Microalgae present a promising pathway for biological hydrogen production due to their ability to harness solar energy and convert water into hydrogen gas through photosynthetic processes. Unlike conventional methods that rely on fossil fuels, microalgae offer a carbon-neutral approach, leveraging natural biochemical pathways. Key strategies to enhance hydrogen production in these organisms include sulfur deprivation, light management, nutrient cycling, and optimized bioreactor designs. The interplay between photosynthesis and hydrogenase activity is central to maximizing yield, while challenges such as photoinhibition and scalability must be addressed for practical implementation.

Sulfur deprivation is a well-documented method to induce hydrogen production in green algae, particularly Chlamydomonas reinhardtii. Under normal conditions, algae prioritize biomass growth and oxygenic photosynthesis. However, sulfur limitation disrupts the repair cycle of Photosystem II (PSII), reducing oxygen evolution and creating an anaerobic environment favorable for hydrogenase activation. Hydrogenases are enzymes that catalyze the reduction of protons to molecular hydrogen, but they are highly sensitive to oxygen. By carefully controlling sulfur availability, the balance between oxygen production and consumption shifts, enabling sustained hydrogen generation. Studies show that sulfur-deprived cultures can achieve hydrogen production rates of up to 3 mL per liter per hour, though prolonged deprivation leads to culture degradation.

Light management is another critical factor in optimizing algal hydrogen production. Microalgae rely on light for photosynthesis, but excessive irradiance causes photoinhibition, damaging PSII and reducing overall efficiency. Conversely, insufficient light limits the energy available for hydrogenase activity. Researchers have explored dynamic light regimes, including alternating light-dark cycles and modulated intensities, to balance these competing demands. For example, a light intensity of 100–200 µmol photons per square meter per second is often optimal for green algae, while higher intensities may benefit cyanobacteria due to their different photosynthetic apparatus. Additionally, spectral composition plays a role; certain wavelengths can preferentially drive photochemical reactions without inducing stress.

Nutrient cycling is essential for maintaining culture health and prolonging hydrogen production. Nitrogen and phosphorus are particularly important, as their availability influences metabolic pathways. Nitrogen limitation, like sulfur deprivation, can promote anaerobic conditions and hydrogenase activity, but it also risks stunting algal growth. Phosphorus scarcity may lead to energy storage as polyphosphates instead of hydrogen. To address this, phased nutrient regimes have been proposed, where algae are first grown under replete conditions to accumulate biomass before transitioning to a nutrient-limited phase for hydrogen production. This approach can extend the productive lifespan of cultures and improve overall yields.

The relationship between photosynthesis and hydrogenase activity is complex and requires careful optimization. In green algae, hydrogen production is primarily linked to the photosynthetic electron transport chain, where electrons derived from water splitting are redirected to hydrogenases under anaerobic conditions. Cyanobacteria, however, often employ nitrogenases for hydrogen production, which consumes ATP and is less energy-efficient. While green algae typically achieve higher peak hydrogen yields, cyanobacteria can produce hydrogen continuously under certain conditions due to their oxygen-tolerant mechanisms. Comparing the two, green algae may be more suitable for high-intensity, short-duration production, whereas cyanobacteria offer advantages in long-term stability.

Bioreactor design is a major determinant of system performance. Open ponds are cost-effective but suffer from low gas retention and contamination risks. Closed photobioreactors provide better control over environmental parameters, such as light exposure and gas exchange, but at higher capital and operational costs. Flat-panel and tubular bioreactors are common designs, with the former offering superior light penetration and the latter enabling scalable modular systems. Innovations like biofilm-based reactors or integrated gas capture systems aim to improve efficiency further. For instance, membrane-based bioreactors can selectively remove hydrogen while retaining other gases, reducing feedback inhibition.

Despite these advances, several limitations hinder large-scale adoption. Photoinhibition remains a significant barrier, as prolonged high-light exposure degrades photosynthetic efficiency. Scalability is another challenge, as laboratory successes often fail to translate to industrial settings due to inconsistent light distribution, contamination, and culture crashes. Additionally, the energy balance of algal hydrogen systems must be favorable; some configurations require more energy input than the hydrogen produced can offset.

Comparing green algae and cyanobacteria reveals trade-offs in hydrogen yield and operational stability. Green algae, such as Chlamydomonas, can achieve higher volumetric production rates under optimal conditions but are more sensitive to oxygen and require precise environmental control. Cyanobacteria, like Synechocystis or Anabaena, exhibit greater robustness and can operate in semi-continuous modes but generally produce less hydrogen per unit volume. The choice between the two depends on the specific application and available infrastructure.

In conclusion, enhancing hydrogen production in microalgae involves a multifaceted approach, integrating physiological manipulation, bioreactor engineering, and process optimization. While challenges like photoinhibition and scalability persist, advances in light management, nutrient cycling, and system design continue to improve feasibility. The selection of algal species and cultivation strategies must align with the intended scale and operational constraints. By addressing these factors, microalgae-based hydrogen production can move closer to becoming a viable component of the renewable energy landscape.