Next Article in Journal
Integrating C-Phycocyanin, and Polyhydroxybutyrate Recovery Using a Triphasic System: Experimental Design and Optimization in Thermotolerant Potamosiphon sp.
Previous Article in Journal
Prospects and Potential for the Use of Microalgae and Cyanobacteria Biomass in Agriculture
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Phycology
Volume 6
Issue 1
10.3390/phycology6010020
phycology-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Getting Attached: A Heterotrophic Nanoflagellate Mingling with Centric Diatoms

by
Gabrielle Corradino
1,2,* and
Astrid Schnetzer
2
1
Department of Biology, Barnard College, Columbia University, New York, NY 10027, USA
2
Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA
*
Author to whom correspondence should be addressed.
Phycology 2026, 6(1), 20; https://doi.org/10.3390/phycology6010020
Submission received: 20 November 2025 / Revised: 20 January 2026 / Accepted: 23 January 2026 / Published: 1 February 2026
(This article belongs to the Special Issue Microbial Interactions in the Phycosphere)
Browse Figures
Versions Notes

Abstract

Heterotrophic nanoflagellates (HNANs) are central components of the microbial loop, transferring carbon from bacteria to higher trophic levels and facilitating nutrient recycling. While many HNANs are free-swimming, some exhibit enhanced feeding efficiency when attached to surfaces, including diatom frustules. Here, we describe the attachment behavior of a novel interception-feeding HNAN affiliated with the order Bicosoecida to centric diatoms common in North Carolina coastal waters. Using growth experiments, live observations, and time-lapse microscopy, we quantified attachment frequency and assessed its influence on diatom growth for three diatom species: Coscinodiscus sp., Odontella sp., and Rhizosolenia sp. HNAN attachment differed significantly among diatom taxa: Coscinodiscus sp. hosted the highest and most sustained numbers per frustule, whereas after normalizing for surface area, Rhizosolenia sp. exhibited the highest attachment efficiency. Diatom peak growth was 1.2 to 2.1-fold higher and occurred earlier in HNAN co-cultures than in controls, indicating microbial recycling by the HNAN stimulated growth. These findings highlight the nuanced ecological role attached HNANs might play as they exploit diatom-associated boundary layers to enhance bacterial encounter rates. The growth trajectories in our lab experiments suggests that attachment behavior in situ can play a role in driving diatom bloom dynamics and, therefore, play an important role for carbon cycling.

1. Introduction

Marine heterotrophic nanoflagellates (HNANs) are a diverse group of protists that (2–20 µm in size) act as primary consumers of picoplankton [1,2,3], shunting carbon towards higher trophic levels or in remineralization of nutrients within the microbial loop [1,4,5,6]. While most phagotrophic HNANs graze while in motion, they were shown to exhibit higher rates of ingestion and particle capture when attached to surfaces (i.e., bacterial mats, the thecal plates of dinoflagellate, diatom frustules) [7,8]. Attachment can enhance the encounter rates by allowing HNANs to exploit boundary-layer hydrodynamics around surfaces, increasing the probability of contacting prey [4,9]. For instance, HNANs like Bodo and Rhynchomonas are dependent on the availability of suspended particles to graze on bacteria [7,8,9,10]. The outside of both dinoflagellates and diatoms is a common substrate for HNAN attachment, allowing the flagellates to take advantage of the increased bacterial abundances often associated with the phycosphere [11,12]. Factors that may impact this environment are complex including nutrient gradients, phytoplankton morphology, or life stage [13,14]. These associations significantly influence carbon cycling, particularly during diatom blooms, when the release of dissolved and particulate organic matter fuels bacterial communities [15].
How HNANs interact with phytoplankton surfaces has been described in only a few studies to date. For instance, some research has shown that HNANs grazing on bacteria, while attached to chain-forming diatoms such as Asterionella and Fragilaria, can reach densities of up to ~35 HNANs per diatom chain [8,16]. For the mixotrophic flagellate Ochromonas, its presence was associated with suppressing algicidal bacteria with the dinoflagellate Alexandrium through bacterial grazing, illustrating how protist and bacteria interactions can shift bloom trajectories [17]. In rare cases, HNANs like Pirsonia can directly consume the cytoplasm of the diatom Guinardia by attaching to the valve of the host cell, with field observations demonstrating >90% of diatoms with the attached parasitic flagellate [18,19,20]. Collectively, these HNAN dynamics influence the phytoplankton population structure, bloom termination, and microbial carbon transfer [21].
Despite research on HNAN grazing ecology, the ecological significance of HNAN attachment to phytoplankton surfaces remains poorly resolved, and the implications of flagellate–diatom interactions remain unaddressed in microbial food web models [7,19,22]. This study documents the attachment behavior of an interception-feeding HNAN to the frustules of several large centric diatoms, including Odontella sp., Rhizosolenia sp., and Coscinodiscus sp., to evaluate how attachment relates to diatom growth. Using both live observations and time-lapse photography in growth experiments, we observed HNAN–diatom interactions, estimated the attachment frequencies, and quantified the impact of HNAN attachment on diatom growth. We observed that attachment seemed to facilitate nutrient remineralization by the HNAN feeding on bacteria and boosted diatom growth by all three genera, with a more pronounced growth response in Odontella sp. and Rhizosolenia sp. compared to Coscinodiscus sp. Attachment densities (HNANs per surface area) differed across the genera but were positively associated with diatom growth during the first phase of the incubations up to ~day 6. Elevated growth did not last beyond this initial phase and sharply decreased towards the end of the three-week experiments indicating a significant but short-lived response. These data provide rare estimates on the number of co-occurring HNAN attaching to diatom taxa across varying growth stages and help to elucidate our understanding of HNAN interactions within the phycosphere.

