REVIEW 3 major objections 6 minor 26 references
3D Holographic Flow Cytometry Measurements of Microalgae: Strategies for Angle Recovery in Complex Rotation Patterns
T0 review · 3 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read The paper claims that one holographic flow cytometer, without engineered rotation control, can reconstruct 3D refractive-index tomograms for transparent microalgae and 3D silhouette shapes for absorbing or scattering ones by tailoring the…
desk verdict A credible proof-of-concept for angle recovery in holographic flow cytometry of non-spherical algae, but the quantitative 3D claim outruns the validation, especially for tumbling motion. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is a rotation-pattern classifier plus a projection-angle estimator. Samples are assigned to Rotation pattern 1, where the alga rotates around its major semiaxis with a quasi-uniform rolling motion, or Rotation pattern 2, where it rotates around a minor semiaxis and alternates sliding with rapid 180-degree tumbles. From each hologram, a binary map is obtained from the quantitative phase map or the amplitude map, and spurious rotations around the y and z axes are compensated by measuring the tilt angle alpha and the apparent kayaking angle lambda. For pattern 1, a spheroid matching the measured axes is numerically projected, and the oscillation of the projected minor axis D is fitted by a sinusoid whose doubled frequency yields the angular step. For pattern 2, the area of the binary maps locates the rotation frames: minima mark 0, 180, and 360 degrees, and the midpoint between consecutive minima gives the 90 and 270 degree quarters, after which the same sinusoid fit is applied per quarter. The recovered angles feed either tomographic refractive-index reconstruction or shape-from-silhouette.
What would settle it
Flow a non-spherical particle with an independently known 3D shape, such as a manufactured bead or a microalga whose shape is measured by confocal or electron microscopy, through the same channel, reconstruct it with this workflow, and check whether the recovered shape, internal refractive-index map, and biovolume match the known ground truth within the reported tolerance; a mismatch concentrated in the tumbling phase of pattern 2 would indicate that the neglected out-of-plane correction is not negligible.
Extended reading notes
Core claim
The central discovery is a workflow that retrieves the projection angles of non-spherical microorganisms rotating in a microfluidic channel, making 3D reconstruction possible across rotation patterns. The paper reports that quasi-transparent and low-scattering diatoms, including Cocconeis sp., Skeletonema pseudocostatum, Skeletonema marinoi, and Navicula sp., can be reconstructed as quantitative 3D refractive-index tomograms, while absorbing or strongly scattering species such as Scrippsiella acuminata, Heterocapsa sp., Prorocentrum sp., and Thalassiosira eccentrica can still be reconstructed as 3D silhouettes. The authors state that this is the first demonstration that 3D quantitative measurements can be obtained for a wide variety of microalgae exhibiting different shapes and uncontrolled rotation patterns, and that a single holographic system can retrieve 3D data independently of shape, size, and intrinsic optical properties.
Load-bearing premise
The pipeline assumes that each alga's motion falls into one of two rotation patterns and that within each rotation quarter the rolling rate can be recovered from the projection of a fitted spheroid, while the out-of-plane tilt during fast tumbling is small enough to be ignored.
Editorial extensions
If this is right
- For weakly scattering diatoms, the workflow yields full 3D refractive-index tomograms showing internal structures such as chloroplasts and nuclei, not just the outer envelope.
- For strongly absorbing or scattering microalgae, the same angle estimates support 3D shape-from-silhouette reconstructions, with volumes and equivalent diameters measured directly in 3D.
- The angle retrieval is automatic once a rotation frame is identified, so the method can scale to the repeatable, high-throughput settings needed for monitoring programs.
- Direct 3D biovolume measurements differ from estimates derived from 2D instruments like the Imaging FlowCytobot, suggesting that 2D-based biovolume can be replaced by a shape-faithful measurement.
- Because the method does not rely on phase values, it applies even when phase estimation is unreliable due to scattering or absorption.
