About

Adrian Marchetti is a biological oceanographer and a professor in the Department of Earth, Marine and Environmental Sciences at the University of North Carolina at Chapel Hill. His research combines physiological and molecular approaches to investigate how phytoplankton are affected by their environment and how they influence ocean biogeochemistry and ecosystem dynamics. His particular interests include studying trace metals, such as iron, which are essential for the nutrition of phytoplankton, and predicting the effects of future climate changes on phytoplankton distribution and abundance. Dr. Marchetti earned his Ph.D. from the University of British Columbia in 2005, specializing in ecophysiology and genomics of marine phytoplankton. His work involves inquiry-based science that integrates laboratory isolates and natural communities to better understand the interactions between phytoplankton and their environment, as well as their role in the broader ocean ecosystem.

Research topics

• Ecology
• Geology
• Biology
• Oceanography
• Environmental science
• Geography
• Physics
• Zoology
• Mathematics
• Fishery
• Demography
• Animal science

Selected publications

• Hyperspectral optical signatures of iron‐limitation in a coastal upwelling system

  Limnology and Oceanography · 2026-03-01

  articleOpen access

  Abstract Iron‐limitation within the California Current System drives changes in phytoplankton taxonomic composition and photo‐physiology over a range of spatial scales. Here we demonstrate how these iron‐dependent signatures can be resolved, with high spatial resolution, using continuous ship‐board optical measurements. Deck‐board incubation experiments, along with discrete measurements of iron and macro nutrient concentrations and molecular diagnostic signatures, demonstrated contrasting levels of iron stress across our study region in the northern California Current System during summer, 2023. Photosynthetic pigment measurements indicated that phytoplankton assemblages were largely dominated by diatoms, but low and high iron waters contained differing relative abundances of phytoplankton genera with different mean cell sizes. High frequency ship‐board measurements of hyperspectral particulate absorption demonstrated the presence of two optically distinct phytoplankton assemblages that were associated with varying levels of iron stress. The distribution of the optical clusters across the cruise track showed strong coherence with a number of oceanographic and physiological variables, including phytoplankton size, nutrient drawdown ratios, photosynthetic efficiency () and absorption cross‐section (), and maximum photosynthetic rates. Notably, optical signatures explained more of the observed photo‐physiological variability than other oceanographic variables, such as sea surface temperature and chlorophyll concentration. Variability in phytoplankton iron stress appeared coupled to the age and source of the upwelling water masses. Our results demonstrate the utility of hyperspectral data to map potential iron‐stress with high spatial resolution in dynamic coastal waters.

  PublisherDOI

• Comparison of advanced methodologies for diatom identification within dynamic coastal communities

  UNC Libraries · 2026-04-14

  articleOpen accessSenior author

  Diatom community composition has a critical influence on global ocean health and ecological processes. Developing accurate and efficient methods for diatom identification under dynamic environmental conditions is essential to understanding the implications of diatom community changes. Two developing methods for identifying and enumerating phytoplankton, cell imaging and molecular sequencing, are experiencing rapid advancements. This study aims to compare diatom taxonomic composition results within natural assemblages derived from rapidly advancing methods, FlowCam imaging and metabarcoding of the V4 region of the 18S rRNA gene, with traditional light microscopy cell counting techniques. All three methods were implemented in tandem to analyze changes in dynamic diatom assemblages within simulated upwelling experiments conducted in the California upwelling zone. The results of this study indicate that, summed across all samples, DNA sequencing detected four times as many genera as morphology‐based methods, thus supporting previous findings that DNA sequencing is the most powerful method for analyzing species richness. Results indicate that all three methods returned comparable relative abundance for the most abundant genera. However, the three methods did not return comparable absolute abundance, primarily due to barriers in deriving quantities in equal units. Overall, this study indicates that at the semi‐quantitative level of relative abundance measurements, FlowCam imaging and metabarcoding of the V4 region of the 18S rRNA gene yield comparable results with light microscopy but at the qualitative and quantitative levels, enumeration metrics diverge, and thus method selection and cross‐method comparison should be performed with caution.

