Although cyanobacteria are oxygenic phototrophic organisms, they often thrive in environments that become periodically anoxic. This is particularly the case in the dark when photosynthetic oxygen evolution does not take place. Whereas cyanobacteria generally utilize endogenous storage carbohydrate by aerobic respiration, they must use alternative ways for energy generation under dark anoxic conditions. This aspect of metabolism of cyanobacteria has received little attention but nevertheless in recent years a steadily increasing number of publications have reported the capacity of fermentation in cyanobacteria. This review summarizes these reports and gives a critical consideration of the energetics of dark fermentation in a number of species. There are a variety of different fermentation pathways in cyanobacteria. These include homo- and heterolactic acid fermentation, mixed acid fermentation and homoacetate fermentation. Products of fermentation include CO2, H2, formate, acetate, lactate and ethanol. In all species investigated, fermentation is constitutive. All enzymes of the fermentative pathways are present in photoautotrophically grown cells. Many cyanobacteria are also capable of using elemental sulfur as electron acceptor. In most cases it seems unlikely that sulfur respiration occurs. The main advantage of sulfur reduction seems to be the higher yield of ATP which can be achieved during fermentation. Besides oxygen and elemental sulfur no other electron acceptors for chemotrophic metabolism are known so far in cyanobacteria. Calculations show that the yield of ATP during fermentation, although it is low relative to aerobic respiration, exceeds the amount that is likely to be required for maintenance, which appears to be very low in these cyanobacteria. The possibility of a limited amount of biosynthesis during anaerobic dark metabolism is discussed.

https://pure.uva.nl/ws/files/3238369/2576_25311y.pdf

Fermentation in cyanobacteria

Forty-four axenic strains of cyanobacteria that synthesize phycoerythrin were screened to ascertain the effect of light quality on pigment synthesis. Cellular pigment compositions were determined after photoautotrophic growth with low light fluxes (7.0 X 10(2) ergs/cm2 per s) of green, red, and white light, and in the case of facultative heterotrophs, after dark growth at the expense of sugars. Twelve strains did not adapt chromatically: the cells contained fixed proportions of phycoerythrin, phycocyanin, and allophycocyanin under the growth conditions used. In the remaining strains, the cellular ratio of phycoerythrin to phycocyanin was much higher after growth in green than in red light. Quantitative data on the cellular pigment contents, supplemented by measurements of the differential rates of pigment synthesis on representative strains, show that chromatic adaptation may involve a light-induced modulation either of phycoerythrin synthesis alone (7 strains) or of both phycoerythrin and phycocyanin synthesis (25 strains). Facultative hetrotrophs able to adapt chromatically have a phycobiliprotein composition after dark growth which closely resembles that after growth in red light. Light quality does not affect the differential rate of chlorophyll synthesis. The physiological and taxonomic implications of these findings are discussed.

Occurrence and nature of chromatic adaptation in cyanobacteria.

