Journal of biological and health sciences http://biotecnia.unison.mx

Universidad de Sonora

ISSN: 1665-1456

Original Article

Physiological and photosynthetic performance

of native microalgae strains isolated from Baja California

Desempeño fisiológico y fotosintético de cepas nativas de microalgas aisladas de Baja California

Ceres A. Molina-Cárdenas1 and M. del Pilar Sánchez-Saavedra1*

1 Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Carretera Tijuana-Ensenada 3918, Zona Playitas, 22860. Ensenada, Baja California, México.

ABSTRACT

To assess the physiological status and photosynthetic activi- ty of sixteen microalgae strains isolated from Baja California, growth estimations and in vivo chlorophyll a (Chl-a) fluores- cence measurements were performed. The bacillariophyte Diploneis sp. exhibited the highest growth rate, while the hig- hest cell densities were observed in Tetraselmis suecica and Navicula sp. strain 2. Most strains showed effective maximum quantum yield (Fv/Fm) values above 0.50. The highest values of maximum electron transport rate (ETRm) and saturation irradiance (Ik) were recorded for Amphora sp. strain 6 and Heterococcus sp. The diatom Navicula sp. strain 4 showed the highest content of chlorophyll a and carotenoids. The culture conditions used in this study were not stressful for the mi- croalgae strains. Notably, T. suecica showed high maximum cell density and Fv/Fm values; Amphora sp. strain 6 exhibited the highest electron transport rate (ETRm) and elevated sa- turation irradiance (Ik). This work highlights the interspecific variability in physiological and photosynthetic traits among native strains, which can be promising candidates for aqua- culture and biotechnology applications requiring robust photosynthetic performance.

Keywords: Microalgae; growth rate; chlorophyll a fluores- cence; photosynthesis performance; electron transport rate.

RESUMEN

Para evaluar el estado fisiológico y la actividad fotosintética de dieciséis cepas de microalgas aisladas de Baja California, se realizaron estimaciones de crecimiento y mediciones de fluorescencia in vivo de clorofila a (Chl-a). La bacilariofita Diploneis sp. presentó la mayor tasa de crecimiento, mientras que las mayores densidades celulares se observaron en Tetraselmis suecica y Navicula sp. cepa 2. La mayoría de las cepas mostró valores efectivos de rendimiento cuántico máximo (Fv/Fm) superiores a 0.50. Los valores más altos de tasa máxima de transporte de electrones (ETRm) e irradiancia de saturación (Ik) se registraron en Amphora sp. cepa 6 y en Heterococcus sp. La diatomea Navicula sp. cepa 4 presentó el mayor contenido de clorofila a y carotenoides. Las condi- ciones de cultivo utilizadas en este estudio no fueron estre- santes para las cepas de microalgas. En particular, T. suecica mostró alta densidad celular máxima y valores elevados de Fv/Fm; Amphora sp. cepa 6 presentó la mayor tasa de trans-

*Author for correspondence: M. del Pilar Sánchez-Saavedra Correo-e: psanchez@cicese.mx

Received: May 21, 2025

Accepted: August 21, 2025

Published: September 18, 2665

porte de electrones (ETRm) e irradiancia de saturación (Ik) elevada. Este trabajo resalta la variabilidad interespecífica en los rasgos fisiológicos y fotosintéticos entre cepas nativas, las cuales pueden ser candidatas promisorias para aplicaciones en acuicultura y procesos biotecnológicos que requieren un desempeño fotosintético robusto.

Palabras clave: microalgas; tasa de crecimiento; fluorescen- cia de clorofila a; desempeño fotosintético.

INTRODUCTION

Marine phytoplankton are diverse communities of microsco- pic photosynthetic organisms that play a fundamental role in global biogeochemical cycles. They account for approxi- mately 50 % of the Earth’s primary productivity (Crockford et al., 2023) and include cells ranging from 0.2 to 2000 µm in size (Haëntjens et al., 2022). The major taxonomic groups contributing to marine productivity include bacillariophytes, dinoflagellates, and coccolithophores. Additional contribu- tors to marine diversity and productivity are green algae, cyanobacteria, haptophytes, cryptophytes, and eugleno- phytes (Simon et al., 2009; Calbet, 2024). Despite the high diversity of phytoplankton, only a fraction of 30,000 existing species have been formally described to date (Thoré et al., 2023). Thus, continued efforts in isolating, describing, and characterizing marine microalgae species are essential to better understand ocean productivity and to explore their potential applications in aquaculture, biotechnology, and the food industry.

