photolithotrophic

Deep research

Generated by just research-trait; source: research/traits/physiology/photolithotrophic-deep-research-falcon.md

# Curation report: photolithotrophic

## Record and recommendation

- **Trait:** photolithotrophic
- **Identifier:** **`METPO:1000658`**
- **Category / kind / status:** PHYSIOLOGY / CLASS / REVIEWED
- **Parent:** `METPO:1000631`
- **Recommended graph design:** retain the trait as a high-level physiological capacity, then represent alternative, taxon-scoped mechanistic branches for sulfur photolithotrophy, photoferrotrophy, hydrogen-based photolithotrophy, and oxygenic water oxidation. The strongest evidence retrieved here supports the sulfur and Fe(II) branches.

## 1. Scope and boundaries

### Operational definition

Photolithotrophy is the capacity to use **light as the energy source** and a **reduced inorganic compound as the electron donor**. In the best-characterized microbial examples, electron transfer supports generation of ATP and reductant, while inorganic carbon is assimilated into biomass. Green sulfur bacteria (GSB) use H₂S as a principal donor, oxidize it initially to elemental sulfur, and fix CO₂ through the reductive—or reverse—TCA cycle. Photoferrotrophy uses light, Fe(II), and inorganic carbon. (kushkevych2024anoxygenicphotosynthesiswith pages 1-2, gupta2020extracellularelectronuptake pages 7-8)

The submitted definition says CO₂ is “typically” the primary carbon source. That qualification is important: **photolithotrophy describes energy and electron sources, whereas autotrophy describes carbon source**. Thus, `photolithotrophic` should not be made logically equivalent to `photolithoautotrophic`. A separate edge such as `photolithotrophic organism — may_fix — CO2` is safer than making CO₂ fixation universally necessary.

### Boundary cases

1. **Photoorganoheterotrophy:** light is used, but electrons and/or carbon are obtained from organic compounds. This is not photolithotrophy.
2. **Chemolithotrophy:** the donor is inorganic, but energy is obtained from chemical oxidation without light. This is not photolithotrophy.
3. **Oxygenic photolithotrophy:** cyanobacteria use water as the donor and produce O₂. It belongs within the broad trait definition, but not within an anoxygenic-sulfur-specific mechanism.
4. **Anoxygenic photolithotrophy:** H₂S, thiosulfate, H₂, or Fe(II) can replace water as donor; oxygen is not evolved. The retrieved 2024 review explicitly lists H₂S, H₂, and ferrous iron as alternative donors. (kushkevych2024anoxygenicphotosynthesiswith pages 16-17)
5. **Mixotrophy:** some GSB can assimilate acetate while retaining light-driven lithotrophic metabolism. Conversely, inability to grow heterotrophically on glucose was reported for the reviewed GSB models. Carbon-substrate utilization therefore must not be inferred solely from the photolithotrophic label. (kushkevych2024anoxygenicphotosynthesiswith pages 13-14)
6. **Genotype-only assignments:** presence of `sqr`, `cyc2`, `pioABC`, `sox`, or photosynthesis genes is mechanistic evidence, but not by itself a demonstrated trait. For example, SQR homologues occur even in *Chlorobium ferrooxidans*, which cannot use sulfur as its sole donor. (kushkevych2021anoxygenicphotosynthesisin pages 3-5)

## 2. Current mechanistic understanding

### Sulfur-based branch

In GSB, chlorosomes collect light and transfer excitation through the Fenna–Matthews–Olson complex to a type-I reaction center. Chlorosomes are exceptionally effective under low irradiance: the 2024 review reports operation below **4 µEinstein m⁻² s⁻¹** and describes assemblies containing hundreds of thousands of bacteriochlorophyll molecules. (kushkevych2024anoxygenicphotosynthesiswith pages 4-6)

H₂S oxidation is initiated by membrane-associated sulfide:quinone oxidoreductase (SQR). SQR oxidizes sulfide while reducing menaquinone to menaquinol; flavocytochrome-c sulfide dehydrogenase can provide an alternative route. Reaction-center electron transfer proceeds through chlorophyll *a*, phylloquinone, and Fe–S centers to ferredoxin. Reduced ferredoxin and ferredoxin:NAD⁺ oxidoreductase systems provide reducing power for biosynthesis. (kushkevych2024anoxygenicphotosynthesiswith pages 9-10)

Thiosulfate use is lineage-dependent. The Sox system supports thiosulfate oxidation in organisms such as *Rhodovulum sulfidophilum*, while some GSB can oxidize thiosulfate and tetrathionate. Stored elemental sulfur and its subsequent oxidation involve additional systems, including reverse/dissimilatory sulfite-reductase machinery, but these modules are not universal among all photolithotrophs. (kushkevych2024anoxygenicphotosynthesiswith pages 9-10, kushkevych2024anoxygenicphotosynthesiswith pages 18-18, kushkevych2021anoxygenicphotosynthesisin pages 3-5)

### Photoferrotrophic branch

Photoferrotrophs couple light energy to oxidation of soluble Fe(II) or reduced iron minerals. In *Rhodopseudomonas palustris* TIE-1, PioA is a periplasmic decaheme cytochrome, PioB an outer-membrane β-barrel, and PioC a periplasmic high-potential Fe–S protein. The reviewed model transfers electrons from Fe(II) through PioAB and PioC toward the photosynthetic reaction center. PioAB-mediated uptake from solid extracellular substrates has direct support in TIE-1. (gupta2020extracellularelectronuptake pages 7-8, gupta2020extracellularelectronuptake pages 8-9)

Cyc2 homologues occur in some *Chlorobium* genomes and are candidates for extracellular Fe(II) oxidation, but the review stresses that molecular mechanisms remain incompletely resolved. Cyc2 presence should therefore be curated as a **candidate mechanism**, not as proof of photoferrotrophy. (gupta2020extracellularelectronuptake pages 7-8, gupta2020extracellularelectronuptake pages 4-5, gupta2020extracellularelectronuptake pages 8-9)

## 3. Candidate nodes

### Trait and biological-process nodes

- `METPO:1000658` — photolithotrophic
- `GO:0015979` — photosynthesis
- `GO:0015977` — carbon fixation
- Anoxygenic photosynthesis — verify the current GO identifier before YAML insertion
- Photoferrotrophy — label-only candidate
- Sulfide-dependent photolithotrophy — label-only candidate
- Thiosulfate-dependent photolithotrophy — label-only candidate
- Reverse/reductive TCA cycle — pathway node; verify pathway CURIE against the target ontology release
- Calvin–Benson–Bassham cycle — pathway node; applies to selected photoferrotrophs and oxygenic phototrophs, not GSB generally

### Chemicals and physical factors

- `CHEBI:16136` — hydrogen sulfide

Showing the first 60 of 214 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.