Cyanobacteria developed oxygenic photosynthesis 2.6–2.5 billion years ago (BYA)
[1]. Approximately 1.9 BYA, after endosymbiotic uptake by a hetetrotrophic eukaryotic host cell of a cyanobacterium, possibly related to the present primitive genus
Gloeomargarita and its evolution into a plastid, the first autotrophic eukaryote arose
[2][3][4][5]. This one, common photosynthetic ancestor without a definite name (one possible name—Protoarchaeplastida), which has not survived to our time after primary endosymbiosis, underwent divergence into three eukaryotic lines
[6][7]. According to the phylogenomic data, first, 1.7–1.6 BYA, Glaucophyta (blue branch) was separated from the common evolutionary tree
[8]; later, 100–200 MYA (million years ago), red algae, or Rhodophyta, (red branch) emerged from the remaining common evolutionary stem of red and green algae; and finally, 1.1 BYA green algae appeared
[9]. These three groups of photosynthetics, having the common name Archaeplastida and also called Plantae (Figure 1A) have double membrane plastids and coexist in the biosphere, together with cyanobacteria, up to the present time. In the subsequent events of secondary endosymbioses, green and red microalgae gave rise to photosynthetics with four-membrane plastids (reduced to three membranes in some algae), resulting in various new algal phyla
[10]. Together with the three Archaeplastida groups, there are approximately ten major algal taxa. Higher plants inherited the Archaeplastida chloroplasts from the green algae
[9].
2. Phycobiliproteins and Phycobilisomes of Cyanobacteria
There are approximately 1500 known species of unicellular, filamentous, and colonial cyanobacteria which are the only prokaryotes that perform oxygenic photosynthesis. Brilliantly colored phycobiliproteins serve as antennas in the pigment apparatus of cyanobacteria, supplying absorbed light energy to PS I and PS II. These chromophorylated water-soluble proteins consist of 16–18 kDa
α and 17–20 kDa
β polypeptides in a 1-to-1 ratio. Such (
αβ)
1-heteromonomers spontaneously aggregate into (
αβ)
3-trimers and then into (
αβ)
6-hexamers
[13]. Four different open-chain phycobilin chromophores, namely, phycocyano-, phycoerythro-, phycourobilin, and cryptoviolin (phycoviolobilin), covalently bind in different ratios via a thioether bond to the Cys residues in both polypeptide subunits. Each
α-subunit attaches one or two chromophore groups; each
β-polypeptide binds one to four tetrapyrrole chromophores, depending on the specific phycobiliprotein. Each phycobiliprotein has chromophores of one (PC, APC, C-PE) or two different varieties (R-PC, PEC, CU-PEs, and R-PE). A unique trichromatic-variety phycocyanin, R-PC V, was revealed in oceanic
Synechococcus sp. WH8102
[14]. As a result, approximately 15 different phycobiliproteins have been described (
Table 1).
Table 1. Phycobiliproteins in various photosynthetic clades (except Cryptophyta).
Colorless linker proteins of about 30 kDa bind disc-shaped hexamers and trimers into cylindrical rods, and these, in turn, assemble into PBSs consisting of two main parts: the APC core and several peripheral rods of other phycobiliproteins surrounding the central core. PBSs of the vast majority of cyanobacteria are hemidiscoidal. Three main morphological types of cyanobacterial PBSs have been observed: (1) with a two-cylindrical core and six fan-shaped lateral cylinders; (2) with a three-cylindrical core and six cylinders; (3) with a five-cylindrical core (three major plus two semicylinders) and eight lateral cylinders (Figure 2A–C).
Figure 2. Morphological types of cyanobacterial PBSs. (A) Hemidiscoidal PBS with the bicylindrical core. (B) The most typical hemidiscoidal PBS with the tricylindrical core and six lateral cylinders. (C) PBS with the pentacylindrical core and eight lateral cylinders. (D) Bundle-shaped PBS (Gloeobacter type). APC —light blue; PC —blue; PE —red color.
Bundle-shaped PBSs of the genus
Gloeobacter have a somewhat unusual architecture with only vertical lateral cylinders (
Figure 2D). This morphology saves space on the membrane surface, since cyanobacteria of this genus have no thylakoids and all PBSs are located on the cytoplasmic membrane
[15]. When, for some reason, altered conventional PBSs in mutant cyanobacterial cells cannot properly attach to the thylakoid membrane, the energy feeding of PS I is provided by newly synthesized small cylindrical PBSs anchored to PS I by the special CpcG2 polypeptide linker
[16][17]. Some cyanobacteria, such as
Calothrix sp., can vary their PC and/or PE ratio in PBS depending on the light quality. This phenomenon, realized in several variants, is known as complementary chromatic adaptation (CCA)
[18].