One approach to characterizing microalgae involves assessing their photosynthetic performance through chlo- rophyll fluorescence measurements. These measurements provide insights into the physiological status of phytoplank- ton cells (Juneau and Harrison, 2005) and are commonly ob- tained using Pulse Amplitude Modulation (PAM) fluorometry (Figueredo et al., 2009). These devices focus on Photosystem II (PSII), and key parameters—such as the maximum quan- tum yield (Fv/Fm), electron transport rate (ETR), saturation irradiance (Ik), and photosynthetic efficiency (α)—which are widely used to evaluate cellular responses under varying en- vironmental conditions (White et al., 2011; Sánchez-Saavedra et al., 2018; Vani et al., 2023; Krivina et al., 2023).

1

A previous study by Jiménez-Valera and Sánchez-Saave- dra (2016) characterized the growth and fatty acid profiles of

Volume XXVII

DOI: 10.18633/biotecnia.v27.2665

21 microalgae strains isolated from the northeastern coastal waters of Baja California, Mexico. That study highlighted the biotechnological potential of these strains, particularly for aquaculture, due to favorable attributes such as small cell size, rapid growth, absence of toxicity, and the presence of polyunsaturated fatty acids (PUFAs), which are essential nutrients for fish, crustaceans, and mollusk larvae. However, no information is yet available on the photosynthetic perfor- mance of these strains.

The objective of this work was to evaluate the pho- tosynthetic response of 16 microalgae strains isolated from Baja California, Mexico, to identify suitable light levels for their cultivation and optimization. These results provide a basis for future studies on the potential applications of these strains in aquaculture and biotechnology.

MATERIAL AND METHODS

Microalgae strain characteristics

We used 16 microalgae strains previously isolated by Jimé- nez-Valera and Sánchez-Saavedra (2016) from coastal waters of Ensenada and San Quintín in Baja California, and Mulegé in Baja California Sur, Mexico. The strains included the chloro- phyte Tetraselmis suecica, the xanthophyte Heterococcus sp., and 14 bacillariophytes: Amphora sp. (strains 1, 2, 4, 5, 6, and

7), Navicula sp. (strains 2, 3, and 4), Cymbella sp. (strains 1 and 2), Nitzschia thermalis, Diploneis sp., and Rhabdonema sp.

Generation time was calculated according to the fo- llowing equation:

TG = 1/μ Eq. (2)

Where TG is the generation time and μ is the growth rate.

Pigment determination

Chlorophyll a (Chl-a), chlorophyll b (Chl-b), chlorophyll c (Chl- c), and total carotenoids were extracted following Parsons et al. (1984). Samples of 5 mL of each microalgae culture were filtered through 25 mm glass fiber filters (GF/C, 1 µm pore size). The filtered samples were extracted with 3 mL of 90 % acetone solution, incubated overnight at 4 °C in darkness, and spectra (400 to 750 nm) were recorded using a spec- trophotometer (HACH DR-6000, HACH, USA). Data obtained were used to calculate pigment concentrations according to Jeffrey and Humphrey (1975):

Chl- a (μg mL-1) = – 0.08 A630 – 1.54 A647 + 11.85 A664 Eq. (3) Chl-b (μg mL-1) = – 2.66 A630 + 21.03 A647 – 5.43 A664 Eq. (4) Chl-c (μg mL-1) = 24.52 A630 – 7.60 A647 – 1.67 A664 Eq. (5)

Carotenes (μg mL-1) = 7.6 A480 – 1.49 A510 Eq. (6)