2. Materials and Methods

2.1. Study Organisms and Culture Conditions

An unidentified HNAN (~5 µm; Figure 1) was collected in surface plankton tows (150 µm mesh size) from the eastern portion of Bogue Sound, North Carolina [23]. The HNAN (Figure 1a) was initially observed attached to diatom cells and was subsequently separated using 20 µm mesh screening combined with serial dilution steps [23]. This culturing approach was paired with single-cell isolations of the co-occurring diatoms Coscinodiscus sp., Rhizosolenia sp., and Odontella sp. [24]. Based on 18S ribosomal RNA sequences and NCBI BLAST (BLASTn, https://blast.ncbi.nlm.nih.gov/, accessed on 1 November 2025) matches (MN818959.1), the HNAN isolate was identified as being most closely affiliated with the order Bicosoecida [23]. Cultures of the HNAN were maintained in artificial seawater (ASW) f/20 medium at 16 °C (Percival Scientific Inc., Perry, IA, USA) under a 14:10 h light: dark cycle and a light intensity of 75 µE m−2 s−1 [23]. The same conditions were selected for the centric diatoms.

2.2. Experimental Setup

Diatom cultures were inoculated during the early exponential phase with the HNAN at a density of 103 cells mL−1. Experiments were conducted in ASW in triplicate 6 mL well-plates, and the abundances of both HNAN and diatoms were recorded at the start of the experiment and every three days for 21 days. At each time point, three aliquots (30 µL each) were analyzed to enumerate both the diatom and HNAN abundances (cells mL−1) and the number of HNANs attached. To account for evaporation losses in our 6-well dishes, an additional 10% of the total volume was added of ASW on day 9 and day 18.
HNAN attachment was defined as one or both flagella of the HNAN in direct contact with the diatom frustule. The diatom growth was estimated by determining the changes in cell abundances over the duration of the experiments (d−1). The instantaneous growth rate (μ) was calculated as μ = [ln(Nₜ) − ln(N0)]/t, where N0 and Nₜ are the diatom abundances at the beginning and end of each 3-day interval, respectively, and t is time (days).
Cell counts, attachment, and behavioral observations were assessed using light microscopy (Olympus BX53, Olympus Corporation, Tokyo, Japan) and differential interference contrast (DIC) microscopy. HNAN attachment was estimated every three days based on observations using time-lapse photography (5 s intervals for 14 frames at 40×), and the video recordings were used to document the attachment behavior and active feeding activity, defined by Boenigk and Arndt (2000) [25] as the HNAN creating a current with their flagella to move particles toward the cell body for possible ingestion. HNAN attachment was observed per cell and was normalized to the diatom surface area. For each diatom genus (n = 3), we calculated its total cell surface area (µm2) using species-specific measurements of diameter, length, and height, assuming a cylindrical geometry (total of 10 observations per genus) to derive the number of HNANs attached per 1000 µm2, allowing for comparison across taxa that differed significantly in size.

2.3. Statistical Analyses

All analyses were performed in JMP Pro 17 (SAS Institute, Cary, NC, USA). Differences in the diatom growth rates across treatments and HNAN attachment densities across and among treatments were tested using one-way ANOVA [26]. Comparisons of means were examined using a non-parametric post hoc Tukey–Kramer HSD test to identify statistically significant differences between groups (p < 0.05). A one-way ANOVA followed by a Tukey–Kramer HSD post hoc test was also used to test for differences in the surface area across genera. A linear mixed-effects model was run to examine the relationship between HNAN attachment (per diatom and per 1000 µm2 at the start of each interval) and subsequent diatom growth rate (day−1) with diatom genus included as a fixed effect and replicate as a random effect.

3. Results

3.1. Growth Rates

The diatom growth rates in the control treatments, in the absence of HNANs, trended lower than those observed in the HNAN addition treatments (Figure 2). In the control treatments, Coscinodiscus sp. growth averaged 0.27 ± 0.32 d−1, Odontella sp. 0.36 ± 0.01 d−1, and Rhizosolenia sp. 0.45 ± 0.34 d−1 (n = 9 each; Figure 2). Comparing over time or between the diatoms did not yield any significant changes during the incubations for controls. With HNANs present, the growth rates for Odontella sp. and Rhizosolenia sp. increased significantly over the first 6 days before a sharp decline during the last phase of the incubations (p < 0.05 for both intervals), while a steep decline during the last phase of the incubations (day 6–day 9) yielded growth rates that resembled those in the control treatment (p > 0.05). Odontella sp. reached its highest mean growth rates with 0.76 ± 0.07 d−1 (overall mean = 0.12 ± 0.07 d−1, n = 9) and Rhizosolenia sp. with 1.0 ± 0.20 d−1 (overall mean = 0.24 ± 0.14 d−1, n = 9) between days 3 and 6 (Figure 2). In contrast, Coscinodiscus sp. grew at significantly lower rates compared to the other two taxa from day 6 to day 9 (p < 0.05), and the peak growth with 0.52 ± 0.05 d−1 in the beginning of the incubations (day 0–3) plummeted to values even lower than in the control treatment (Figure 2). HNAN growth was not determined, since only the attached cells were counted.

