Question: What is found in Prochlorococcus and essential for photosynthesis?
Context: A bicarbonate indicator (hydrogencarbonate indicator) is a type of pH indicator that is sensitive enough to show a color change as the concentration of carbon dioxide gas in an aqueous solution increases.  The indicator is used in photosynthesis and respiration experiments to find out whether carbon dioxide is being liberated.  It is also used to test the carbon dioxide content during gaseous exchange of organisms.  When the carbon dioxide content is higher than 0.04%, the initial red color changes to yellow as the pH becomes more acidic.  If the carbon dioxide content is lower than 0.04%, it changes from red to magenta and, in relatively very low carbon dioxide concentrations, to purple.  Carbon dioxide, even in the concentrations found in exhaled air, will dissolve in the indicator to form carbonic acid, a weak acid, which will lower the pH and give the characteristic colour change.  A colour change to purple during photosynthesis shows a reduction in the percentage of carbon dioxide and is sometimes inferred as production of oxygen, but there is not actually any direct evidence for it. "Candidatus" Atelocyanobacterium thalassa (previously cyanobacterium UCYN-A,) is a species of cyanobacteria commonly found throughout the world's oceans.  Unlike many other cyanobacteria, "A. thalassa" lacks the ability to perform photosynthesis.  Instead, it is found in a symbiotic relationship with photosynthetic algae.  "A. thalassa" fixes nitrogen for the algae, while the algae provides carbon for "A. thalassa" through photosynthesis. Aerobic anoxygenic phototrophic bacteria (AAPBs) are alphaproteobacteria and gammaproteobacteriathat are obligate aerobes that capture energy from light by anoxygenic photosynthesis.  Anoxygenic photosynthesis is the phototrophic process where light energy is captured and stored as ATP.  The production of oxygen is non-existent and, therefore, water is not used as an electron donor.  They are widely distributed marine plankton that may constitute over 10% of the open ocean microbial community.  They can be particularly abundant in oligotrophic conditions where they were found to be 24% of the community.  Aerobic anoxygenic phototrophic bacteria are photoheterotrophic (phototroph)microbes that exist in a variety of aquatic environments.  Photoheterotrophs(Gk: photo = light, hetero = (an)other, troph = nourishment), are heterotrophic organisms that use light to produce energy, but are unable to utilize carbon dioxide as their primary carbon source.  Most are obligately aerobic, meaning they require oxygen to grow.  One remarkable aspect of these novel bacteria is that they, unlike other similar bacteria, are unable to utilize BChl (bacteriochlorophyll) for anaerobic growth.The only photosynthetic pigment that exists in AAPB is BChl a. Anaerobic phototrophic bacteria, on the contrary, can contain numerous species of photosynthetic pigments like bacteriochlorophyll a, b, c, d, e, f, etc.  There is still a large void in the areas regarding the abundance and genetic diversity of the AAPB, as well as the environmental variables that regulate these properties. Jan Ingenhousz or Ingen-Housz FRS (8 December 1730 – 7 September 1799) was a Dutch physiologist, biologist and chemist.  He is best known for showing that light is essential to photosynthesis and thus having discovered photosynthesis.  He also discovered that plants, like animals, have cellular respiration.  In his lifetime he was best known for successfully inoculating the members of the Habsburg family in Vienna against smallpox in 1768 and subsequently being the private counsellor and personal physician to the Austrian Empress Maria Theresa. Chlorophyll "a" is a specific form of chlorophyll used in oxygenic photosynthesis.  It absorbs most energy from wavelengths of violet-blue and orange-red light.  It also reflects green/yellow light, and as such contributes to the observed green color of most plants.  This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain.  Chlorophyll "a" also transfers resonance energy in the antenna complex, ending in the reaction center where specific chlorophylls P680 and P700 are located. Chromatium okenii is a Gram-negative bacterium found in water.  It belongs to the Purple sulfur bacteria.  These bacteria are capable of photosynthesis and use Hydrogen sulfide (HS) as an electron donor for CO reduction and so do not produce oxygen.  This type of photosynthesis is called anoxygenic photosynthesis. An action spectrum is the rate of a physiological activity plotted against wavelength of light.  It shows which wavelength of light is most effectively used in a specific chemical reaction.  Some reactants are able to use specific wavelengths of light more effectively to complete their reactions.  For example, chlorophyll is much more efficient at using the red and blue spectrums of light to carry out photosynthesis.  Therefore, the action spectrum graph would show spikes above the wavelengths representing the colours red and blue.  Engelmann split light into its components by the prism and then illuminated Cladophora placed in a suspension of aerobic bacteria.  He found that bacteria accumulated in the region of blue and red light of the split spectrum.  He thus discovered the effect of the different wavelengths of light on photosynthesis and plotted the first action spectrum of photosynthesis. Chlorophyll (also chlorophyl) is any of several closely related green pigments found in cyanobacteria and the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός, "chloros" ("green") and φύλλον, "phyllon" ("leaf").  Chlorophyll is essential in photosynthesis, allowing plants to absorb energy from light. Prochlorococcus is a genus of very small (0.6 µm) marine cyanobacteria with an unusual pigmentation (chlorophyll "a2" and "b2").  These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth.  "Prochlorococcus" microbes are among the major primary producers in the ocean, responsible for a large percentage of the photosynthetic production of oxygen.  Analysis of the genome sequences of 12 "Prochlorococcus" strains show that 1,100 genes are common to all strains, and the average genome size is about 2,000 genes.  In contrast, eukaryotic algae have over 10,000 genes. Auxiliary metabolic genes (AMGs) are found in many bacteriophages but originated in bacterial cells.  AMGs modulate host cell metabolism during infection so that the phage can replicate more efficiently.  For instance, bacteriophages that infect the abundant marine cyanobacteria "Synechococcus" and "Prochlorococcus" (cyanophages) carry AMGs that have been acquired from their immediate host as well as more distantly-related bacteria.  Cyanophage AMGs support a variety of functions including photosynthesis, carbon metabolism, nucleic acid synthesis and metabolism.
Answer: Chlorophyll