Reading between the lines
- If the workflow generalizes, the same angle-recovery logic could be applied to other non-spherical particles in flow, such as blood-cell aggregates or sediment particles, wherever rotation is uncontrolled but laminar.
- The uncompensated out-of-plane tilt during the tumbling phase sets a limit on tomogram fidelity for pattern 2, and a dual-view or multi-angle holographic setup could remove that bias as a natural test of how much the neglected correction matters.
- A quantitative comparison against ground-truth shapes, for example from confocal or electron microscopy, for a set of species would turn the demonstrated feasibility into an error-bounded measurement protocol.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents a workflow for recovering the 3D structure of non-spherical microalgae imaged with digital holographic microscopy in flow-cytometry mode. Depending on the object's rotation pattern, the authors retrieve per-frame rolling angles either by fitting a sinusoid to the minor-axis length of binary projections (Rotation pattern 1) or by combining area-based identification of rotation quarters with the same sinusoidal angle estimation within each quarter (Rotation pattern 2). From the recovered angles, they compute 3D refractive-index tomograms for weakly scattering diatoms and 3D shape-from-silhouette reconstructions for absorbing or strongly scattering dinoflagellates and diatoms. The paper reports tomograms and morphometric parameters (volume, eccentricity, equivalent diameter) for eight microalgal samples and compares these with 2D estimates from a commercial Imaging FlowCytobot.
Significance. If the angle-recovery pipeline is quantitatively accurate, the work would constitute a valuable extension of holographic flow cytometry to non-spherical, uncontrolled-rotation objects, which is a recognized bottleneck in label-free 3D imaging of phytoplankton. The paper's main strengths are its automatic pipeline (no manual angle annotation), its dual use of phase and amplitude information (enabling both FHT and SFS from the same instrument), and the breadth of demonstrated morphologies. The authors also provide extensive supplementary material with culture, optics, and data-processing details. However, the central 'quantitative 3D' claim currently rests on an angle-recovery step that is validated only by internal consistency (the tomograms look similar to expected shapes) rather than by an independent ground-truth orientation or a quantitative error bound. Because the same measured projections are used both to estimate the angles and to produce the reconstruction, the risk of bias is real and needs to be addressed before the quantitative claims can be accepted.
major comments (3)
- [Sec. 2.2, Fig. 4] The angular step in Rotation pattern 1 is derived by fitting a sinusoid to the minor-axis length D measured from the very same binary maps that are later used in the tomographic reconstruction. This is not an independent measurement: any out-of-plane tilt or morphological asymmetry that distorts the D curve directly biases the estimated angular step and, consequently, the entire angular sequence. The paper reports a y-axis tilt up to α=18.7° (Sec. 2.2, Fig. 3d) and states that this oscillation 'is not possible to compensate.' A tilt of this magnitude modulates the projected length of the major axis by about 5.3%, which will contaminate the D(t) signal used for the sinusoidal fit. The authors should provide a quantitative sensitivity analysis (e.g., a simulation of a spheroid with superimposed y-tilt showing the resulting error in the recovered angular step) or an experimental calibration against a known orientation (e.g., a bead with an embedded marker). Without such a bound, the 'quantitative' descriptor for the Rotation-pattern-1 tomograms is not fully justified.
- [Sec. 2.3, Sec. 3.2, Fig. S3] For Rotation pattern 2, the authors explicitly state that the tumbling-phase out-of-plane correction 'should be made in the xz plane... not possible in our experimental setup' and that this correction 'can be neglected' because the tumbling phase is 'generally fast.' This is a load-bearing approximation for the central claim that quantitative 3D data can be obtained for uncontrolled rotation patterns. The approximation is not bounded: Sec. 3.2 acknowledges that Cocconeis sp. motion is 'often turbulent and non-stable among consecutive 360° rotations,' and the validation shown in Fig. S3 compares the proposed method only against the spherical-assumption method of Ref. [15], which is already known to be inappropriate for non-spherical samples. The correct comparison would be against an independent orientation measurement or a phantom with a known rotation trajectory. The authors should either provide such a benchmark or clearly downgrade the claim from 'quantitative 3D measurements' to 'qualitative 3D reconstructions' for the Rotation-pattern-2 class.
