Growth and phosphate uptake drivers revealed in Acinetobacter tjernbergiae

Phosphorus is one of those elements that rarely makes headlines, yet civilization depends on it almost entirely. The world’s food supply is built on phosphate fertilizers, and those fertilizers are mined from phosphate rock, a finite resource concentrated in a handful of countries and steadily depleting in quality. Meanwhile, vast quantities of phosphorus flow every day through wastewater treatment plants and out into rivers, lakes, and oceans, where it fuels harmful algal blooms and dead zones. Closing this loop—recovering phosphorus from waste streams and returning it to agricultural use—has become a central challenge of environmental biotechnology. A new study from researchers at Technische Universität Berlin and Aalborg University Esbjerg, published open access in Applied Microbiology and Biotechnology, brings that goal a significant step closer by systematically identifying the conditions under which a remarkable bacterium can be pushed to hoard phosphate at record-breaking speeds.

The microorganism in question is Acinetobacter tjernbergiae, a species belonging to a genus famous in wastewater engineering circles for its role in enhanced biological phosphorus removal. Certain Acinetobacter strains can accumulate astonishing amounts of phosphate inside their cells—as much as 24 to 30 percent of their dry weight—storing it in the form of polyphosphate, a polymer of dozens to hundreds of phosphate linked by high-energy bonds. In principle, this makes them ideal candidates for a microbial phosphorus recovery process: let the bacteria feast on phosphate in a tank, harvest the biomass, and release or process the stored phosphorus into a reusable fertilizer product. In practice, however, earlier studies kept running into the same frustrating bottleneck. The bacteria could store plenty of phosphate per gram of cell, but they failed to produce enough biomass quickly enough to make the overall process fast and productive. A culture that takes phosphate up slowly, no matter how skillfully, cannot treat wastewater at industrial rates.

That is the problem the Berlin team, led by Jan Brück, Simon Täuber, Leonie Ackermann, Henry Schittkowski, Peter Neubauer, and corresponding author Stefan Junne, set out to solve empirically. Their starting hypothesis was that something as mundane as the trace metal composition of the growth medium might be silently limiting biomass formation. Iron, in particular, plays a host of essential roles in microbial physiology: it sits at the active centers of the cytochromes that drive respiration, in the iron-sulfur clusters of numerous enzymes, and in the machinery of DNA synthesis. When iron is scarce, cells cannot build the respiratory capacity needed to sustain rapid growth, even when carbon and phosphate are abundant.

The results of their shake flask experiments confirmed the suspicion in dramatic fashion. Simply increasing the iron concentration in the medium by one milligram per liter enhanced bacterial growth six-fold. A single milligram—an almost trivial quantity of metal—was the difference between a sluggish, barely dividing culture and a vigorous one. It is a striking demonstration of how trace element supply can mask or reveal the true physiological potential of a production organism, and a reminder that “optimal” media formulations inherited from decades-old literature deserve constant re-examination. For a process aiming to recover phosphorus at scale, the finding translates directly into economics, because higher biomass concentrations mean more cellular machinery available to capture phosphate per liter of reactor volume.

With iron supply corrected, the researchers moved from flasks to controlled bioreactor cultivation, where pH, oxygen transfer, and feeding can be managed precisely. There, A. tjernbergiae reached a maximum biomass concentration of 12 plus or minus 0.7 grams per liter within just 14 hours of cultivation. That figure matters enormously for the downstream concept. Phosphate capture by microorganisms is, at first approximation, the product of how much biomass you have and how fast each gram of that biomass can take up phosphate. Raising the biomass term from the insufficient levels of previous studies to more than 10 grams per liter in half a day transforms the throughput of the entire operation.

And the uptake rates the team measured were indeed record-setting. The maximum volumetric phosphate uptake rate reached 280.2 plus or minus 40.8 milligrams per liter per hour—the highest value reported to date for any Acinetobacter species. Volumetric rate is the number a plant engineer cares about, since it dictates how much wastewater can be processed per unit of reactor volume and time. The specific uptake rate, normalized to biomass, peaked at 41.0 plus or minus 1.1 milligrams per gram of biomass per hour, a measure of the intrinsic enzymatic and transport capacity of the cells themselves. Together, the two numbers show that the earlier bottleneck was not an inherent limitation of the organism but a consequence of suboptimal cultivation conditions.

Beyond the headline numbers, the study resolves questions about the underlying physiology of phosphate accumulation. The researchers demonstrated that A. tjernbergiae exhibits both classical polyphosphate accumulation and the so-called overplus phenomenon. Overplus, a term with roots in early observations of phosphorus dynamics in algae, describes a curious behavior: when cells that have been starved of phosphate are suddenly re-exposed to it along with an available carbon source, they transiently take up phosphate far in excess of their immediate needs, overshooting even their normal luxury uptake capacity. The mechanism is linked to the mobilization of stored carbon reserves—polyhydroxyalkanoates in many organisms—which supply the energy and reducing power needed to drive polyphosphate synthesis. In effect, a phosphorus-starved, carbon-loaded cell becomes a phosphate vacuum for a short, intense period.

That coupling points to one of the study’s most practically important findings: phosphate uptake was strongly correlated with carbon uptake. Whenever the bacteria were consuming their carbon substrate, they were simultaneously drawing phosphate into their cells and polymerizing it. Conversely, when carbon was absent, the direction of flow reversed—the cells began releasing phosphate back into the medium. This bidirectional dependence has direct consequences for how a microbial phosphorus recovery process would be designed and operated. It means that a robust carbon supply must be guaranteed during the uptake phase, whether as the organic matter naturally present in wastewater or as an added carbon source, and that any process interruption that leaves the biomass starved of carbon risks undoing the captured phosphorus. The same release behavior, however, could be exploited deliberately: a controlled carbon starvation step could be used to strip phosphorus from the biomass into a small, concentrated liquid volume from which phosphate salts could be precipitated, closing the recovery loop.

The context of this work is a shifting regulatory landscape. Across Europe and beyond, legislation increasingly mandates phosphorus recovery from wastewater and sewage sludge, driven simultaneously by concerns over the depletion and geopolitics of phosphate rock and by the environmental damage caused by phosphorus discharge. Conventional chemical recovery routes, such as precipitating struvite from sludge liquors, are established but face limitations in selectivity, reagent costs, and the quality of the recovered product. Biological recovery offers an alternative in which the phosphorus is concentrated inside living cells under mild conditions, powered not by industrial reagents but by the metabolism of the organism itself. The Berlin and Esbjerg team’s demonstration that the right trace metal regime, careful bioreactor control, and exploitation of the overplus phenomenon can push volumetric uptake to unprecedented levels strengthens the case for this biological route considerably.

There is, of course, still distance between a well-controlled bioreactor experiment and a full-scale wastewater plant. Real wastewater arrives with fluctuating composition, competing microbial communities, inhibitory compounds, and variable carbon loads, all of which will test the performance demonstrated here under laboratory conditions. Scaling up a process that depends on precise iron supplementation and tightly managed carbon availability will require engineering ingenuity. But the study provides something essential for that journey: a quantitative, empirically grounded map of the key factors governing growth and phosphate uptake in A. tjernbergiae, and proof that the organism’s celebrated phosphate-storing capacity can be unleashed rather than merely admired. As phosphate rock dwindles and regulations tighten, the bacteria that quietly learned to bank phosphorus in their cells may prove to be among the most important microorganisms in the transition to a circular phosphorus economy.