In the PBS, absorbed solar energy is funneled from the lateral cylinders to the core and eventually to the terminal longwavelength emitters - chromophorylated ApcD and ApcF varieties of APC subunits and the ApcE (Lcm) - chromophorylated large core-membrane linker protein located in the bottom APC cylinders. From collectively acted terminal emitters, the energy rapidly rolls to PS I and PS II
[19][20][21]. Regarding PS II, there has long been a general consensus that in the plane of the thylakoid membrane, PBSs are disposed on the outer surface of PS II dimers
[22].
The situation with PS I is more complicated
[23][24][25][26]. In cyanobacteria, PS I mainly forms trimers with a certain share of monomers
[27][28]. In contrast to the PS II dimers
[22], the surface of PS I shows a major protrusion of three hydrophilic polypeptide subunits (PsaC, PsaD, and PsaE), which extends into the cytoplasm
[27][28] and prevents a tight binding of PBS to PS I trimer due to its threefold symmetry. Computer modeling demonstrated the docking of PBS to the PS I monomer only, corresponding to a realization of a highly likely energy transfer between these two pigment–protein complexes
[29].
Along with the “common” PBSs and the “common” cyanobacteria, there are other species that have caught everyone’s attention.
2.1. Prochlorophyta: Chlorophyll b-Containing Cyanobacteria
Prochlorophyta, or Prochlorales, is a mixed group of cyanobacteria containing Chl
a/b-proteins instead of PBS as a photosynthetic antenna. Few known Prochlorophyta members form three genera, namely,
Prochlorococcus, Prochloron,
and Prochlorothrix, with cells having green or greenish color. The genus
Prochlorothrix is represented by two filamentous freshwater species,
P. hollandica [30] and
P. scandica [31].
P. didemni, the only one known species of the
Prochloron genus, is an obligate symbiont of the didemnid ascidia
Lissoclinum patella [32]. Various described ecotypes of the genus
Prochlorococcus marinus inhabit different geographical areas of the ocean and belong to the most abundant representatives of oceanic photosynthetic picoplankton.
Light-harvesting membrane Chl
a/b-binding proteins in Prochlorophyta are derived from the CP43 polypeptide of PS II and are named prochlorophyte Chl-binding (Pcb) proteins
[33] or, less frequently, designated CBP
[34], while the Chl
a/b-containing antenna in the pigment apparatus of Viridiplantae consists of LHC-polypeptides. According to this type of antenna, Prochlorophyta have stacked regions of thylakoids (
Figure 3), making them different from other cyanobacteria
[35]. Pcb-proteins form in thylakoid membrane supercomplexes with both photosystems. The PS I trimers are surrounded by a single ring of Pcbs, while dimers of PS II are in contact with two or three divided by a gap Pcb arches
[36]. Interestingly, cells of
Prochlorococcus contain their divinyl derivatives with the same absorption properties instead of Chls
a and
b [37]. In addition to the presence of Chls
a and
b, very small amounts of Chl
c variety in Pcb of
P. didemnii also cannot be excluded
[38].
Figure 3. Three types of Archaeplastida double membrane chloroplasts. In glaucophytic algae (as well as in photosynthetic amoebae), the chloroplast retains PBSs and the peptidoglycan layer (PG). Red algae retain PBSs, but the peptidoglycan layer is eliminated. In plastids of Viridiplantae, PBSs and peptidoglycan layer are lost while thylakoids form grana regions (similarly to Prochlorophyta).
Of all Prochlorophyta,
Prochlorococcus attracts the most attention because of its prominent role in marine primary productivity and the presence of PE
[39][40], in addition to its Pcb-antenna proteins. Cyanophages infecting
Prochlorococcus contain genes for phycobilin-synthesizing enzymes, and these are expressed in
Prochlorococcus, raising further questions of how the PE genes could have been acquired by this species
[41]. Various described marine strains of
Prochlorococcus belong to one of two ecotypes that are specifically adapted to either low-light or high-light conditions
[42][43]. Low-light-adapted ecotypes carry a gene cluster with the
cpeA and
cpeB genes of PE to produce a small amount of a functional light-absorption pigment. Differences in the sequences of the
α and
β polypeptide chains and the revealed presence of only one phycourobilin-like attachment site on the
α subunit distinguish this red pigment from the known PEs of PBS-containing cyanobacteria. However, the supramolecular organization and aggregation state of PE molecules due to their very low content in thylakoids remain unknown. According to electron micrographs of immunogold-labeled cells, PE is attached to the thylakoid membrane at its lumen side
[44].
