The enlightened secrets across the ocean

Abstract

An amazing 70% of the Earth's surface is water. Some creatures that live in the oceans depend on light as a primary source of energy. For example, phytoplankton is a common oceanic photosynthetic organism. Bacteria are also found in the ocean, but it was assumed that oceanic phototrophic bacteria were found in small niches and were absent in oceanic surface waters. Two recent reports now put that assumption to rest: phototrophic oceanic bacteria are more widespread than previously thought.The first report focuses on a new phototrophy in bacteria. Light-absorbing pigments are essential for generating responses from light sources. One pigment, rhodopsin, consists of a retinal chromophore bound to membrane proteins, and was originally found in the Animal and Archaeal kingdoms. Archaeal rhodopsins perform similar functions, such as photosensing or chloride pumps, but rhodopsins have not been reported in bacteria until now.Oded Beja and colleagues1xBacterial rhodopsin: evidence for a new type of phototrophy in the sea. Beja, O. et al. Science. 2000; 289: 1902–1906Crossref | PubMed | Scopus (810)See all References1 describe a bacterial rhodopsin that is a functional light-driven proton pump. Sequence analysis of a marine Proteobacteria uncovered a putative rhodopsin gene, encoding proteorhodopsin (PR). Phylogenic comparison put PR in a new family that is distinct both from animal and from Archaeal rhodopsins. PR was expressed in E. coli to determine whether PR binds to retinal. E. coli-expressing PR turns red in the presence of retinal and its membranes show an absorbance spectra that is consistent with bound retinal. PR is also a light-dependent proton pump. In PR-expressing E. coli, light induces an increase in pH and right-side-out vesicles transport tritium in a light-dependent fashion. PR has a rapid photochemical cycle, which indicates that it acts as a transporter, but it is not known whether it is involved in energy generation. The proteobacteria from which Beja et al.1xBacterial rhodopsin: evidence for a new type of phototrophy in the sea. Beja, O. et al. Science. 2000; 289: 1902–1906Crossref | PubMed | Scopus (810)See all References1 isolated PR is widespread, found both in the Atlantic and in the Pacific Oceans. Their study suggests that light-dependent bacterial energy generation is prevalent in the oceans.Z.S. Kobler and colleagues2xBacterial photosynthesis in surface waters of the open ocean. Kobler, Z.S. et al. Nature. 2000; 407: 177–179Crossref | PubMed | Scopus (202)See all References2 report that oceanic bacterial photosynthesis is widespread in the eastern Pacific and northwestern Atlantic oceans. The group detected bacterial photosynthesis using fluorescence variance in oceanic water samples. The fluorescence variance signal is specific to bacterial photosynthetic electron transport. Phytoplankton give a signal that is distinguishable from photosynthetic bacteria, and they show daily photosynthetic fluctuations that are absent in photosynthesizing bacteria. Bacterial photosynthesis is three times less efficient than in phytoplankton. Phytoplankton are ∼100 times more prevalent in the Pacific Ocean, but along a 1000 km Pacific Ocean transect, both organisms are detected at the surface.Both of these reports are important and complementary, uncovering a new possible component of the world's carbon cycle. Beja et al.1xBacterial rhodopsin: evidence for a new type of phototrophy in the sea. Beja, O. et al. Science. 2000; 289: 1902–1906Crossref | PubMed | Scopus (810)See all References1 have identified a new phototrophy in a bacteria that is widespread at the oceanic surface and Kobler et al.2xBacterial photosynthesis in surface waters of the open ocean. Kobler, Z.S. et al. Nature. 2000; 407: 177–179Crossref | PubMed | Scopus (202)See all References2 have biophysical evidence that bacterial photosynthesis occurs in the upper ocean. What is the total biomass of ocean-surface photosynthetic bacteria and how does it contribute to global carbon cycles? What determines the global positioning and abundance of phototrophic bacteria? These reports suggest how rich in bacterial life the oceans might be.