- [Conclusions] The conclusions assert that 'we are the first to show that 3D quantitative measurements can be obtained for a wide variety of microalgae presenting different shapes and exhibiting various uncontrolled rotation patterns' and that 'a single holographic system can retrieve 3D data independently of the shape, size, and intrinsic optical properties of the cells.' These claims are stronger than the evidence supports. The study demonstrates a proof of concept on a small number of cells (one or a few per species), with no statistics on reproducibility across multiple individuals of the same species and no error bars on the reported volumes and morphometric parameters in Table 1. In addition, because the out-of-plane tilt is left uncompensated (Secs. 2.2 and 2.3), the word 'quantitative' is not yet earned. I recommend tempering the novelty claim to 'a workflow that can retrieve 3D reconstructions for a diverse set of microalgae' and adding a quantitative validation or a clear statement of the expected error range.
minor comments (6)
- [Fig. 5 caption] The caption of Figure 5 reads 'proposed angle retrieval method for non-spherical samples for Rotation pattern 1,' but the figure and its description in Sec. 2.3 concern Rotation pattern 2. The caption should be corrected to 'Rotation pattern 2'.
- [Sec. 2.2] The relative variation (a_max − a_min)/a_max is reported as '0.053 μm.' The quantity is dimensionless (0.053, i.e., 5.3%), not 0.053 μm. Please correct the units.
- [Sec. 2.3] The word 'thumbling' appears twice ('thumbling phase', 'thumbling-sliding'); it should be 'tumbling'.
- [Sec. 3.1] The phrase 'the tomograms have been calculated through the correct angle sequence estimation that compensate the compensation of the apparent kayaking' is grammatically unclear. It should read something like '...that includes the compensation of the apparent kayaking.'
- [References [18–20]] The rotation of non-spherical samples in Sec. 2.1 is attributed to references [18–20], but reference [18] is R. A. Fisher's 1922 statistics paper and references [19] and [20] are about optical imaging, not spheroid rotation in flow. The authors should cite the relevant literature on particle rotation in shear flow (e.g., G. B. Jeffery, Proc. R. Soc. Lond. A 102, 161–179, 1922, or a modern review) instead. This is a significant citation error that should be fixed.
- [Table 1] Table 1 lists morphometric parameters without indicating the number of cells measured per species or any measure of variability. Given the paper's 'quantitative' claim, at least the number of replicates and, where possible, a standard deviation should be provided.
Circularity Check
No significant circularity; angle retrieval is self-consistent estimation from the data, and the 3D claims rest on in-paper reconstruction, not on a by-construction equivalence or load-bearing self-citation.
full rationale
The central derivation is not circular. In Section 2.2, the angular step is estimated by fitting a sinusoid to the minor-axis length D measured from the binary maps, with the spheroid used only as a forward model to relate D to rotation angle; in Section 2.3, rotation quarters are identified from area extrema and then the same sinusoid fit is applied per quarter. The resulting angle sequence is then used as input to tomographic inversion or shape-from-silhouette. The reconstructed 3D volume is not equal to the spheroid model by construction: internal RI features such as chloroplasts and nuclei, and the distortions visible when angles are wrong (Fig. S3), emerge from the measured projections rather than from the fitted parameters. The absence of an independent orientation ground truth is a real validation/robustness limitation—explicitly acknowledged in Section 2.3 ('not possible in our experimental setup') and Section 3.2 ('motion is often turbulent and non-stable')—but it is a correctness risk, not a circular reduction. Self-citations, including the prior spherical angle-retrieval method [15] used as a baseline and the tomographic-flow-cytometry context [14], are not load-bearing for the new non-spherical angle-recovery strategy. No equation or fitted parameter is renamed as an independent prediction. Accordingly, no specific circular step meets the required evidentiary threshold.