𝐶𝐶 𝑥𝑥 𝑣𝑣

Non-axenic, monospecific cultures were maintained in 250 mL Erlenmeyer flasks containing 100 mL of “f” medium

Final concentration = mg pigment m-3 =

𝑉𝑉 𝑥𝑥 10

Eq. (7)

(Guillard and Ryther, 1962) at 20 °C, salinity of 33 ± 1 ‰, and under continuous light (24 h) at an irradiance of 50 µmol photons m-2 s-1, provided by cool white fluorescent lamps. After 10 d of preculture, the microalgae were inoculated into fresh 100 mL“f” medium in new 250 mL flasks and maintained under identical culture conditions with daily manual stirring. All microalgae strain cultures were carried out in triplicate. Initial cell densities were 0.05 x106 cells mL-1 for Rhabdonema sp. and Diploneis sp., 0.1 x106 cells mL-1 for Navicula sp. strain 3 and Amphora species, 0.25 x106 cells mL-1 for N. thermalis and Cymbella sp. strains 1 and 2, 0.4 x106 cells mL-1 for Heterococ- cus sp., and 1.0 x106 cells mL-1 for T. suecica.

Cell density and maximum cell density (MCD) were measured every 48 h over 12 d using direct cell counts with a hemocytometer and compound microscope (Olympus CX-31, Japan). Growth rate (µ) and generation time (GT) were calculated using the equations described by Fogg and Thake (1987). For the growth rate:

where: A is the corrected absorbance at the wavelength

indicated; C is the concentration of each pigment calculated according to equations 3 to 6; v is the 90 % acetone volume used for the extraction (expressed in mL), and V is the sample volume filtered (expressed in liters). Wavelength corrections were applied by subtracting 1x the absorbance of 750 nm from the absorbances of 630, 647, and 664 nm; 2x from the absorbance at 510 nm, and 3x from the absorbance at 480 nm. Pigment concentrations were expressed in µg mL-1 to represent the content for each microalgae strain. For absorp- tion measurements used in the estimation of photosynthetic parameters, pigment concentrations were expressed as mg m-3.

In vivo chlorophyll a fluorescence measurements Photosynthetic activity was assessed on day 3 by measuring in vivo chlorophyll a fluorescence. Triplicates of 10 mL sam- ples, a sample from each flask, were dark-adapted for 20 min to oxidize the PSII reaction centers. Rapid light curves (RLC)

(log2𝑁𝑁2) − (log2𝑁𝑁1)

𝜇𝜇 =

𝑡𝑡2 − 𝑡𝑡1

Eq. (1)

were obtained with a pulse-amplitude modulation fluorome- ter (Junior-PAM, Heinz Walz, GmbH, Germany) operated with

Where, μ is the specific growth rate; N2 is the cell con- centration at the end of the exponential growth phase; N1 is the cell concentration at the beginning of the exponential growth phase; Log2 is the logarithm base 2 of the cell con- centration; t2 the final time of the exponential growth phase; and t1 initial time of the exponential growth phase.

WinControl software. To ensure optimal signal quality across replicates and species, settings of intensity, frequency, and gain of actinic light were adjusted to achieve a fluorescence yield (Ft) between 200 and 400 mV. The RLC measurements followed the Universal Light curve protocol (WinControl), and electron transport rate (ETR) was calculated according to Schreiber et al. (1995):

𝐸𝐸𝑇𝑇𝑇𝑇 =

Δ𝐹𝐹

𝐹𝐹´𝑚𝑚

∗ 𝑎𝑎∗(𝜆𝜆) * E * FII (µmol e- (mg Chl-a)-1 m-2 s-1) (Eq. 8)

divisions d-¹), and Amphora sp. strain 5 (0.35 ± 0.04 divi- sions d-¹). In contrast, the xanthophyte Heterococcus sp. showed the lowest growth rate (0.16 ± 0.01 divisions d-¹). An

For this, the effective quantum yield (ΔF/F´m) is calculated according to Schreiber et al. (1995) as:

inverse trend was observed for generation time (GT): Hetero- coccus sp. had the longest GT (6.19 ± 0.25 ds), while Diploneis sp. showed the shortest (1.69 ± 0.38 ds). On average, most

Δ𝐹𝐹 = (F´

- F ) / F´

(Eq. 9)

strains remained in exponential growth for 8 ds; however,

𝐹𝐹´𝑚𝑚

m t m

Amphora sp. strain 26, Nizschia thermalis, and Cymbella sp.

