Influence des conditions hivernales sur les productions phyto-et zooplanctoniques en Méditerranée Nord-Occidentale. III. Caractérisation des eaux de surface au moyen de cultures d'algues

Bioassays were performed on surface-water samples, collected during the Médiprod I cruise of the R.V. “Jean Charcot” in the northern part of the Mediterranean Sea. Physiological responses of axenic algal cultures were measured in order to characterize the different water masses. The responses obtained are specific to each algal strain: Phaeodactylum tricornutum grows in all waters sampled during the first part of the cruise, while Chaetoceros lauderi does not develop because of its different nutritional requirements; Chlamydomonas magnusii does not grow in any of the tested samples. No clear relationship exists between the results obtained in vitro and those obtained in situ but, on the whole, the waters collected during the second part of the cruise allow a higher growth of the test algae in vitro. This higher fertility is probably related to the algal bloom which occurred between the first and the second part of the cruise. In the authors' opinion the great difference in uptake rates observed between the high-turbulence area (low production) and the adjacent area (high production), may be related more to biological conditioning than to any physical process. However, physical mixing seems to be responsible for inhibition of primary productivity at the two stations where mixing attains maximum values.     

Variations de quelques facteurs estimatifs de la biomasse,et en particulier de l'ATP,chez plusieurs algues marines planctoniques

ATP, carbon, and nitrogen content, and cell volume have been measured in 7 marine algae in culture. Intraspecific differences are negligible during the phase of exponential growth; interspecific differences in ATP and carbon content are slight during this phase compared with those observed in the same cultures between the exponential and senescent phases. As the interspecific differences agree well with those reported for algae in situ, this leads the authors to believe that the greater part of the biomass in situ is always in a state of physiological youth. ATP content is higher in diatoms, and seems linked with silica shell synthesis. In non-silicified species, there is a significant correlation between the ATP: plasma ratio and the division rate, although the cellular volumes are quite different. In the author's opinion, ATP content allows a good estimation of biomass to be made, as well as, under controlled conditions, a suitable estimation of primary productivity; however, because of their high silica content, diatoms should be considered separately from other phytoplankters.     

Study of bacteria associated with marine algae in culture

Inhibitory action of some usual antibiotic, substances have been tested on 25 species of bacteria isolated from marine algae cultures. Penicillin, kanamycin, neomycin and streptomycin appear to be the most convenient antibiotics for eliminating or inhibiting polluting bacteria populations.     

Study of bacteria associated with marine algae in culture

Seventy seven organic compounds have been tested for receptivity to attack by bacteria strains isolated from marine algae cultures. Bacteria utilize amino and organic acids more frequently than sugars and derivatives, especially when growth substances are not included in the experimental medium.     

Etude des bactéries associées aux Algues marines en culture

Some bacteria strains have been isolated from marine algae cultures and assigned tentative genera and species. Observations and tests were conducted using Bergey's manual and specific papers on marine bacteria as taxonomic keys. The genera Pseudomonas, Flavobacterium and Achromobacter appear to be the most important, as far as the number of species is concerned.     

Are some bacteria toxic for marine algae?

It was observed in vitro that some bacteria from different origins, including the sea, are capable of inhibiting the growth of several marine algae. However, there is too much difference between in vitro experiments and in situ environmental conditions to assume that bacterial poisons may rule a great deal of algac-bacteria relationships in the sea as a whole. In the author's opinion the importance of bacterial poison is limited to special and rare instances.     

Nocturnal synthesis and diurnal degradation of phytoplankton biomass in surface waters

Natural populations of phytoplankton were collected near the Bay of Bourgneuf, France, in spring 1982, and were subjected to natural surface irradiance outdoors. They exhibited exponential growth on time scales of a week, but significant decreases in biomass indicators such as chlorophyll a and particulate nitrogen were observed during daytime. At night, these decreases were more than compensated by increases in the same biomass variables, which could double over 12 h of darkness. These features are characteristic of phytoplankton populations in surface waters which cannot escape high irradiances, and may be representative of situations in incubation bottles held at fixed depths near the surface. Under such conditions, a decrease in biomass during daytime should not necessarily be interpreted as irreversible damage unless growth measurements are carried out over the following night hours to check for possible recovery.