3.2. Attachment

Coscinodiscus sp. had the largest mean surface area (110,182 ± 19,563 µm−2, p < 0.05; Figure 3), followed by Odontella sp. (12,855 ± 5122 µm−2) and Rhizosolenia sp. (6917 ± 1335 µm−2); the latter did not differ significantly from each other. Video observations corroborated the microscopy-based attachment counts by demonstrating sustained flagellar anchoring and active feeding (Supplementary Videos S1–S3). The HNAN was observed both free-swimming surrounding the diatoms and attaching with one flagellum anchored and the other creating a feeding current (Figure 1 and Supplementary Video S3). The HNAN was not observed to penetrate or consume the diatom at any time throughout any of the experiments as described for parasitic forms. By day 3, HNAN attachment was observed for all diatoms (Figure 1 and Table 1), and when normalized to diatom surface area, the HNAN attachment densities (HNAN per 1000 µm2) differed significantly among diatom genera (two-way ANOVA, p < 0.05, Figure 4).
On average, Coscinodiscus sp. had the lowest number of HNANs attached with 0.06 ± 0.7 1000 µm−2, Odontella sp. had 0.13 ± 0.09 1000 µm−2, and the highest attachment was Rhizosolenia sp. averaging 0.19 ± 0.1 1000 µm−2 (n = 21 per genus; secondary y-axes in Figure 4). The overall maximum was seen on day 21 for Odontella sp. with 0.39 HNANs 1000 µm−2. The HNAN attachment increased over time (p < 0.05), and an interactive effect was observed for diatom genus and observation time point (p = 0.0497). The linear mixed-effects model showed HNAN attachment significantly correlated with the diatom growth rates (p < 0.05). The strength and direction of this effect varied among species (p < 0.05), with a negative association in Rhizosolenia sp. and no significant change with Coscinodiscus sp. or Odontella sp. Since previous studies on attachment behavior have listed densities per cell or chain (Table 2), they are also provided here. Due to the considerable size differences among the three genera, using this measure, Coscinodiscus sp. would exhibit the highest number of attached HNAN with an average of 6.36 ± 7.18 HNAN diatom−1. Odontella sp. followed with 1.67 ± 1.20, and finally, Rhizosolenia sp. had 1.28 ± 0.72 (n = 21 per species). Estimated as per cell attachment, the maximum was seen on day 18 for Coscinodiscus sp. with 22 HNAN per diatom−1 (Table 1).

4. Discussion

4.1. Diatom Growth and Nanoflagellate Attachment

The mean diatom growth rates increased by 1.2 to 2.1-fold, in the presence of HNANs across all genera. We propose that this difference in diatom growth was driven by the HNAN facilitating microbial recycling, which benefitted additional algal growth [29]. However, the responses varied over time and across diatoms, where two of the three species, Odontella sp. and Rhizosolenia sp., displayed significant increases in growth throughout the initial period of the experiment followed by a steep decline after day 6 (Figure 2). In contrast, the elevated initial growth rates for Coscinodiscus sp. compared to the control were not followed by an increase, but Coscinodiscus sp. growth took an immediate downturn in the continued presence of the HNAN. The HNAN did attach to all the frustules independent of diatom type within several days, but species-specific differences were observed in the HNAN attachment densities. Rhizosolenia sp. exhibited the highest HNAN attachments (range = 0.15 to 0.31 1000 µm−2), followed by Odontella sp. (range = 0.8 to 0.29 1000 µm−2), with Coscinodiscus sp. showing the lowest (range = 0.01 to 0.10 1000 µm−2, Figure 4).
Flagellate attachment to varying surfaces including diatoms has been reported numerous times; however, only a few studies have estimated flagellate densities normalized to the cell surface area, per cell, colony, or chain (Table 2). Representing our findings on a per-cell basis, we find that the attachment numbers for our study (1.3–6.4 HNAN diatom cell−1 across Coscinodiscus sp., Odontella sp., and Rhizosolenia sp.) fell well within the general range compared to the limited number of reports from previous studies (Table 2). We did not estimate the densities per colony observed for other taxa (e.g., 10–35 cells colony−1 in Fragilaria and 0–21 cells colony−1 in Asterionella, Table 2). The densities also aligned with parasitic nanoflagellates observed in prior work: Kühn (1997) [27] documented at least one Pirsonia cell per infected Coscinodiscus, while Peacock (2014) [22] reported one to eight parasitic HNANs per infected Guinardia (Table 2). These values fall within the same magnitude as our observed attachment densities. Differences among flagellate species have also been documented in prior work; for example, Caron (1987) [7] reported that Bodo and Rhynchomonas exhibited 40–80% attachment to detrital particles, whereas Monas and Cafeteria attached far less frequently, underscoring that attachment efficiencies reflect intrinsic species-level behaviors rather than host characteristics alone. Comparisons across studies, however, must be made with caution, given the substantial differences in experimental design, host species, and observational windows. Much of the existing work on HNAN–diatom interactions spans < 2 days or is based on limited temporal coverage (Table 2), which restricts our ability to assess longer-term attachment dynamics and, thereby, evaluate their broader ecological consequences.
The live observations, videos, and timelapse pictures demonstrated how the HNAN continually anchored to the diatom frustules, with no observed penetration of host cells or visible damage, indicating a non-parasitoid relationship likely leveraging more efficient interception feeding (Figure 1b–d and Supplemental Videos S1–S3). Prior observations of anchored flagellates that generate feeding currents while tethered to surfaces [9,25] have shown how attachment allows for the exploitation of the boundary layer hydrodynamics and enhances the encounter rates with suspended bacteria and organic particles [30,31,32]. While previous laboratory studies with the same HNAN used here [22,33] showed that the HNAN can hunt efficiently in its free-swimming stage, the fact that bacteria are more highly concentrated within and around the phycosphere [11,34,35] was a likely driver for attachment in this study. Seymour et al. (2017) [13] demonstrated that diatom surfaces exhibit microscale chemotactic signaling, and Amin et al. (2015) [36] showed that diatom-derived metabolites can actively recruit and stimulate the growth of specific bacteria. Although these studies did not examine Odontella sp., Rhizosolenia sp., or Coscinodiscus sp., each is known to harbor distinct bacterial partners and metabolically active phycospheres [37,38,39], suggesting similar signaling may impact microbial dynamics.