Assumptions & free parameters
free parameters (4)
- Rotation frequency f_hat from sinusoidal fit =
Not reported numerically; taken as the spectral peak of the fitted sinusoid
- Sliding-phase area threshold =
90% of the maximum BM area
- Spheroid semiaxes a, b, c =
Measured per specimen from binarized maps
- Isolevel thresholds for outer-shell extraction =
10%, 20%, 50% of maximum RI
assumptions (5)
- domain assumption Microalgal motion in the microfluidic channel can be classified into Rotation pattern 1 (rotation about the major semiaxis) or Rotation pattern 2 (minor-axis rotation with tumbling-sliding alternance).
- domain assumption A spheroid with the measured semiaxes is an adequate generative model for the projection sequence.
- domain assumption The spurious out-of-plane y-rotation cannot be compensated but is small enough that the resulting 3D approximation is acceptable; the apparent kayaking can be corrected in the BM plane.
- domain assumption During Rotation pattern 2 tumbling, correction in the xz plane can be neglected because the tumbling phase is short.
- domain assumption Binarized amplitude or phase maps faithfully represent the outer silhouette of the cell for SFS, and the HOTV reconstruction used in [23] is correct.
Cite this review
Pith. "Pith review of 3D Holographic Flow Cytometry Measurements of Microalgae: Strategies for Angle Recovery in Complex Rotation Patterns." pith.science (2026). https://pith.science/paper/GU6YX36R
@misc{pith2026250603738,
author = {Pith},
title = {Pith review of: 3D Holographic Flow Cytometry Measurements of Microalgae: Strategies for Angle Recovery in Complex Rotation Patterns},
year = {2026},
howpublished = {\url{https://pith.science/paper/GU6YX36R}},
note = {Machine review of arXiv:2506.03738}
}
read the original abstract
Marine ecosystems are in the spotlight, because environmental changes are threatening biodiversity and ecological functions. In this context, microalgae play key ecological roles both in planktonic and benthic ecosystems. Consequently, they are considered indispensable targets for global monitoring programs. However, due to a high spatial and temporal variability and to difficulties of species identification (still relying on microscopy observations), the assessment of roles played by these components of marine ecosystems is demanding. In addition, technologies for a 3D assessment of their complex morphology are scarcely available. Here, we present a comprehensive workflow for retrieving 3D information on microalgae with diverse geometries through holographic microscopy operating in flow-cytometry mode. Depending on the rotation patterns of samples, a tailored approach is used to retrieve their rolling angles. We demonstrate the feasibility of measuring 3D data of various microalgae, contingent to the intrinsic optical properties of cells. Specifically, we show that for quasi-transparent and low-scattering microorganisms, the retrieved angles permit to achieve quantitative 3D tomographic Refractive Index (RI) mapping, providing a full characterization of the alga in terms of its inner structure and the outer shape. Moreover, even in the most challenging scenarios, where microalgae exhibit high light absorption or strong scattering, quantitative 3D shape reconstructions of diatoms and dinoflagellates can be at least achieved. Finally, we compare our direct 3D measurements with 2D inferences of 3D properties, obtained using a commercially available microscopy system. The ability to non-invasively obtain 3D information on microalgae marks a fundamental advancement in the field, unlocking a wealth of novel biological insights for characterizing aquatic ecosystems.
