Oligotrophy and pelagic marine bacteria:Facts and fiction

Oligotrophy, or the inability of bacterial cells to propagate at elevated nutrient concentrations, is a controversial phenomenon in microbiology. The exact cause of the unculturability of many indigenous marine bacteria on standard laboratory media has still not been resolved. Unfortunately the physiology of such cells is difficult to investigate as long as high cell density cultures cannot be obtained. An extensive evaluation of experiments relating to oligotrophy and the cultivation of marine bacteria is presented in this review. When incorporating the findings of studies performed with molecular biological methods, the picture emerges that indigenous marine bacteria can be cultivated under certain conditions and that the 'oligotrophic way of life' is a transient characteristic. Although strong generalisations should not be made with respect to a biological system as diverse as the world's oceans, it should be anticipated that cells with unique physiological characteristics appear to exist in the oceanic system. When combining conventional physiological approaches with molecular biological techniques it is feasible to unveil the phenotypes that go with the encountered genotypes. In view of the enormous complexity of the oceanic system this will prove an ambitious, yet resourceful undertaking

Carbohydrates in the North Sea during spring blooms of <i>Phaeocystis</i>: a specific fingerprint

Regional and temporal variation in the composition of water-soluble carbohydrates from Phaeocystis colonies sampled in the southern North Sea was small during spring 1994, except for a high variability in the contribution of glucose. Glucose is universally present in storage products of microalgae; the relative constancy of the carbohydrate pattern of the other monosaccharides suggests that these are part of the more refractory colony mucus. In all Phaeocystis samples arabinose dominated, followed by xylose (Belgian coast) or galactose and mannose (Dutch coast). Rhamnose, glucuronate and O-methylated sugars were present in lower amounts. The latter, always present in samples containing Phaeocystis, may be typical for North Sea strains. The sugar patterns we report here differ from those presented in the literature concerning Phaeocystis-derived material, and also from the sugar fingerprint in the preceding diatom bloom. The Phaeocystis mucus apparently behaves as particulate matter since it was retained on filters of over 1 um. This characteristic together with its refractory nature, typical of 'transparent exopolymer particles' (TEPs), must have consequences for the heterotrophic microbial community in terms of adherence and substrate availability

High acrylate concentrations in the mucus of Phaeocystis globosa colonies

Acrylate produced from dimethylsulphoniopropionate (DMSP) by Phaeocystis has been claimed to inhibit bacterial growth. However, the concentrations of acrylate measured in seawater during Phaeocystis blooms are not high enough to expect inhibition of bacterial growth. In this study, the total acrylate in Phaeocystis cultures free from bacteria was measured. The concentration found in the exponential phase of growth was similar (0.1 to 1.0 mu M) to earlier field reports, but the amount found in the stationary phase of growth was much higher (1 to 4 mu M). Acrylate in cultures, as well as in field samples. was found to be located in the mucous layer of the colony. 'Microscale' concentrations in that layer were more than 1000-fold higher (1.3 to 6.5 mM) than the total concentration found in the unfractionated culture. Such high concentrations could have an antimicrobial effect. However, acrylate appears to be adsorbed to the mucus and may be inaccessible to bacteria. including those that consume acrylate. As soon as the colonies started to decay, acrylate was released into the surrounding environment, and since it is not detected in bloom samples, it is apparently consumed by bacteria

Isolation and characterisation of the marine ultramicrobacterium Sphingomonas sp. strain RB2256

Indigenous heterotrophic marine bacteria are of great importance to global nutrient cycling. Predominant native bacteria are of ultramicrobacterial dimensions, are not associated with aggregates and must have truly remarkable abilities for substrate capture. Agar media are unsuited for the isolation of the dominant oceanic bacterioplankton. In contrast, the dilution culture technique [Button et al. (1993) Appl. Environ. Microbiol. 59, 881-891; Schut et al. (1993) Appl. Environ. Microbiol. 59, 2150-2160] leads to a successful enrichment of the dominant cell types. Up to 50% of the indigenous bacterial population in water obtained from Resurrection Bay was able to grow in dilution tubes containing only filtered, autoclaved natural sea water (FAS). Ultramicrobacteria (UMB), very small bacteria with small genomes, predominate in such cultures. Generally, dilution factors that resulted in inocula of approximately 2 cells per tube were optimal and prevented outgrowth of atypical large bacteria. Strain RB2256 [Schut et al. (1993) Appl. Environ. Microbiol. 59, 2150-2160], one of the UMB isolated by this dilution culture technique and tentatively identified as a marine Sphingomonas sp., was investigated in more detail. Although reverted from obligately oligotrophic to facultatively oligotrophic upon isolation, this strain possessed a number of traits assigned to a 'model oligotroph' and some unpredicted novel properties. The cells showed no miniaturisation upon starvation but consistently exhibited low cell volumes. They had a very low DNA content, were rich in protein, and contained only one copy of the rRNA operon. The cells were well adapted to the simultaneous utilisation of mixed substrates. A constitutive, high-affinity and binding protein-dependent uptake system for mixed amino acids was found that would allow realistic in situ generation times at the prevailing amino acid concentrations. Further studies on this same organism revealed that the cells appeared to be extremely resistant to various stress-inducing agents. High survival rates were observed after high-intensity heat shocks, treatments with H2O2 or with ethanol. Moreover, no marked differences were observed between starved or actively growing cells in this respect, particularly when cells were grown in chemostat. Application of the dilution culture technique to the field of subsurface microbiology could be adopted to study the occurrence of UMB in groundwater with a comparable and stable 'low-nutrient-conditioned' phenotype