4.2. Diatom Bloom Impacts

Laboratory findings cannot simply be extrapolated to field settings; so, it is unknown whether the observed increases in diatom growth paired with a swift decrease would translate to a boosted bloom and its faster decline in situ. Nevertheless, the connection between the diatom bloom decline with increasing colonizing bacteria and HNAN abundances surrounding the phycosphere has been established [13,16,40]. The blooms can result in a detritus-dominated phase, which can provide nutrients and a refuge to certain groups of marine bacteria [40,41]. For instance, in a mesocosm study by Riemann et al. (2000) [41], the diatom bloom decay period coincided with a strong rise in bacterivorous HNAN abundances, and these increases were paired with elevated contributions from particle-attached bacteria to the total bacterial numbers before the bloom peak compared to the late post-bloom phase. While major progress has been made in documenting bacterial behavioral aspects [42,43], such as the settling of bacteria on surfaces within the phycosphere, studies to date have not been designed to decipher what proportion of HNANs directly colonize or attach to diatoms during these critical bloom phases. Given that buoyancy regulation may be heavily affected, and that the loss of buoyancy control is a major factor in the onset of diatom sinking, further attention should be paid to flagellate attachment [44]. At a minimum, our results do support previous findings that HNAN activity can alter diatom growth trajectories and, thereby, potentially bloom evolution [28,45]. In our laboratory experiments, attaching HNANs affected the timing of the initial growth, the growth maxima, and the onset of growth decline for the three diatoms in varying ways. These HNAN interactions are indicative of how attachment could affect in situ bloom composition as well as the duration and magnitude of a bloom.
A study by Kühn (1996) [19] found over 40% of Eucampia had parasitic flagellates attached during a bloom in the North Sea, which contributed to diatom mortality. The deciphering of complex microbial connections that may underly bloom dynamics will continue to require the combination of carefully derived laboratory studies and field observations [46]. An example that demonstrates the potential implications of algae–bacteria–flagellate interactions on harmful algal bloom dynamics was shown for a dinoflagellate during a co-culturing experiment with Alexandrium tamarense and algicidal bacteria, where the addition of a bacterivorous flagellate benefited the toxic dinoflagellate [17]. Such interactions are rarely documented, but together with the studies summarized in Table 2, they illustrate how flagellate activity can restructure microbial associations at the phytoplankton’s cell surface. The ways that flagellates within the phycosphere can impact diatom growth and bloom dynamics are diverse and may range from impacting the composition of algae-associated bacteria, facilitating nutrient recycling, or affecting diatom fitness by influencing the buoyancy, drag, or sinking rate [47]. Consequently, HNAN attachment plays a potentially important role in the phycosphere with carbon cycling in the surface ocean and for export flux. By modifying diatom growth trajectories and bloom persistence, HNAN attachment has the potential to affect the timing and magnitude of carbon transfer between phytoplankton and heterotrophic microbes to higher trophic levels. This highlights the importance of considering behavioral aspects that surround diatom–bacteria–flagellate interactions when examining bloom dynamics and carbon flux in marine systems.

5. Conclusions

This study demonstrated that the attachment of a HNAN to centric diatoms can measurably alter the diatom growth trajectories, potentially in a taxon-specific manner. While all three diatoms hosted attached HNANs, the attachment rates and diatom growth responses differed among species. These results suggested that HNAN attachment can enhance localized nutrient recycling within the phycosphere, likely through intensified, more efficient, grazing by HNANs on diatom-associated bacteria. The results of our study further indicated that attached HNANs influenced the magnitude and the timing of diatom growth and decline. Together these findings highlight HNAN attachment as an important mechanism linking microbial interactions at the cell surface to broader patterns of diatom bloom dynamics.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/phycology6010020/s1, Videos S1–S3: Videos showing attachment behavior of the heterotrophic nanoflagellate interacting with Video S1: Coscinodiscus sp., Video S2: Odontella sp., and Video S3: Rhizosolenia sp., respectively. The videos are representative of early (Video S1), intermediate (Video S2), and final conditions (Video S3) during experiments.

