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S U M M A R YThe sorption of two marine bacteria to surfaces involved an instantaneous reversible phase, and a time-dependent irreversible phase. Reversible sorption of the non-motile Achrornobacter strain R 8 decreased to zero as the electrolyte concentration decreased, or as the thickness of the electrical double-layer increased. The electrolyte concentration at which all bacteria were repelled from the glass surface depended on the valency of the cation. The reversible phase is interpreted in terms of the balance between the electrical double-layer repulsion energies at different electrolyte concentrations and the van der Waals attractive energies. Even at the electrolyte concentration of seawater, the bacteria probably are held at a small distance from the glass surface by a repulsion barrier. Reversible sorption often led to rotational motion of the motile Pseudornonas sp. strain ~3 at a liquid-glass interface.Pseudomonas ~3 produced polymeric fibrils in artificial seawater; these may be concerned in the irreversible sorption of the bacteria to surfaces. Sorption and polymer production were stimulated by 7 mg./l. glucose but higher levels inhibited irreversible sorption. Omission of Ca2+ and Mg2+ from the artificial seawater prevented growth, polymer production, and sorption to surfaces by Pseudomonas R 3 -I N T R O D U C T I O NThe sorption of bacteria to surfaces is a general phenomenon encountered in natural environments with important ecological implications (Wood, 1967; Marshall, 197 I ) . Primary microbial film formation on surfaces immersed in seawater is considered by some investigators to be a prerequisite to fouling by larger organisms such as barnacles (Wood, 1967). The mechanism whereby marine bacteria sorb to surfaces has received scant attentjon.ZoBell (1943) suggested that, once bacteria are attracted to a surface, firm attachment requires incubation for several hours. He attributed this delay to the need for the synthesis of extracellular adhesive materials. Recently, Corpe (1970~2) has reported the production of an extracellular acid polysaccharide by a primary film-forming bacterium, Pseiidomonas atlantica. Glass slides coated with this polymer became fouled with micro-organisms more rapidly than uncoated slides. Corpe (Ig70b) has reviewed the literature on attachment of bacteria to surfaces immersed in marine environments.The present investigation combines a study of some of the colloidal and biological properties of pure cultures of marine bacteria to obtain information on processes involved in the sorption to surfaces.

It has been known for many years that filtrates from axenic algal cultures may be enriched with organic compounds. These materials, including simple amino acids and peptides, sugars, polyalcohols, and occasionally vitamins, enzymes, and toxins, are usually lumped under the term "extracellular products" (Fogg, 1966). Studies using natural populations of phytoplankton have shown that extracellular products are not mere laboratory artifacts, and that, depending upon environmental conditions, they account for 1-20% of the total photoassimilated carbon (Hellebust, 1965;Nalewajko, 1966; Samuel, Shad and Fogg, 1971;Thomas, 1971).The potential significance of extracellular organic material in marine food chains is extremely interesting. Many authors (Fogg, 1966;Brock, 1966;Alexander, 1971 ; Whittaker and Feeney, 1971) have suggested that these products may play an important role in marine food chains, especially as potential nutrients for bacteria. However, to our knowledge, there is no direct evidence that this is so although the ability of bacteria to grow in algal cultures (Vela and Guerra, 1966;Berland, Bianchi and Maestrini, 1969) might be interpreted to support such conclusions.If, in fact, algal extracellular products are important contributors to bacterial food chains, it would seem possible to construct an aquatic counterpart of the wellknown "rhizosphere" of terrestrial ecosystems (Rovira, 1965). A zone may exist, extending outward from an algal cell or colony for an undefined distance, in which bacterial growth is stimulated by extracellular products of the alga. For purposes of discussion in this paper, we will term this region the "phycosphere."Motile bacteria commonly exhibit chemotaxis to concentration gradients of organic material (Weibull, 1960;Adler, 1969). The ecology of chemotaxis by organotrophic bacteria has not been well studied, but highly species-specific responses to certain carbohydrates, amino acids, and nucleotide bases have been observed (Fogel, Chet and Mitchell, 1971), and certain predatory microorganisms have been shown to be chemotactic to their prey (Chet, Fogel and Mitchell, 1971).

Mucus from selected Red Sea coelentcrates was analyzed for protein, polysaccharide, lipid, monosaccharides, and amino acids. While the proportions of the macromolecular fractions of the different mucins varied widely, the individual makeup of the component proteins and polysaccharides was more uniform. The appearance and transformation of liquid mucus into mucus floc or web material was revealed by scanning electron microscopy (SEM). Observations on the mucus web-forming coral Porites astreoides from Barbados suggested that sediment capture is at least partly responsible for the apparent denaturation of secreted mucus and the transformation of liquid mucus into particulate detritus.

The external mucus layers of the stony coral Porites astreoides and the soft corals Palythoa sp. and Heteroxeniu fuscesens are inhabited by communities of marinc heterotrophic bacteria. Population levels of bacteria in coral mucus may be regulated by the self-cleaning behavior of the host. Bacterial populations in coral mucus respond to stresses applied to the host coral by growing to higher population levels in the mucus, indicating that these are populations of viable organisms closely attuned to host metabolism.IMembers of these microbial populations utilize the mucus compounds and may play a role in processing coral mucus for reef detritus feeders. One such species, Vibrio ulginolyticus, grows rapidly on Heteroxeniu mucus, is attracted to dissolved mucus, and possesses a mechanism to maintain itself on the coral surface.

Nitrogen stable-isotope compositions (δ15N) can help track denitrification and N2O production in the environment, as can knowledge of the isotopic discrimination, or isotope effect, inherent to denitrification. However, the isotope effects associated with denitrification as a function of dissolved-oxygen concentration and their influence on the isotopic composition of N2O are not known. We developed a simple steady-state reactor to allow the measurement of denitrification isotope effects in Paracoccus denitrificans. With [dO2] between 0 and 1.2 μM, the N stable-isotope effects of NO3 − and N2O reduction were constant at 28.6‰ ± 1.9‰ and 12.9‰ ± 2.6‰, respectively (mean ± standard error,n = 5). This estimate of the isotope effect of N2O reduction is the first in an axenic denitrifying culture and places the δ15N of denitrification-produced N2O midway between those of the nitrogenous oxide substrates and the product N2 in steady-state systems. Application of both isotope effects to N2O cycling studies is discussed.

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