In the genome of the high-light-adapted strains, only a single and free-standing
cpeB gene occurs. This gene encodes a derived form of
β-PE, the function of which has remained enigmatic thus far
[45]. Interestingly, degenerated forms of the
α and
β subunits of other cyanobacterial PEs without perturbing their folding, structural stability, and fluorescence functionality were demonstrated to be stable in vitro
[46]. Correspondingly, phycobilins are known to serve as single chromophores in cyanobacteriochromes
[47]. Therefore, such a degenerated form of PE could change its purpose to being a photoreceptor pigment in
Prochlorococcus.
2.2. Chlorophyll d-Containing Cyanobacteria, Acaryochloris marina
Acaryochloris marina is a widespread free-living cyanobacterium that exploits special marine habitats depleted in visible solar radiation and characterized accordingly by an increased proportion of infrared penetrated light. These shady sieve site conditions fully correspond to the microlayer of seawater beneath the body surface of didemnid ascidians, where this species was first described in 1996
[48]. Since then, numerous free-living
Acaryochloris strains have been reported to be distributed in various environments, including marine stromatolites, saline lake epilithic biofilms, and some special freshwaters
[49].
The pigment apparatus of
A. marina is characterized by the presence of Chl
d, with absorption shifted to the long-wavelength spectral region compared to Chl
a. Different kinds of Chls can readily be distinguished by their S
0 → S
1 long-wavelength absorption peak positions. This feature helps gain insight into the evolved mechanism for the realization of infrared region photosynthesis. The polypeptide compositions and aggregation states of both the PS I and PS II core complexes (dimers of PS II and monomers/trimers of PS I) of
A. marina are similar to those of well studied Chl
a-containing cyanobacteria, but in contrast, each photosystem uses Chl
d as the main photosynthetic pigment, including the special chlorophyll pairs in both their RCs
[50]. The light-harvesting antenna of
A. marina is a special Pcb-protein of ca. 34 kDa incorporated by Chl
d in concert with some part of Chl
a, indicating that this membrane antenna is similar to the Chl
a/b-protein of prochlorophytes
[51]. Pcb-proteins encircle in the form of a ring the PS II dimers, but data on Pcb composition and function in PS I are currently absent. In summary, Chl
d can account for ~95% of total Chls (
a +
d) in the contents of PS I and PS II
[52].
All the known strains of
Acaryochloris are free from phycobiliproteins except the first described and most extensively studied “under ascidian niche found”
A. marina MBIC11017. Classic cyanobacterial PBSs in this strain are eliminated. Instead,
A. marina MBIC11017 contains the rod-shaped phycobiliprotein aggregates located on the cytoplasmic side of the thylakoid membranes along with the intrinsic membrane Pcb-antenna. Each rod is composed of four phycobiliprotein hexamers equal in size to lateral rods in a typical PBS from other cyanobacteria. Rods consist of PC (
αβ)
6-hexamers with some admixture of pigment, whose absorption spectrum resembles that of APC. The complete chromosome sequence of
A. marina demonstrates that, along with PC-related genes, only the
apcB gene is present for coding the
β-chain of APC, while the corresponding
apcA is absent
[52]. This means that very unusual (
αβ)
1-heteromonomers mixed from
α-PC and
β-APC polypeptides most likely compose the last trimer in the bottom hexamer of a cylinder adjoining the outer thylakoid membrane surface. The presence of both PC and APC, however, within one hexamer seems to be a unique feature of the phycobiliprotein aggregates in
A. marina.
The architecture of native cylindrical phycobiliprotein aggregates is much less complex than that of complete PBS. However, the energy transfer to Chl
d is very efficient and takes about 70 ps
[53]. In isolated preparations
[54][55] and in photosynthetic action spectra
[56], it has been found that the biliproteins are physically attached to the PS II complexes. Comparison of the surface areas for the rounded bottom of phycobiliprotein cylinders and PS II shows that each PSII dimer can anchor one or two cylinders. Currently, it is not clear whether such an attachment needs special linker protein(s) or not. Due to the cytoplasmic surface area of the PS II dimer
[22], there is enough place for the ends of two cylinders. Dimers of PS II in
A. marina are paired in tetramers
[50]. As a consequence, the phycobiliproteins form separate patches on the thylakoid surface that, highly probably, consist of four cylinders feeding by absorbed energy these PS II tetramers.