Author Contributions

A.S. and G.C. contributed to the design of the study and data interpretation. G.C. performed the incubation experiment and ran sample analyses. G.C. wrote the initial draft of the manuscript, and A.S. contributed to its revision. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support for this work was provided by the National Science Foundation Chemical Oceanography award 1459406.

Data Availability Statement

The raw sequence data utilized for this study was deposited to the National Center for Biotechnology Information (NCBI) MN818959.1.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
HNANHeterotrophic Nanoflagellate

References

  1. Azam, F.; Fenchel, T.; Field, J.G.; Gray, J.S.; Meyer-Reil, L.-A.; Thingstad, F. The Ecological Role of Water-Column Microbes in the Sea. Mar. Ecol. Prog. Ser. 1983, 10, 257–263. [Google Scholar] [CrossRef]
  2. Logares, R.; Audic, S.; Santini, S.; Pernice, M.C.; De Vargas, C.; Massana, R. Diversity Patterns and Activity of Uncultured Marine Heterotrophic Flagellates Unveiled with Pyrosequencing. ISME J. 2012, 6, 1823–1833. [Google Scholar] [CrossRef]
  3. Nakayama, T. Biology, Diversity and Ecology of Free-Living Heterotrophic Flagellates. In Marine Protists: Diversity and Dynamics; Springer: Berlin/Heidelberg, Germany, 2015; pp. 63–87. [Google Scholar]
  4. Fenchel, T. Ecology of Heterotrophic Microflagellates. IV. Quantitative Occurrence and Importance as Bacterial Consumers. Mar. Ecol. Prog. Ser. 1982, 9, 35–42. [Google Scholar] [CrossRef]
  5. Sherr, E.B.; Sherr, B.F.; Verity, P.G. Distribution and Relation of Total Bacteria, Active Bacteria, Bacterivory, and Volume of Organic Detritus in Atlantic Continental Shelf Waters off Cape Hatteras NC, USA. Deep Sea Res. Part II Top. Stud. Oceanogr. 2002, 49, 4571–4585. [Google Scholar] [CrossRef]
  6. Yang, J.W.; Wu, W.; Chung, C.-C.; Chiang, K.-P.; Gong, G.-C.; Hsieh, C. Predator and Prey Biodiversity Relationship and Its Consequences on Marine Ecosystem Functioning—Interplay between Nanoflagellates and Bacterioplankton. ISME J. 2018, 12, 1532–1542. [Google Scholar] [CrossRef]
  7. Caron, D.A. Grazing of Attached Bacteria by Heterotrophic Microflagellates. Microb. Ecol. 1987, 13, 203–218. [Google Scholar] [CrossRef] [PubMed]
  8. Sanders, R.; Porter, K. Phagotrophic Phytoflagellates. In Advances in Microbial Ecology; Nelson, K.E., Ed.; Springer Nature: Berlin/Heidelberg, Germany, 1988; pp. 167–192. [Google Scholar]
  9. Willkomm, M.; Schlüssel, A.; Reiz, E.; Arndt, H. Effects of Microcurrents in the Boundary Layer on the Attachment of Benthic Heterotrophic Nanoflagellates. Aquat. Microb. Ecol. 2007, 48, 169–174. [Google Scholar] [CrossRef]
  10. Artolozaga, I.; Valcárcel, M.; Ayo, B.; Latatu, A.; Iriberri, J. Grazing Rates of Bacterivorous Protists Inhabiting Diverse Marine Planktonic Microenvironments. Limnol. Oceanogr. 2002, 47, 142–150. [Google Scholar] [CrossRef]
  11. Smriga, S.; Fernandez, V.I.; Mitchell, J.G.; Stocker, R. Chemotaxis toward Phytoplankton Drives Organic Matter Partitioning among Marine Bacteria. Proc. Natl. Acad. Sci. USA 2016, 113, 1576–1581. [Google Scholar] [CrossRef]
  12. Daly, G.; Decorosi, F.; Viti, C.; Adessi, A. Shaping the Phycosphere: Analysis of the EPS in Diatom-bacterial Co-cultures. J. Phycol. 2023, 59, 791–797. [Google Scholar] [CrossRef]
  13. Seymour, J.R.; Amin, S.A.; Raina, J.-B.; Stocker, R. Zooming in on the Phycosphere: The Ecological Interface for Phytoplankton–Bacteria Relationships. Nat. Microbiol. 2017, 2, 17065. [Google Scholar] [CrossRef]
  14. Johansson, O.N.; Pinder, M.I.; Ohlsson, F.; Egardt, J.; Töpel, M.; Clarke, A.K. Friends with Benefits: Exploring the Phycosphere of the Marine Diatom Skeletonema Marinoi. Front. Microbiol. 2019, 10, 1828. [Google Scholar] [CrossRef]
  15. Azam, F.; Malfatti, F. Microbial Structuring of Marine Ecosystems. Nat. Rev. Microbiol. 2007, 5, 782–791. [Google Scholar] [CrossRef]
  16. Grossart, H. Ecological Consequences of Bacterioplankton Lifestyles: Changes in Concepts Are Needed. Environ. Microbiol. Rep. 2010, 2, 706–714. [Google Scholar] [CrossRef]