Where, F´m is the maximum fluorescence induced by a saturating light pulse; Ft is the steady-state fluorescence of light-adapted algae; a*(λ) is the chlorophyll a (Chl a) specific

absorption of phytoplankton based on the chlorophyll a con- tent (expressed in mg m-3); E is the photosynthetically active radiation (PAR) (expressed in µmol m-2 s-1); FII is the fraction of light absorbed by photosystem II. The FII values were obtained from Johansen and Sakshaug (2007) and were 0.8 for bacillariophytes and 0.5 for chlorophyte and xantophyte. To calculate the absorption coefficient a(λ) was obtained as follows:

𝑎𝑎(𝜆𝜆)=(2.303 ODλl-1) / Chl a content (mg m-3) (Eq. 10)

where ODλ is the spectral optical density in the visible range (400 to 750 nm), and 2.303 is the conversion factor from base-10 logarithm to natural logaritm (log10/loge). The maximum quantum yield of PSII (Fv/Fm) was calculated using the following equation:

Fv/Fm= (Fm-F0)/Fm ( Eq. 11)

where Fm is the maximmun fluorescence and F0 is the minimum fluorescence.

Photosynthetic parameters—maximum electron trans- port rate (ETRm), photosynthetic efficiency (α), and saturation irradiance (Ik)—were estimated from Fo an d Fm´ values obtained from rapid light curves, from absrotance a*(λ) as calculated as previously described, and from the fraction of light absorbed by photosystem II (FII), dependig on the microalgae group analyzed. This information was integrated, and photosynthetic parameters were calculated using the hyperbolic tangential function of Eilers and Peeters (1988).

Statistical analysis

Normality and homoscedasticity of data were verified. Diffe- rences in growth, pigment concentrations, and photosynthe- tic parameters were analyzed using the Kruskal-Wallis test, followed by a Tukey a posteriori test when significant diffe- rences were found. Statistical significance was set at p < 0.05. Data were analyzed using Statistica 7.0, and graphs were generated with Origin Pro 8.0.

RESULTS AND DISCUSSION

Microalgae strains exhibited significant differences in growth parameters (p < 0.05) (Table 1, Figure 1). The highest growth rates (µ) were observed in the diatoms Diploneis sp. (0.52 ± 0.02 divisions d-1), Navicula sp. strain 3 (0.41 ± 0.04

strain 2 showed extended exponential phases of 13, 11, and 11 d, respectively. The shortest exponential growth phases were observed in Navicula sp. strain 3, Cymbella sp. strain 1, and Amphora sp. strain 2, with durations of 7, 7, and 5 d, respectively.The growth rates of all microalgae strains in this study were lower than those reported by Jiménez-Valera and Sánchez-Saavedra (2016), who conducted a preliminary cha- racterization of the same strains. This reduction may be attri- buted to longer exponential growth phases observed in our cultures, which typically lower the calculated growth rate. For Navicula sp. (0.35 divisions day-¹) and Cymbella sp. (0.30 divi- sions day-¹), growth rates were similar to those reported by other authors (Correa-Reyes et al., 2001; Khatoon et al., 2010). The chlorophyte Tetraselmis suecica and the diatoms Navicula sp. strain 2 and Nizschia thermalis reached the highest cell densities (46.79 ± 0.38, 46.37 ± 2.18, and 44.49 ± 1.59 × 10⁵

cells mL-1, respectively). Conversely, Cymbella sp. strain 1 and Diploneis sp. exhibited the lowest densities (3.65 ± 0.25 × 10⁵ cells mL-¹ for both). In this study, Tetraselmis suecica, Navicula sp. strain 2, and Nizschia thermalis reached values close to 4

× 10⁶ cells mL-¹, which were higher than those observed by Jiménez-Valera and Sánchez-Saavedra (2016). Temperature, medium, and agitation conditions used in this study were the same as those used by Jimenez-Valera and Sánchez Saa- vedra (2016), except for the irradiance level. Thus, differences in growth rate may be attributed to variations in irradiance conditions: we used 50 µmol m-2 s-¹, whereas the previous study used 100 µmol m-2 s-¹.