  PublisherDOI

• Prokaryotic bias in surface ocean particles

  Proceedings of the National Academy of Sciences · 2026-04-01

  articleOpen access

  While the ocean's photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean's microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

  PublisherOA PDFDOI

• Contrasting the Marine Biogeochemical Cycles of Iron and Scandium in the California Current System

  UNC Libraries · 2026-03-07

  articleOpen access

  The oceanic biogeochemical cycling of iron is globally important yet difficult to fully understand due to the many chemical processes involved. There is potential to use scandium, which has a similar ionic size and charge density to trivalent iron but lacks redox cycling, as a simpler analog for specific parts of the iron cycle, if we can sufficiently develop our understanding of scandium's reactivity. Here we move closer to this understanding. We look at particle reactivity and solubility through a 24‐hr incubation experiment: 5 nmol/kg of dissolved scandium and/or iron were added to filtered and unfiltered California Current System water. Particulate scandium formed only in the unfiltered treatments, at a quantity unlikely to have been taken up biologically. This is the first direct observation of scavenging of scandium, an attribute shared with iron. Our results also serve as the first test of scandium solubility in seawater: 1.9 nmol/kg of dissolved scandium was stable in the filtered treatment, 50 times more than the highest natural concentrations so far observed. This indicates that, in contrast to iron, scandium's oceanic cycling is unlikely to be influenced by solubility limits. We also compare particulate depth profiles: labile particulate iron was disproportionally higher than that of scandium in shelf‐influenced samples, likely due to iron reductively dissolving in the sediments, which scandium cannot do, and then precipitating in oxic seawater. Due to this combination of behaviors, our results suggest that paired observations of scandium and iron may help distinguish between iron sourced from sediment resuspension and reductive dissolution. Iron is an essential nutrient required for growth, yet it is as trace levels in the ocean: in roughly ∼1/3 of the surface ocean, low levels of iron are what prevent phytoplankton from growing more. Therefore, it's important to understand what factors influence its availability in the surface ocean. However, the chemical cycling of iron in the oceans is very complex. Here we investigate a new potential tool to help learn about oceanic iron cycling. Scandium is an element with some similar attributes to iron. If we can learn enough about the similarities and differences in the chemical cycling of these elements, we may be able to use scandium as a simpler analog for iron to learn more about iron cycling. Here we investigate their reactivities in the California Current System. We present the first direct evidence of scandium adsorbing to the surface of particles, like iron. We also show that, in contrast to iron, the solubility of scandium in seawater is likely much higher than natural concentrations. Finally, we compare the source of iron from the sediment just offshore of California and Oregon. Iron can come from two sedimentary pathways and scandium may allow us to distinguish between them. We present the first direct evidence of scandium scavenging onto natural marine particles We perform the first test of scandium solubility in seawater and find that scandium is soluble well above natural concentrations Paired measurements of iron and scandium near margin‐sources may allow discrimination between redox and sediment resuspension iron sources We present the first direct evidence of scandium scavenging onto natural marine particles We perform the first test of scandium solubility in seawater and find that scandium is soluble well above natural concentrations Paired measurements of iron and scandium near margin‐sources may allow discrimination between redox and sediment resuspension iron sources

  PublisherDOI

• Distinct iron acquisition strategies in oceanic and coastal variants of the mixotrophic dinoflagellate <i>Karlodinium</i>

  The ISME Journal · 2025-01-01 · 2 citations

  articleOpen accessSenior author

  The availability of the micronutrient iron is important in regulating phytoplankton growth across much of the world's oceans, particularly in the high-nutrient, low-chlorophyll regions. Compared to known mechanisms of iron acquisition and conservation in autotrophic protists (e.g. diatoms), those of dinoflagellates remain unclear, despite their frequent presence in offshore iron-limited waters. Here, we investigate the strategies of an ecologically important mixotrophic dinoflagellate to coping with low iron conditions. Coupled gene expression and physiological responses as a function of iron availability were examined in oceanic and coastal strains of the dinoflagellate Karlodinium. Under iron-replete conditions, grazing was only detected in coastal variants, resulting in faster growth rates compared to when grown autotrophically. Under iron-limited conditions, all isolates exhibited slower growth rates, reduced photosynthetic efficiencies, and lower cellular iron quotas than in iron-replete conditions. However, oceanic isolates exhibited higher relative growth rates compared to coastal isolates under similar low iron concentrations, suggesting they are better adapted to coping under iron limitation. Yet the oceanic isolates did not exhibit the ability to appreciably reduce cell volume or increase iron-use efficiencies compared to the coastal isolates to cope with iron limitation, as often observed in oceanic diatoms. Rather, molecular pathway analysis and corresponding gene expression patterns suggest that oceanic Karlodinium utilizes a high-affinity iron uptake system when iron is low. Our findings reveal cellular mechanisms by which dinoflagellates have adapted to low iron conditions, further shedding light on how they potentially survive in variable iron regions of the world's oceans.