  17. Hu, L.; Peng, X.; Zhou, J.; Zhang, Y.; Xu, S.; Mao, X.; Gao, Y.; Liang, J. Characterizing the Interactions among a Dinoflagellate, Flagellate and Bacteria in the Phycosphere of Alexandrium tamarense (Dinophyta). Front. Mar. Sci. 2015, 2, 100. [Google Scholar] [CrossRef]
  18. Schnepf, E.; Drebes, G.; Elbrächter, M. Pirsonia Guinardiae, Gen. et Spec. Nov.: A Parasitic Flagellate on the Marine Diatom Guinardia flaccida with an Unusual Mode of Food Uptake. Helgoländer Meeresunters. 1990, 44, 275–293. [Google Scholar] [CrossRef]
  19. Kühn, S.; Drebes, G.; Schnepf, E. Five New Species of the Nanoflagellate Pirsonia in the German Bight, North Sea, Feeding on Planktic Diatoms. Helgoländer Meeresunters. 1996, 50, 205–222. [Google Scholar] [CrossRef]
  20. Kühn, S.; Medlin, L.; Eller, G. Phylogenetic Position of the Parasitoid Nanoflagellate Pirsonia Inferred from Nuclear-Encoded Small Subunit Ribosomal DNA and a Description of Pseudopirsonia n. Gen. and Pseudopirsonia mucosa (Drebes) Comb Nov. Protist 2004, 155, 143–156. [Google Scholar] [CrossRef] [PubMed]
  21. Lin, S.; Huang, L.; Zhu, Z.; Jia, X. Changes in Size and Trophic Structure of the Nanoflagellate Assemblage in Response to a Spring Phytoplankton Bloom in the Central Yellow Sea. Deep Sea Res. Part II Top. Stud. Oceanogr. 2013, 97, 93–100. [Google Scholar] [CrossRef]
  22. Peacock, E.E.; Olson, R.J.; Sosik, H.M. Parasitic Infection of the Diatom Guinardia delicatula, a Recurrent and Ecologically Important Phenomenon on the New England Shelf. Mar. Ecol. Prog. Ser. 2014, 503, 1–10. [Google Scholar] [CrossRef]
  23. Corradino, G.L.; Schnetzer, A. Grazing of a Heterotrophic Nanoflagellate on Prokaryote and Eukaryote Prey. Mar. Ecol. Prog. Ser. 2022, 682, 65–77. [Google Scholar] [CrossRef]
  24. Tomas, C.R. Identifying Marine Phytoplankton; Elsevier: Amsterdam, The Netherlands, 1997. [Google Scholar]
  25. Boenigk, J.; Arndt, H. Particle Handling during Interception Feeding by Four Species of Heterotrophic Nanoflagellates. J. Eukaryot. Microbiol. 2000, 47, 350–358. [Google Scholar] [CrossRef] [PubMed]
  26. Zar, J.H. Simple Linear Correlation. Biostat. Anal. 1984, 4, 377–383. [Google Scholar]
  27. Kühn, S.F. Infection of Coscinodiscus spp. by the Parasitoid Nanoflagellate Pirsonla diadema: I. Behavioural Studies on the Infection Process. J. Plankton Res. 1997, 19, 791–804. [Google Scholar] [CrossRef]
  28. Šimek, K.; Jezbera, J.; Hornák, K.; Vrba, J.; Sed¹a, J. Role of Diatom-Attached Choanoflagellates of the Genus Salpingoeca as Pelagic Bacterivores. Aquat. Microb. Ecol. 2004, 36, 257–269. [Google Scholar] [CrossRef]
  29. Zöllner, E.; Hoppe, H.-G.; Sommer, U.; Jürgens, K. Effect of Zooplankton-mediated Trophic Cascades on Marine Microbial Food Web Components (Bacteria, Nanoflagellates, Ciliates). Limnol. Oceanogr. 2009, 54, 262–275. [Google Scholar] [CrossRef]
  30. Pepper, R.E.; Roper, M.; Ryu, S.; Matsudaira, P.; Stone, H.A. Nearby Boundaries Create Eddies near Microscopic Filter Feeders. J. R. Soc. Interface 2010, 7, 851–862. [Google Scholar]
  31. Nguyen, H.; Koehl, M.; Oakes, C.; Bustamante, G.; Fauci, L. Effects of Cell Morphology and Attachment to a Surface on the Hydrodynamic Performance of Unicellular Choanoflagellates. J. R. Soc. Interface 2019, 16, 20180736. [Google Scholar] [CrossRef]
  32. Suzuki-Tellier, S.; Andersen, A.; Kiørboe, T. Mechanisms and Fluid Dynamics of Foraging in Heterotrophic Nanoflagellates. Limnol. Oceanogr. 2022, 67, 1287–1298. [Google Scholar] [CrossRef]
  33. Corradino, G.L.; Schnetzer, A. Impact of Temperature and Diet on Feeding and Growth of a Marine Heterotrophic Nanoflagellate. J. Plankton Res. 2025, 47, fbaf022. [Google Scholar] [CrossRef]
  34. Bell, W.; Mitchell, R. Chemotactic and Growth Responses of Marine Bacteria to Algal Extracellular Products. Biol. Bull. 1972, 143, 265–277. [Google Scholar] [CrossRef]
  35. Ohno, H.; Endo, Y.; Sato-Okoshi, W.; Nishitani, G. Feeding and Growth Characteristics of a Diatom-Feeding Flagellate Isolated from the Bottom Sediment of Onagawa Bay, Northeastern Japan. Open J. Mar. Sci. 2013, 3, 9–14. [Google Scholar] [CrossRef]
  36. Amin, S.A.; Hmelo, L.; Van Tol, H.; Durham, B.; Carlson, L.; Heal, K.; Morales, R.; Berthiaume, C.; Parker, M.; Djunaedi, B. Interaction and Signalling between a Cosmopolitan Phytoplankton and Associated Bacteria. Nature 2015, 522, 98–101. [Google Scholar] [CrossRef]