Tetraselmis suecica is a widely studied species due to its importance in aquaculture and biotechnology (Grabowsky, 2017; Rentería-Mexía et al., 2022). In aquaculture, T. suecica is one of the most commonly used microalgae species as feed for fish, crustaceans, and mollusk larvae due to its cell size, biochemical composition, ease of culture, and digestibility (Yiğitkurt et al., 2025). This microalgae species is widely used in wastewater bioremediation (Andreotti et al., 2020), as a startategy to control the density of pathogenic Vibrio species (Smahajcsik et al., 2025), and for the production of com- pounds with anticancer, antibacterial, and anti-inflamatory activities (Rentería-Mexía et al., 2022). The higher cell den- sities observed here, compared with previous studies, are likely related to differences in nutrient availability, salinity, photoperiod, and possibly strain variation.

Another relevant factor influencing growth is the initial inoculum density. In our study, higher inoculum concentra- tions may have contributed to lower growth rates, consistent with findings by Jiménez-Valera and Sánchez-Saavedra (2016) and Michels et al. (2012), who reported that high initial

Table 1. Mean values and standard deviation of growth rate (µ: divisions day-1), generation time (GT: d), maximum cell density (MCD: cells mL-1 x105) and days in exponential growth phase (EGP: days) for 16 microalgae strains isolated from Baja California, Mexico. Letters indicate significant differences by non-parametric ANOVA Kruskal Wallis, n = 3, α = 0.05, a>b>c>d>e>f>g>h>i.

Tabla 1. Valores promedio y desviación estándar de la tasa de crecimiento (µ: divisiones día-1), tiempo de generación (GT: días), densidad celular máxima (MCD: células mL-1 x105) y días en fase de crecimiento exponencial (TEP: días) de 16 cepas de microalgas aisladas de Baja California, México. Letras indican diferencias significativas por la prueba no paramétric ANOVA Kruskal Wallis, n = 3, α = 0.05, a>b>c>d>e>f>g>h>i.

cell densities reduce light availability within bioreactors, limi- ting cell division. For Navicula sp., the growth rate obtained in this work was 0.35 divisions day-1, which is similar to the 0.29 divisions day-1 reported by Correa-Reyes et al. (2001) and 0.37 divisions day-1 obtained by Khatoon et al. (2010). For Cymbella sp. strain 1, the growth rate was 0.31 divisions day-1, whereas for Cymbella sp. strain 2 was 0.27 divisions day-1. These results are consistent with the 0.35 divisions day-1reported by Kha- toon et al. (2010). It is important to note that the irradiance used by Correa-Reyes et al. (2001) was 150 µmol m-2 s-1, whe- reas the culture medium and temperature conditions were the same as those used in this study. In the study of Khatoon et al. (2010), cultures were maintained at 28 °C, using Conway medium, and an irradiance of 32 µmol m-2 s-1, which is similar to that used in this study.

Pigment content and photosynthetic activity

Chlorophyll a (Chl-a) was the most abundant pigment across all strains, with concentrations ranging from 2.42 ± 0.35 µg mL-1 in Navicula sp. strain 4 to 0.18 ± 0.03 µg mL-1 in Navicula sp. strain 3. Chlorophyll b (Chl-b) was consistently lower, with the highest concentration found in Heterococcus sp. (0.52 ± 0.03 µg mL-1). The highest carotenoid content was recorded in Navicula sp. strain 4 (0.71 ± 0.00 µg mL-1), while the lowest values were observed in Tetraselmis suecica (0.04 ± 0.00), Amphora sp. strain 4 (0.05 ± 0.00), Cymbella

sp. strain 1 (0.05 ± 0.04), and Nizschia thermalis (0.05 ± 0.01 µg mL-1) (Table 2). Pigments found in diatom species stu-

died here are in consistent with those reported for diatoms (Sharma et al., 2023), whereas the pigments detected in T. suecica and Heterococcus sp. are similar to those reported in the literature for these microalgae species (Serive et al., 2017; Casian-González, 2020).