  PublisherOA PDFDOI

• Photosynthetic electron, carbon and oxygen fluxes within a mosaic of Fe limitation in the California Current Upwelling System

  2025-01-06 · 1 citations

  preprintOpen accessCorresponding

  Abstract. We compare primary productivity estimates based on different photosynthetic ‘currencies’ (electrons, O2 and carbon) collected from the dynamic coastal upwelling waters of the California Current. Fast Repetition Rate Fluorometry and O2/N2 measurements were used to collect high-resolution underway estimates of photosynthetic electron transport rates and net community productivity, respectively, alongside on-station 14C uptake experiments to measure gross carbon fixation rates. Our survey captured two upwelling filaments at Cape Blanco and Cape Mendocino with distinct biogeochemical signatures and iron availabilities, enabling us to examine photosynthetic processes along a natural iron gradient. Significant differences in photo-physiology, cell sizes, Si:NO3- draw-down ratios, and molecular markers of Fe-stress indicated that phytoplankton assemblages near Cape Mendocino were Fe-stressed, while those near Cape Blanco were Fe-replete. Upwelling of O2-poor deep water to the surface complicated O2-based net community productivity estimates, but we were able to correct for these vertical mixing effects using continuous [N2O] surface measurements and depth-profiles of ∂[O2]∂[N2O]. Vertical mixing corrections were strongly correlated to sea surface temperature, which serves as an N2O-independent proxy for upwelling. Following vertical mixing corrections, all three productivity estimates reflected trends in Fe-stress physiology, indicating greater productivity near Cape Blanco compared to Cape Mendocino. For all assemblages, carbon fixation varied as a hyperbolic function of electron transport rates, but the derived parameters of this relationship were highly variable and significantly correlated with physiological indicators of Fe-stress (σPSII, FV/FM, Si:NO3- and diatom-specific PSI gene expression), suggesting that iron availability influenced the coupling between photosynthetic electron transport and subsequent carbon fixation. Net community productivity showed strong coherence with daily-integrated photosynthetic electron transport rates across the entire cruise track, with no apparent relationship with Fe-stress. This result suggests that fluorescence-based estimates of gross photochemistry are still a good indicator for bulk primary productivity, even if Fe-limitation influences the stoichiometric relationship between productivity currencies.

  PublisherDOI

• Evaluating acidotropic dyes for detecting mixotrophy in protists: Insights from cultures and field communities

  bioRxiv (Cold Spring Harbor Laboratory) · 2025-10-01 · 1 citations

  preprintOpen access

  Abstract Mixotrophic protists combine photoautotrophic primary production with heterotrophic phagotrophy, and distinctly impact nutrient cycling and microbial food web dynamics in aquatic environments. Despite their biogeochemical importance, detecting and quantifying mixotrophic presence and grazing in situ remains challenging, preventing a comprehensive understanding of their ecology and biogeography. Fluorescently labeled particle (FLP) incubations are commonly used to quantify mixotroph abundance and ingestion but may underestimate activity due to prey and size preferences of grazers. Acidotropic dyes that stain acidic vacuoles associated with phagotrophy have emerged as an alternative to FLP incubations for estimating mixotroph abundance, yet have not been thoroughly tested among a diverse suite of marine eukaryotes. Here, we evaluate the effectiveness and specificity of two dyes, LysoTracker Green and LysoSensor Blue, in laboratory cultures and natural marine communities. In laboratory cultures, both dyes correctly did not stain one photoautotrophic species. However, LysoSensor failed to stain several known mixotrophs, indicating false negatives, while both dyes stained photoautotrophic diatoms, indicating false positives. In the field, LysoTracker staining broadly tracked with FLP-derived results in the North East Shelf (NES) and the diatom-rich California Current System (CCS). Both methods indicated lower mixotroph abundance and proportion in the CCS, suggesting acidotropic dyes may more reliably reflect mixotrophy in the field than in monoculture. This study highlights the utility and limitations of acidotropic dyes for detecting mixotrophy and underscores the importance of incorporating community composition and complementary grazing estimates for reliable interpretation.

  PublisherOA PDFDOI

• Corrigendum: Diatom transcriptional and physiological responses to changes in iron bioavailability across ocean provinces

  UNC Libraries · 2025-04-10

  articleOpen access

  <span style="font-size: 11pt; font-weight: normal; font-style: normal;" data-sheets-root="1">Corrigendum to: "Diatom transcriptional and physiological responses to changes in iron bioavailability across ocean provinces"</span>

  PublisherDOI

• Corrigendum: Diatom transcriptional and physiological responses to changes in iron bioavailability across ocean provinces