  37. Foster, R.A.; Zehr, J.P. Characterization of Diatom–Cyanobacteria Symbioses on the Basis of nifH, hetR and 16S rRNA Sequences. Environ. Microbiol. 2006, 8, 1913–1925. [Google Scholar] [CrossRef] [PubMed]
  38. Sison-Mangus, M.P.; Kempnich, M.W.; Appiano, M.; Mehic, S.; Yazzie, T. Specific Bacterial Microbiome Enhances the Sexual Reproduction and Auxospore Production of the Marine Diatom, Odontella. PLoS ONE 2022, 17, e0276305. [Google Scholar] [CrossRef]
  39. Deng, Y.; Mauri, M.; Vallet, M.; Staudinger, M.; Allen, R.J.; Pohnert, G. Dynamic Diatom-Bacteria Consortia in Synthetic Plankton Communities. Appl. Environ. Microbiol. 2022, 88, e01619-22. [Google Scholar] [CrossRef] [PubMed]
  40. Worden, A.Z.; Seidel, M.; Smriga, S.; Wick, A.; Malfatti, F.; Bartlett, D.; Azam, F. Trophic Regulation of Vibrio cholerae in Coastal Marine Waters. Environ. Microbiol. 2006, 8, 21–29. [Google Scholar] [CrossRef]
  41. Riemann, L.; Steward, G.F.; Azam, F. Dynamics of Bacterial Community Composition and Activity during a Mesocosm Diatom Bloom. Appl. Environ. Microbiol. 2000, 66, 578–587. [Google Scholar] [CrossRef]
  42. Cirri, E.; Pohnert, G. Algae−Bacteria Interactions That Balance the Planktonic Microbiome. New Phytol. 2019, 223, 100–106. [Google Scholar] [CrossRef] [PubMed]
  43. Di Costanzo, F.; Di Dato, V.; Romano, G. Diatom–Bacteria Interactions in the Marine Environment: Complexity, Heterogeneity, and Potential for Biotechnological Applications. Microorganisms 2023, 11, 2967. [Google Scholar] [CrossRef]
  44. Behrenfeld, M.J.; Halsey, K.H.; Boss, E.; Karp-Boss, L.; Milligan, A.J.; Peers, G. Thoughts on the Evolution and Ecological Niche of Diatoms. Ecol. Monogr. 2021, 91, e01457. [Google Scholar] [CrossRef]
  45. Schnepf, E.; Schweikert, M. Pirsonia, Phagotrophic Nanoflagellates Incertae Sedis, Feeding on Marine Diatoms: Attachment, Fine Structure and Taxonomy. Arch. Protistenkd. 1997, 147, 361–371. [Google Scholar] [CrossRef]
  46. Geesink, P.; ter Horst, J.; Ettema, T.J.G. More than the Sum of Its Parts: Uncovering Emerging Effects of Microbial Interactions in Complex Communities. FEMS Microbiol. Ecol. 2024, 100, fiae029. [Google Scholar] [CrossRef] [PubMed]
  47. Smayda, T.J.; Trainer, V.L. Dinoflagellate Blooms in Upwelling Systems: Seeding, Variability, and Contrasts with Diatom Bloom Behaviour. Prog. Oceanogr. 2010, 85, 92–107. [Google Scholar] [CrossRef]
Phycology 06 00020 g001
Figure 1. (a) A HNAN cell at 60× (n = 1) from the original inoculation culture, (b) Odontella sp., (c) Cosinodiscus sp., and (d) Rhizosolenia sp. cultures on day 6 with an attached (n = 1) HNAN. The scale bars indicate 10 µm.
Figure 1. (a) A HNAN cell at 60× (n = 1) from the original inoculation culture, (b) Odontella sp., (c) Cosinodiscus sp., and (d) Rhizosolenia sp. cultures on day 6 with an attached (n = 1) HNAN. The scale bars indicate 10 µm.
Phycology 06 00020 g001
Phycology 06 00020 g002
Figure 2. Growth rates (µ day−1, mean ± SD) of three diatoms Coscinodiscus sp. (green), Odontella sp. (yellow), and Rhizosolenia sp. (black) in (a) control incubations and (b) incubations with the HNAN. The hash symbols (#) indicate significant changes over time within the HNAN treatment for a given taxon. Asterisks (*) denote significantly lower growth of Coscinodiscus relative to the other two taxa within the HNAN treatment during the same time interval (p < 0.05). No significant differences were detected among control treatments.
Figure 2. Growth rates (µ day−1, mean ± SD) of three diatoms Coscinodiscus sp. (green), Odontella sp. (yellow), and Rhizosolenia sp. (black) in (a) control incubations and (b) incubations with the HNAN. The hash symbols (#) indicate significant changes over time within the HNAN treatment for a given taxon. Asterisks (*) denote significantly lower growth of Coscinodiscus relative to the other two taxa within the HNAN treatment during the same time interval (p < 0.05). No significant differences were detected among control treatments.
Phycology 06 00020 g002
Phycology 06 00020 g003
