSP) Oxygen Deficient Zone (ODZ) comparing
them to a suboxic station. Our results revealed that metazoan DNA
concentrations peaked within the anoxic and suboxic zones at the suspected
depth of diel vertical migration and that oxygen strongly structured the
community. Mollusk DNA increased in relative proportion at the suboxic station,
but otherwise anoxic and suboxic metazoan communities were similar. In chapter
3, using samples from Hood Canal, WA., a seasonally hypoxic Fjord, we created
new techniques for examining metazoan DNA. First, we compared results from COI
eDNA amplicons to phylogenetic read placement of COI from metagenomes from the
same water samples, revealing how taxonomic identification varied between these
complementary approaches. Additionally, we highlight a long-read COI amplicon
approach sequencing net tows from Hood Canal to help improve our reference
database. The use of phylogenetic read placement of metagenomes allowed us to
statistically link between bacteria (RNA polymerase) and zooplankton (COI). We
revealed that the total zooplankton reads, mostly comprised of hits that placed
to Cnidaria, were statistically linked to Alphaproteobacteria SAR11 but also to
Gammaproteobacteria SAR86, two free-living oligotrophic heterotrophic bacteria.
Together, these results show how modern sequencing approaches can reveal
ecological responses spanning from viruses, bacteria and metazoans across
environmental gradients, improving our understanding of how marine ecosystems
may respond to a changing ocean.