ETR curves showed high variability among species, with values ranging from 3 to 25 µmol e- mg Chl-a-1 s-1. The light intensities used did not lead to photoinhibition (Figue 2). Variations in ETR curves are related to differences in the photosynthetic apparatus, pigment content, and light adap- tation of the analyzed microalgae strain. For example, the light-harvesting antenna of diatoms differs from green algae due to the presence of fucoxanthin-chlorophyll a/c protein complexes, and there is evidence that the organization and structure of the photosynthetic apparatus can vary among different diatom species. It is plausible that this heteroge- neity in pigment composition and architecture of the pho- tosynthetic machinery may lead to wide photosynthetic res- ponses among different microalgae groups or even among species within the same group (Arshad et al., 2021).

Significant differences were observed among strains in Fv/Fm, photosynthetic efficiency (α), maximum electron transport rate (ETRm), and saturation irradiance (Ik) (p < 0.05) (Table 3). Fv/Fm ranged from 0.74 ± 0.01 in Tetraselmis suecica to 0.47 ± 0.03 in Navicula sp. strain 3. Most strains exhibited photosynthetic efficiency values (α) between 1.0 and 2.0 × 10-2, although Tetraselmis suecica, Rhabdonema sp., and Nizschia thermalis recorded higher values (7.0, 3.0, and

Figure 1. Mean values and standard deviation of cell density (cells mL-1 x106) for 16 microalgae strains isolated from Baja California, Mexico. Figura 1. Valores promedio y desviación estándar de la densidad celular (células mL-1 x106) de 16 cepas de microalgas aisladas de Baja California, México.

Table 2. Mean values and standard deviations of chlorophyll a, b, c, and carotenoids (Chl-a, Chl-b, Chl-c, and carotenoids, respectively; µg mL⁻¹) in 16 microalgae strains isolated from Baja California, Mexico. Letters indicate significant differences based on non-parametric ANOVA (Kruskal-Wallis test), n = 3, α

= 0.05; a > b > c > d > e > f > g > h > i > j.

Tabla 2. Valores promedio y desviación estándar del contenido de clorofila a, b, c y carotenoides (Chl-a, Chl-b, Chl-c y carotenes, respectivamente, en µg mL-1) de 16 cepas de microalgas aisladas de Baja California, México. Letras ins¿dican diferencias significativas por ANOVA no paramétrico Kruskal Wallis, n = 3, α = 0.05, a>b>c>d>e>f>g>h>i>j.

3.0 × 10-2, respectively). The highest ETRm was obtained in Amphora sp. strain 6 (44.34 ± 1.51 µmol e- mg Chl-a-1 s-1), fo- llowed by Heterococcus sp. and Rhabdonema sp. (34.37 ± 1.24 and 30.72 ± 4.27 µmol e- mg Chl-a-1 s-1, respectively). The lowest ETRm values (3–4 µmol e- mg Chl-a-1 s-1) were ob- served in Amphora sp. strains 1, 2, and 7, and Navicula sp. strain 2. Regarding saturation irradiance (Ik), most strains had values between 400 and 1000 µmol photons m-2 s-1. Exceptions included Heterococcus sp. (2582.12 ± 5.77 µmol photons m-2 s-1), Amphora sp. strain 6 (2059.59 ± 249.21), and Navicula sp. strain 4 (1653.13 ± 321.15), which exhibited the highest Ik values. The lowest Ik values were recorded in Tetraselmis suecica (235.81 ± 24.59) and Navicula sp. strain 2 (310.17 ± 162.08 µmol photons m-2 s-1) (Table 3).

