FEP microcapillary strips enable high-resolution analysis of algal growth dynamics under limited nutrient and light regimes

Harmful algal blooms (HABs) threaten ecosystems, aquaculture, and public health worldwide. However, current monitoring tools are limited to satellite imaging, which is too slow and spatially coarse, or chlorophyll assays which lack specificity for species and sensitivity to early physiological shifts. Anticipating bloom onset requires real-time, species-specific probes that can reproduce how environmental drivers shape algal growth. Here, we introduce uncoated FEP microcapillary strips as an advanced cultivation platform that enables cell-level tracking of microalgal growth under controlled nutrient and light conditions. Using Parachlorella kessleri as a model, we systematically tested the effects of light intensity, gas permeability, and nitrogen-to‑phosphorus (N:P) stoichiometry on growth dynamics. As expected, light availability governed both growth rates and exponential onset of growth, with cultivation in dark conditions fully suppressing cell proliferation. Gas-permeable uncoated FEP strips supported higher densities than encapsulated or PVOH-coated strips, highlighting the critical role of O₂/CO₂ exchange. Nutrient stoichiometry further modulated the kinetics of exponential growth for cultures under balanced conditions. A nitrogen to phosphorous, N:P ratio of 11:1 yielded a high cell density of 4.1 × 10⁷ ± 5.6 × 10⁶ cells/mL, whereas excess nitrogen, corresponding to a N:P ratio of 47:1, suppressed expansion by nearly 50 %. Although maximum specific growth rates were found stable across the range of conditions tested, the timing of exponential initiation and the final yields were strongly dependent on the microenvironment. This positions uncoated FEP microcapillary strips as a sensitive probe for analysis of environmental drivers of HAB dynamics, enabling early, high-resolution detection of bloom-favouring conditions and advancing predictive bloom surveillance.