Figure 3. Diatom surface area µm2 (mean ± SD) of the three diatoms: Coscinodiscus sp. (green), Odontella sp. (yellow), and Rhizosolenia sp. (black) (n = 10).
Figure 3. Diatom surface area µm2 (mean ± SD) of the three diatoms: Coscinodiscus sp. (green), Odontella sp. (yellow), and Rhizosolenia sp. (black) (n = 10).
Phycology 06 00020 g003
Phycology 06 00020 g004
Figure 4. Average number of diatoms per ml and HNAN attached per 1000 µm2 surface area (black line) from the beginning (day 0) to the end of the incubations (day 21) for (a) Coscinodiscus sp. (green), (b) Odontella sp. (yellow), and (c) Rhizosolenia sp. (black). The error bars indicate the standard deviation of the HNAN attachment rates. Note differences in scale of primary y-axes.
Figure 4. Average number of diatoms per ml and HNAN attached per 1000 µm2 surface area (black line) from the beginning (day 0) to the end of the incubations (day 21) for (a) Coscinodiscus sp. (green), (b) Odontella sp. (yellow), and (c) Rhizosolenia sp. (black). The error bars indicate the standard deviation of the HNAN attachment rates. Note differences in scale of primary y-axes.
Phycology 06 00020 g004
Table 1. Average HNAN attachment per diatom cell over time for Coscinodiscus sp., Odontella sp., and Rhizosolenia sp. (n = 3 for each observation) with ranges.
Table 1. Average HNAN attachment per diatom cell over time for Coscinodiscus sp., Odontella sp., and Rhizosolenia sp. (n = 3 for each observation) with ranges.
DiatomDayHNANPer Cell–1
AverageRange
Coscinodiscus00.10.0–0.2
31.00.7–1.2
62.90.0–4.8
97.51.0–12.2
1211.42.0–17.1
159.92.0–17.1
1810.82.0–22.9
217.32.0–18.8
Odontella00.00.0
31.01.0
61.01.0
91.01.0
122.01.0–3.0
152.32.0–3.0
183.73.0–5.0
212.32.0–3.0
Rhizosolenia00.00.0
31.01.0
61.01.0
91.31.0–2.0
121.31.0–2.0
151.31.0–2.0
182.12.0–2.3
212.22.0–2.5
Table 2. Summary table on previously published attachment observations for nanoflagellates (HNANs) and this study. Note that there are differences in substrate nature (diatoms, bacterial mats, and marine snow), flagellate species, and observation approaches. For each study, the dominant flagellates, feeding mode, substrate type, attachment densities (HNAN Attach. in cell/colony/chain−1 or µm−2), and, when available, context on how often or for how long observations lasted are listed.
Table 2. Summary table on previously published attachment observations for nanoflagellates (HNANs) and this study. Note that there are differences in substrate nature (diatoms, bacterial mats, and marine snow), flagellate species, and observation approaches. For each study, the dominant flagellates, feeding mode, substrate type, attachment densities (HNAN Attach. in cell/colony/chain−1 or µm−2), and, when available, context on how often or for how long observations lasted are listed.
FlagellateFeeding ModeSubstrate TypeHNAN Attach. (Per Cell/Colony/Chain)HNAN Attach. (Per Unit Surface Area)DurationStudy
Bicosoecida sp.InterceptionCoscinodiscus6.4 ± 4.2 per cell−10.06 ± 0.0421-day batch
cultures		This Study
Odontella1.7 ± 0.5 per cell−10.13 ± 0.04
Rhizosolenia1.3 ± 0.3 per cell−10.19 ± 0.04
Pirsonia sp.ParasiticCoscinodiscus1+ per infected cell−1–3–10 min batch cultures[27]
Unknow HNANParasiticGuinardia1–8 per infected cell–1–In situ of 3 h
observations collected over 1 month		[22]
Salpingoeca sp.FilterAsterionella0–21 cells colony–1–4-day incubations, batch cultures[28]
Fragilaria0–35 cells colony–1
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Corradino, G.; Schnetzer, A. Getting Attached: A Heterotrophic Nanoflagellate Mingling with Centric Diatoms. Phycology 2026, 6, 20. https://doi.org/10.3390/phycology6010020

AMA Style

Corradino G, Schnetzer A. Getting Attached: A Heterotrophic Nanoflagellate Mingling with Centric Diatoms. Phycology. 2026; 6(1):20. https://doi.org/10.3390/phycology6010020

Chicago/Turabian Style

Corradino, Gabrielle, and Astrid Schnetzer. 2026. "Getting Attached: A Heterotrophic Nanoflagellate Mingling with Centric Diatoms" Phycology 6, no. 1: 20. https://doi.org/10.3390/phycology6010020

APA Style

Corradino, G., & Schnetzer, A. (2026). Getting Attached: A Heterotrophic Nanoflagellate Mingling with Centric Diatoms. Phycology, 6(1), 20. https://doi.org/10.3390/phycology6010020

Article Metrics

Back to TopTop