Reference graph
Works this paper leans on
-
[18]
On the mathematical foundations of theoretical statistics,
R. A. Fisher, “On the mathematical foundations of theoretical statistics,” Proc. R. Soc. Lond. A 222(1149), 309–368 (1922), https://doi.org/10.1098/rspa.1922.0078
arXiv 1922
-
[23]
Quantification of circulating tumor cells using AI and label-free imaging,
M. P. Matos, J. R. dos Santos, et al., “Quantification of circulating tumor cells using AI and label-free imaging,” Cells 11(16), 2591 (2022), https://doi.org/10.3390/cells11162591
-
[15]
3D morphometry of red blood cells by digital holography,
P. Memmolo, F. Merola, L. Miccio, et al., “3D morphometry of red blood cells by digital holography,” Appl. Opt. 59(4), A65–A73 (2020), https://doi.org/10.1364/AO.404376
-
[1]
Coastal ecosystem-based management,
E. B. Barbier, “Coastal ecosystem-based management,” Ecological Applications 15(6), 1869– 1873 (2005), https://doi.org/10.1890/04-0922
doi:10.1890/04-0922 2005
-
[2]
The ecological role of water -column microbes in the sea,
F. Azam, T. Fenchel, J. Field, J. Gray, L. Meyer -Reil, and F. Thingstad, “The ecological role of water -column microbes in the sea,” Mar. Ecol. Prog. Ser. 10, 257 –263 (1983), https://doi.org/10.3354/meps010257
-
[3]
European Union, “Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy,” Off. J. Eur. Communities L 327, 1–72 (2000)
work page 2000
-
[4]
S. D. Batten, W. W. Gregg, R. G. Henson, et al., “Global plankton monitoring,” Global Ecology and Biogeography 28, 727–745 (2019)
work page 2019
-
[5]
Phytoplankton phenology in the Northeast Atlantic,
J. F. Tweddle, M. D. Tilstone, T. Smyth, et al., “Phytoplankton phenology in the Northeast Atlantic,” Remote Sens. Environ. 215, 1–14 (2018); E. R. Eriksen, J. G. Paasche, L. Strand, et al., “Monitoring marine microplastics,” Sci. Total Environ. 675, 1–12 (2019)
work page 2018
Show all 26 references
-
[6]
Ecological status of transitional waters based on phytoplankton,
L. Roselli, G. Caroppo, D. Stanca, et al., “Ecological status of transitional waters based on phytoplankton,” Estuarine Coastal and Shelf Science 265, 107581 (2022). (EI Indexed)
2022
-
[7]
Zur Vervollkommnung der quantitativen Methodik,
H. Utermöhl, “Zur Vervollkommnung der quantitativen Methodik,” SIL Commun. 9(1), 1–38 (1958), https://doi.org/10.1080/05384680.1958.11904091
1958
-
[8]
Deep learning- based classification of microalgae using light and scanning electron microscopy images,
M. E. Sönmez, B. Altınsoy, B. Y. Öztürk, N. E. Gümüş, and N. Eczacıoğlu , “Deep learning- based classification of microalgae using light and scanning electron microscopy images,” Micron 172, 103506 (2023). https://doi.org/10.1016/j.micron.2023.103506
2023
-
[9]
Identification of eukaryotic microalgal strains,
M. W. Fawley and K. P. Fawley, “Identification of eukaryotic microalgal strains,” J. Appl. Phycol. 33, 971–984 (2021). https://doi.org/10.1007/s10811-020-02190-5
2021 doi
-
[10]
Taxonomy in a changing world: Seeking solutions for a science in crisis,
I. Agnarsson and M. Kuntner, "Taxonomy in a changing world: Seeking solutions for a science in crisis," Syst. Biol. 56(3), 531–539 (2007), https://doi.org/10.1080/10635150701424546
2007 doi
-
[11]
Global distribution patterns of phytoplankton chlorophyll a: functional types and biogeography,
A. Pierella Karlusich, et al., “Global distribution patterns of phytoplankton chlorophyll a: functional types and biogeography,” Global Biogeochem. Cycles 36, e2021GB007235 (2022). 21