In vivo chlorophyll a fluorescence is a rapid, non-invasive method to assess photosynthetic performance and physio- logical status in algae. Light intensity, temperature, and nutrient levels affect the photosynthetic apparatus and, consequently, fluorescence (Malapascua et al., 2014; Gebara et al., 2023). To our knowledge, only one study (Mercado et al., 2004) has examined the photosynthetic activity of microalgae strains from the Baja California Peninsula. That study measured Ik values ranging from 12 to 43 µmol pho- tons m-2 s-1 in benthic diatoms, consistent with values repor- ted for other subtidal communities. Species such as T. suecica and N. thermalis showed high values of photosynthetic effi- ciency (α) and lower values of Ik. This response is expected for low-light acclimatation cells, which adjust their physiology

to optimize light-harvesting efficiency (Perkins et al., 2006). High values of α, ETRm, and Ik can be associated with efficient light utilization and the capacity of photoadaptation to high irradiances (Pérez-Varillas and Sánchez-Saavedra, 2025). This suggests that the diversity of photosynthetic strategies in the microalgae studied here is species-specific and can be linked to the environmental sites from which they were isolated. In

the benthic diatom Navicula phyllepta, Ik values around 800 µmol photons m-2 s-1 were obtained under light intensities of 25 and 400 µmol photons m-2 s-1, and no photoinhibition was detected in the rapid light curves (Perkins et al., 2006). Similar Ik values were obtained in this study for Amphora sp. strains 2, 4, 5, 6, and 7, Navicula sp. strains 3 and 4, Cymbella sp. strains 1 and 2, and for Rhabdonema sp. ETRm values observed in Amphora sp. strain 6 and Rhabdonema sp. in this study were comparable to the 31.6 ± 1.1 µmol e – m-2 s-1 reported for Amphora coffeaeformis (Torres et al., 2013).

The Fv/Fm ratio, widely used to assess cellular health, ty- pically ranges from 0.5 to 0.8 under non-stressful conditions; lower values suggest stress or cell death (Bobco, 2014). Tan et al. (2019) reported average Fv/Fm values by algal group: Chlorophyta (0.71) > Cryptophyta (0.62) > Bacillariophyta

≈ Chrysophyta (0.60) > Xanthophyceae (0.54) > Pyrrophyta (0.51). Our Fv/Fm results for the chlorophyte T. suecica (0.74

± 0.01) align with these values. Most strains studied here had Fv/Fm values between 0.50 and 0.72, suggesting that our culture conditions were generally non-stressful. Only Navicu- la sp. strains 2 and 3 showed slightly lower values (0.49 and

Figure 2. Mean values and standard deviation of electron transport rate (ETR, µmol e- mg Chl-a s-1) versus Photosynthetic Active Radiation (PAR, µmol m-2 s-1) for 16 microalgae strains isolated from Baja California, Mexico.

Figura 2. Valores promedio y desviación estándar de al tasa de trasporte de electrones (ETR, µmol e- mg Chl-a s-1) contra la radiación fotosintéticamente activa (PAR, µmol m-2 s-1) de 16 cepas de microalgas aisladas de Baja California, México.

Table 3. Mean values and standard deviation of maximmum quantum yield of photosystem II (Fv/Fm), photosynthetic efficiency (α (x10-2) µmol photon m-2 s-1), maximum electron transport rate (ETRm: µmol e- mg Chl-a s-1) and irradiance of saturation (Ik: µmol photon m-2 s-1) for 16 microalgae strains isolated from Baja California, Mexico. Letters indicate significant differences by non-parametric ANOVA Kruskal Wallis, n = 3, α = 0.05, a>b>c>d>e>f>g>h>i.

Tabla 3. Valores promedio y desviación estándar del rendimeinto cuántico máximo del fotosistema II (Fv/Fm), eficiencia fotosintética (α (x10-2) µmol photon m-2 s-1), tasa de transporte de electrones máxima (ETRm: µmol e- mg Chl-a s-1) e irradiancia de saturación (Ik: µmol photon m-2 s-1) de 16 cepas de microalgas aisladas de Baja California, México. Letras indican diferencias significativas por ANOVA no paramétrico Kruskal-Wallis, n = 3, α = 0.05, a>b>c>d>e>f>g>h>i.

