Study Shows Oxygenic Photosynthesis Possible Using Only One Photosystem

LMU researchers have reported a striking challenge to a long-standing textbook rule of photosynthesis: oxygenic photosynthesis can, in principle, occur without photosystem I. The study, published in Nature Communications, shows that certain cyanobacteria continued to grow, fix carbon dioxide, and release oxygen even after photosystem I disappeared.

For more than half a century, the dominant model has emphasized the coordinated roles of two protein complexes—photosystem II and photosystem I. In that framework, photosystem I is widely treated as indispensable for producing NADPH, the reducing power required for carbon fixation.

What made the finding especially surprising is that the work began with a different goal. Dario Leister’s team aimed to introduce a plant-like photosystem I component into the cyanobacterium Synechocystis. Instead, through genetic engineering combined with adaptive laboratory evolution, they generated strains in which the photosystem I machinery vanished.

Yet these evolved organisms did not lose oxygenic photosynthesis. They maintained electron flow and preserved the ability to perform the full suite of biochemical steps leading to oxygen production and carbon assimilation, contradicting the assumption that photosystem I is universally required.

The researchers traced this resilience to a major reorganization of the photosynthetic electron transport chain. A steep proton gradient emerges under these new conditions, and the NDH-1 complex appears to operate in reverse.

In reverse mode, NDH-1 can generate NADPH, effectively replacing the function that had previously been attributed exclusively to photosystem I. This mechanistic workaround suggests that the cell can reroute energy conversion chemistry when key components are removed.

The findings carry implications beyond cyanobacteria. They reshape hypotheses about how oxygenic photosynthesis may have evolved, and they highlight the ability of biological systems to achieve essential outputs via alternative biochemical pathways.

In the bigger picture, the work also raises questions about robustness and adaptability in one of biology’s best-studied processes. If nature can re-engineer NADPH supply without photosystem I, it may be possible to exploit similar flexibility when designing more efficient photosynthetic systems.

The study underscores a viral-science takeaway: even entrenched paradigms can collapse when organisms reveal unexpected routes through evolution, and new experiments can expose surprising “missing pieces” in what we thought we understood.