2022
-
[12]
Length, width, shape regularity, and chain structure: time series analysis of phytoplankton morphology from imagery,
V. Sonnet, L. Guidi, C. B. Mouw, G. Puggioni, and S. D. Ayata, “Length, width, shape regularity, and chain structure: time series analysis of phytoplankton morphology from imagery,” Limnol. Oceanogr. 67, 1850–1864 (2022). https://doi.org/10.1002/lno.12147
2022 doi
-
[13]
Holo-tomographic flow cytometry: a new paradigm in diagnostics by high-throughput and stain-free single-cell imaging,
L. Miccio, G. Coppola, P. Memmolo, et al., “Holo-tomographic flow cytometry: a new paradigm in diagnostics by high-throughput and stain-free single-cell imaging,” Proc. SPIE 12464, Optical Biopsy XXI, 124640I (2023), https://doi.org/10.1117/12.2675702
2023 doi
-
[14]
Tomographic flow cytometry by digital holography,
F. Merola, L. Miccio, P. Memmolo, et al., “Tomographic flow cytometry by digital holography,” Light Sci. Appl. 6(4), e16241 (2017), https://doi.org/10.1038/lsa.2016.241
2017 doi
-
[16]
Distinctive image features from scale-invariant keypoints,
D. G. Lowe, “Distinctive image features from scale-invariant keypoints,” Comput. Vis. Image Underst. 110(3), 346–359 (2008), https://doi.org/10.1016/j.cviu.2008.02.006
2008 doi
-
[17]
S. J. D. Prince, Computer Vision: Models, Learning, and Inference, Cambridge University Press (2012)
2012
-
[19]
Optical imaging of cell mass and growth dynamics,
G. Popescu, Y. Park, N. Lue, et al., “Optical imaging of cell mass and growth dynamics,” Micromachines 12(3), 277 (2021), https://doi.org/10.3390/mi12030277
2021 doi
-
[20]
Label -free optical imaging of single -cell morphology and dynamics,
J. J. Ferraro, C. R. Moncayo, A. J. Berger, “Label -free optical imaging of single -cell morphology and dynamics,” Rev. Sci. Instrum. 93, 083702 (2022), https://doi.org/10.1063/5.0100963
2022 doi
-
[21]
High-throughput single-cell analysis using microfluidics and optical imaging,
Y. C. Eldridge, A. J. Walsh, et al., “High-throughput single-cell analysis using microfluidics and optical imaging,” Lab Chip 13, 4463–4473 (2013), https://doi.org/10.1039/C3LC50515D
2013 doi
-
[22]
Deep learning -based label-free cell classification using refractive index tomograms,
Y. Kim, S. Shim, J. S. Park, et al., “Deep learning -based label-free cell classification using refractive index tomograms,” Light Sci. Appl. 10, 153 (2021), https://doi.org/10.1038/s41377- 021-00626-2
2021 doi
-
[25]
Label -free imaging and analysis of subcellular parts of a liv ing diatom Cylindrotheca sp. using optical diffraction tomography,
K. Umemura, Y. Matsukawa, Y. Ide, and S. Mayama, "Label -free imaging and analysis of subcellular parts of a liv ing diatom Cylindrotheca sp. using optical diffraction tomography," MethodsX 7, 100889 (2020). https://doi.org/10.1016/j.mex.2020.100889
2020
-
[26]
ifcb -analysis,
H. Sosik, “ifcb -analysis,” GitHub repository, 2022. [Online]. Available: https://github.com/hsosik/ifcb-analysis
2022
-
[27]
Distance maps to estimate cell volume from two-dimensional plankton images,
E. A. Moberg and H. M. Sosik, “Distance maps to estimate cell volume from two-dimensional plankton images,” Limnol. Oceanogr. Methods 10, 278 –288 (2012). https://doi.org/10.4319/lom.2012.10.278 23 Supplementary material IFCB characterization of microalgae IFCB was used to cha...
2012 doi
Reviewed August 7, 2026 · model on record in the stance chip above.
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