  Frontiers in Marine Science · 2025-04-01

  erratumOpen accessSenior authorCorresponding

  A corrigendum refers to a change to their article that the author wishes to publish after publication. The publication of this article is subject to Frontiers Editorial approval. Instructions:• please read through all the templates before choosing • pick the most relevant text template(s) from the following page and delete all others.• edit the text as necessary, ensuring that the original incorrect text is included for the record, please see the below. • please do not use any extra formatting when editing the templates, and only modify the red text unless absolutely necessary • submit to Frontiers following the instructions on this page.When the original text contained incorrect information, to preserve the scientific record, please include that text when editing the below templates. For example:There was a mistake in the Funding statement, an incorrect number was used. The correct number is &quot;2015C03Bd051.&quot;. The publisher apologizes for this mistake.The original version of this article has been updated. In the published article, there was an error in Figure 3 as published. The initial (T0) phytoplankton community composition at site C2 was incorrectly displayed, with prasinophytes erroneously appearing as relatively abundant taxa. The corrected Figure 3 appears below.A text correction has been made to Results, Community Composition across Sites, paragraph number 1. This sentence previously stated:&quot;In contrast, CUZ site C2 initially yielded a phytoplankton community transcript pool dominated equally by diatoms (30%) and prasinophytes (28%), with diatoms remaining a dominant taxa following incubation (26-28%) and prasinophyte transcripts substantially decreasing from 28 to 3-8% in both Fe and DFB incubations&quot;Has been removed.The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been updated.Reminder: Figures, tables, and images will be published under a Creative Commons CC-BY licence and permission must be obtained for use of copyrighted material from other sources (including re-published/adapted/modified/partial figures and images from the internet). It is the responsibility of the authors to acquire the licenses, to follow any citation instructions requested by third-party rights holders, and cover any supplementary charges.End of template, if you would like to request a correction for a reason not seen here, please contact the journal&#39;s Editorial Office

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• Photosynthetic electron, carbon, and oxygen fluxes within a mosaic of Fe limitation in the California Current upwelling system

  Biogeosciences · 2025-10-02 · 1 citations

  articleOpen access

  Abstract. We compare primary productivity estimates based on different photosynthetic “currencies” (electrons, O2, and carbon) measured in the dynamic coastal upwelling waters of the California Current. Fast repetition rate fluorometry and O2/N2′ measurements were used to collect high-resolution underway estimates of photosynthetic electron transport rates and net community productivity, respectively, alongside on-station 14C uptake experiments to measure gross carbon fixation rates. Our survey captured two upwelling filaments at Cape Blanco and Cape Mendocino with distinct biogeochemical signatures and iron availabilities, enabling us to examine photosynthetic processes along a natural iron gradient. Significant differences in photophysiology, cell sizes, Si:NO3- draw-down ratios, and molecular markers of Fe stress indicated that phytoplankton assemblages near Cape Mendocino were Fe stressed, while those near Cape Blanco were Fe replete. Upwelling of O2-poor deep water to the surface complicated O2-based net community productivity estimates, but we were able to correct for these vertical mixing effects using continuous [N2O] surface measurements and depth-profiles of ∂[O2]∂[N2O]. Vertical mixing corrections were strongly correlated to sea surface temperature, which serves as an N2O-independent proxy for upwelling. All three productivity estimates reflected trends in Fe-stress physiology, indicating greater productivity near Cape Blanco compared to Cape Mendocino. For all phytoplankton assemblages, carbon fixation varied as a hyperbolic function of photosynthetic electron transport rates, but the derived parameters of this relationship were variable and significantly correlated with physiological indicators of Fe stress (σPSII, Fv/Fm, Si : NO3-, and diatom-specific PSI gene expression), suggesting that iron availability influenced the coupling between photosynthetic electron transport and carbon fixation. Net community productivity showed strong coherence with daily integrated photosynthetic electron transport rates across the entire cruise track, with no apparent relationship with Fe stress. This result suggests that fluorescence-based estimates of gross photochemistry are still a good indicator for bulk primary productivity, even if Fe limitation influences the stoichiometric relationship between different productivity currencies.

  PublisherDOI

Recent grants

• Collaborative Research: Investigating the Ecological Importance of Iron Storage in Diatoms

  NSF · $435k · 2013–2017

• CAREER: An integrated molecular and physiological approach to examining the dynamics of upwelled phytoplankton in current and changing oceans

  NSF · $1.1M · 2018–2025

• Collaborative Research: Antarctic Diatom Proteorhodopsins: Characterization and a Potential Role in the Iron-limitation Response

  NSF · $607k · 2018–2023

• Iron and Light Limitation in Ecologically Important Polar Diatoms: Comparative Transcriptomics and Development of Molecular Indicators

  NSF · $260k · 2014–2018

Frequent coauthors

• Robert Lampe

  University of California, San Diego

  38 shared

• Natalie R. Cohen

  25 shared

• Nicolas Cassar

  Duke University

  24 shared

• Paul J. Harrison

  19 shared

• Benjamin S. Twining

  16 shared

• Sarah W. Davies

  16 shared

• Claire P. Till

  California State Polytechnic University

  16 shared

• Weida Gong

  14 shared

Labs

• Earth, Marine and Environmental SciencesPI