Complete sequencing of the
Acariochloris genome showed that phycobiliprotein genes are not located in the chromosome but in large cell plasmids. This led to the conclusion that genes for PBS appear to have been previously lost in the common ancestor of
Acaryochloris and some other marine cyanobacteria.
A. marina MBIC11017 is the only
Acaryochloris strain which is thought to have reacquired a set of the phycobiliprotein genes via horizontal gene transport
[57].
2.3. Chlorophyll f-Containing Cyanobacteria and Their Involvement in the Process of Photoacclimation to Far Red Light
Chl
f was discovered as a minor pigment in a filamentous cyanobacterium
Halomicronema hongdechloris collected from living columnar marine stromatolites
[34] and has since been found in a number of other cyanobacteria, both filamentous and uni-cellular, including terrestrial species
[58]. Compared to
A. marina, Chl
f-containing cyanobacteria can photosynthesize further into the far-red and near-infrared regions (Chl
f gains enhanced absorbance by up to 800 nm)
[59]. The amount of Chl
f in white light is very small or absent
[60], and its biosynthesis is induced only under FaRLiP (far-red light photoacclimation) conditions. In contrast to Chl
d in
A. marina, it was shown that Chl
f is not part of a special pair in both RCs and functions only in a light-harvesting role in the core antennae of PS I and PS II. The precise cryo-EM data have shown that the FaRLip-PS II pigment–protein complex from
Synechococcus sp. PCC 7335 binds one Chl
d and four Chl
f in place of several of 35 Chl
a molecules present in PS II when grown in visible light
[61]. Analogously, the high-resolution structure of the far-red PS I core from
H. hongdechloris showed that in far-red light, it binds 83 Chl
a plus 7 Chl
f associated at the periphery of the PS I core antenna
[62]. Simultaneously with these pigment replacements, several polypeptide subunits of PS I and PS II cores are remodeled using subunits paralogous to those produced in white light
[61][62]. In doing so, even then Chl
f constitutes only a small part (less than 10%) of the remaining predominant Chl
a but expands the spectral region of solar light absorption.
The architecture of PBS from
Leptolyngbia sp. strain JSC-1 exhibits an extensive response to growth in far-red light simultaneously with the stimulation of Chl
f biosynthesis. The hemidiscoidal pentacylindrical PBS of this species corresponds to the one of varities of chromatic adaptation synthesizing PE in green and white light and the PC only under red light conditions. In far-red light, the PBS core diminishes to only two APC cylinders, retains PE disks in lateral cylinders while absent in red light, and produces another ApcE linker (ApcE2 instead of ApcE1) with noncovalently bound terminal phycobilin chromophor
[59]. This long-wavelength-shifted terminal chromophore better corresponds to effective energy transfer from PBS to Chl
f-modified complexes of PS I and PS II. It has been suggested that
apcE2 could be used as a marker of FARLiP adaptability in different tested species of cyanobacteria
[63]. The genome of
Leptolyngbya sp. JCC-1 has a 21-gene cluster that encodes paralogues of most of the core polypeptides of PS I, PS II, and PBSs; similar clusters are found in other FaRLiP-cyanobacteria
[59]. All photoacclimation processes usually take approximately one day. The restructured pigment apparatus (i.e., PS I, PS II, PBS) acquires the ability to absorb at longer light wavelengths, which enables cyanobacteria to grow photoautotrophically in far-red light.
To summarize somewhat, it is stated that changes in the composition of phycobiliproteins occur differently in cyanobacteria possessing various kinds of Chl pigments. The vast majority of cyanobacterial species contain only chl a, and the form of their phycobiliprotein antenna is phycobilisomes, which can exist in several morphological variants. There is a complete loss of PBSs and phycobiliproteins in Chl b-containing phototrophs, a reduction from whole PBS to phycobiliprotein cylinders in Chl d-containing species, and only a partial rearrangement of the PBS core and replacement of individual phycobiliproteins in Chl f-containing cyanobacterial cells.