0.47, respectively). Based on Fv/Fm and Ik values, the light intensity used in this study (50 µmol photons m-2 s-1) was not a limiting or stressful factor for most strains. However, some strains, such as T. suecica and Navicula sp. strain 2, did not reach high Ik values, indicating potential for adaptation to lower irradiance. In contrast, Heterococcus sp., Amphora sp. strain 6, and Navicula sp. strain 4 exhibited higher Ik values,

consistent with adaptation to high-light environments, such

as the shallow coastal waters of Ensenada, San Quintín, and Mulegé, where they were originally isolated (Bermúdez-Con- treras et al., 2008; Perea-Moreno and Hernández-Escobedo, 2016).

ETRm values varied among strains and were likely influenced by pigment composition, cell size, and environ- mental origin. For example, Amphora sp. strain 6, a small benthic diatom (5.70 µm length, 3.68 µm width), showed the highest ETRm (44.34 ± 1.51 µmol e-1 mg Chl-a-1 s-1) and a moderate Chl-a content (0.43 ± 0.06 µg mL-1). In contrast, Amphora sp. strain 7, a larger-celled strain (13.83 µm length, 3.99 µm width), had the lowest ETRm (4.13 ± 1.34), indicating that larger cells may be less efficient in light capture due to self-shading and greater pigment packaging (Yun et al., 2010). An interesting exception was the xanthophyte Hetero- coccus sp., which exhibited high photosynthetic activity and moderate pigment content despite being the largest strain (1903.13 µm in length, 13.44 µm in width). Heterococcus species are known for their morphological plasticity, capable of shifting between spherical, elongated, and irregular forms depending on their life cycle stage, even under controlled

conditions (Darling et al., 1987). The results of growth and photosynthetic parameters suggest that the culture con- ditions applied in this study did not impose stress on the microalgae strains analyzed. Therefore, these conditions can serve as a reference for laboratory cultivation of microalgae in aquaculture or biotechnology facilities.

CONCLUSION

This work provides a comprehensive physiological and pho- tosynthetic characterization of 16 native microalgae strains isolated from coastal environments of the Baja California Pe- ninsula, México. The results reveal marked interspecific varia- bility in growth, pigment composition, and photosynthetic performance, supporting their potential for diverse biotech- nological applications.

Among the analyzed strains, Tetraselmis suecica stood out for its high Fv/Fm value (0.74 ± 0.01), indicating excellent physiological status under the tested conditions, and its ele- vated maximum cell density (46.79 × 10⁵ cells mL-1), reinfor- cing its suitability for aquaculture and biomass production. Similarly, the benthic diatom Amphora sp. strain 6 exhibited the highest maximum electron transport rate (ETRm) at

44.34 ± 1.51 µmol e- mg Chl-a-1 s-1 and one of the highest Ik values (2059.59 ± 249.21 µmol photons m-2 s-1), suggesting strong adaptation to high-irradiance environments. In con- trast, strains such as Navicula sp. strain 3 exhibited both low Chl-a content (0.18 ± 0.03 µg mL-1) and low Fv/Fm (0.47 ± 0.03), pointing to a reduced photosynthetic efficiency under the tested conditions.

The strain Heterococcus sp., despite its unusually large cell size, displayed notable photosynthetic capacity (ETRm: 34.37 ± 1.24 µmol e- mg Chl-a-1 s-1) and high light saturation (Ik: 2582.12 ± 5.77 µmol photons m-2 s-1), highlighting its morphological plasticity and potential adaptability to fluc- tuating environmental conditions.

Taken together, these findings not only broaden our understanding of the physiological diversity among native microalgae from this region, but also provide a valuable baseline for selecting strains with optimal traits for targeted uses in aquaculture, sustainable bioresource development, and industries such as pharmacology and cosmeceuticals. Future studies should evaluate these strains under stress conditions or in large-scale culture systems to confirm their robustness and commercial applicability.

CONFLICTS OF INTEREST

The authors declare that they have not conflicts of interest.

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