Resume Dans leur environnement naturel, les cyanobacteries ont a faire face, non seulement aux fluctuations d'aliments nutritifs mais aussi a l'alternance journaliere de lumiere et d'obscurite. Leur capacite d'adaptation a de telles variations implique un degre de flexibilite metabolique considerable. L'adaptabilite des cyanobacteries en ce qui concerne leur metabolisme carbone est particulierement interessant. La photoautotrophie en tant que metabolisme fondamental des cyanobacteries a fait l'objet de nombreuses etudes. Les differents marquages de metabolites derivant de l'assimilation de 14CO2 dans diverses conditions ont ete etablis et interpretes en termes de reactions de carboxylation specifiques. Chez la plupart des cyanobacteries, la D-ribulose-1,5-bisphosphate carboxylase, l'enzyme-cle intervenant dans la fixation de CO2, est tres semblable dans ses proprietes moleculaires a celle des plantes et des algues eucaryotes, bien que quelques exceptions aient ete relevees. Le metabolisme photosynthetique des cyanobacteries depend de la capture de l'energie lumineuse par un systeme complexe de pigments, parmi lesquels les phycobiliproteines et la chlorophylle a jouent un role majeur. Les etudes recentes concernant les premieres etapes de biosynthese des noyaux tetrapyrroliques ont etabli que l'acide δ-amino-levulinique, premier intermediaire specifique de leur biosynthese, est forme par la voie en C5, c'est-a-dire a partir de l'acide glutamique; cette voie de biosynthese est caracteristique des plantes et des algues superieures chez lesquelles elle est probablement compartimentalisee dans les chloroplastes. Le metabolisme photoautotrophe des cyanobacteries fournit non seulement les biopolymeres cellulaires mais egalement des materiaux de reserve necessaires a la survie des cellules dans des conditions peu favorables. Parmi ceux-ci, le plus important, dans le contexte du metabolisme carbone, est un polyoside contenant du D-glucose qui est utilise quand les cellules se trouvent a l'obscurite. La synthese et la degradation du polyoside se produisent d'une facon cyclique lorsque les organismes sont maintenus sous un regime alternatif de lumiere et d'obscurite. Dans des conditions aerobies, le catabolisme des polyosides endogenes se fait par le cycle oxydatif des pentoses phosphates. Le couplage avec une chaine respiratoire de transporteurs d'electrons sert a maintenir le niveau cellulaire d'ATP. Certaines especes de cyanobacteries catabolisent les hydrates de carbone en absence d'oxygene par un processus essentiellement respiratoire utilisant le soufre comme accepteur terminal des electrons, ou par fermentation avec production de lactate. La quantite relativement faible de reserves polyosidiques qui est vitale pour maintenir la viabilite des cyanobacteries dans des conditions de non-croissance est de peu d'importance pour la croissance a l'obscurite; cette croissance a l'obscurite dependra de l'exploitation de substrats exogenes. Cela est egalement vrai lorsque le metabolisme photoautotrophe ne procure que les besoins de maintenance de l'organisme. Les hydrates de carbone sont les seuls substrats organiques compatibles avec le metabolisme intermediaire des cyanobacteries. A peu pres la moitie des souches en culture a l'heure actuelle sont capables d'utiliser certains de ces composes comme source de carbone pour la croissance. L'utilisation de ces composes hydrophiles necessite la possession de systemes specifiques pour leur penetration dans les cellules. Trois cyanobacteries capables de pousser aux depens du D-glucose possedent un systeme de transport actif specifique pour ce substrat et certains de ses analogues; leur penetration dans les cellules fonctionne grâce a un gradient electrochimique de protons a travers la membrane cellulaire. Les caracteristiques de ce processus ont procure des renseignements indirects sur les relations spatiales entre les transporteurs d'electrons actifs dans la respiration d'une part et la photosynthese d'autre part. Alors qu'un grand nombre de souches sont heterotrophes a des degres divers, il n'existe, jusqu'a present, aucune correlation entre la capacite d'utiliser les sucres pour une espece donnee et la presence de ces sucres dans le milieu naturel d'ou cette souche a ete isolee. Bien que les parametres du systeme de transport du D-glucose aient ete determines pour un certain nombre d'especes, le benefice que la possession de tels mecanismes peut conferer dans les conditions competitives de l'environnement naturel n'est pas encore defini.

Modes of cyanobacterial carbon metabolism.

Thirty eight axenic strains belonging to the Chroococcales were screened for the ability to grow photoheterotrophically in the light with glucose in the presence of 10-5 M DCMU, which inhibits photoautotrophic growth. Seven strains could do so, and four of them could also grow chemoheterotrophically, though more slowly, with glucose in the dark. Six are members of the genus Aphanocapsa and one of the genus Chlorogloea. The remaining 31 strains examined appear to be obligate photoautotrophs. A quantitative study of the growth behaviour of one strain, Aphanocapsa 6714, fully confirmed its facultative photoheterotrophy and chemoheterotrophy.

Photoheterotrophy and chemoheterotrophy among unicellular blue-green algae

Dissolved free amino acids (DFAA) form a significant proportion of dissolved fixed nitrogen in marine waters and could provide an additional source of nitrogen for the growth of marine microalgae. With the advent of DFAA analysis by HPLC, recent studies of algal physiology, and an increased awareness of potential experimental errors, doubts are cast over the conclusion that marine microalgae are not net users of DFAA. Results obtained by testing the use of a few amino acids cannot be extrapolated; the full range of amino acids should be used in field experiments. A high rate of uptake of any one amino acid is not to be expected. It is more probable that a simultaneous uptake of several amino acids will occur at a lower rate. It is essential that environmental conditions be taken into account in interpretation of results from field experiments. Results from laboratory and field studies suggest that maximum rates of DFAA uptake would occur in dark conditions in waters depleted of dissolved inorganic nitrogen. Turbid estuaries and coastal waters would also be environments likely to induce a significant uptake of amino acids, because levels of nFA.4 i.n. sl~ch WB!P~S S ~ C :~!st;l.~.~!y high and the periods of enforced darkness due to turbidity are likely to induce the development of microalgal amino acid uptake systems.

/pdf/nitrogen-sources-for-the-growth-of-marine-microalgae-role-of-1vi15fxj1h.pdf

Nitrogen sources for the growth of marine microalgae: role of dissolved free amino acids

0768 09

1.

An axenic culture of a species of Nostoc from the coralloid roots of the cycad Macrozamia lucida, grows heterotrophically in complete darkness at measurable and reproducible rates.




2.

Heterotrophic growth was supported by glucose, fructose and sucrose over the pH range 6 to 9. The growth rate with glucose was increased if casamino acids were added.




3.

Heterotrophic growth on glucose proceeded under both aerobic and anaerobic conditions.




4.

Cultures grown in the dark were pigmented. There was no significant difference in the ultrastructure of photoautotrophically-grown and dark heterotrophically grown cells.

Dark heterotrophic growth of an endophytic blue-green alga

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Dark heterotrophic growth of an endophytic blue-green alga