Dissolved organic matter and heterotrophic microneuston in the surface microlayers of the north atlantic

Highly active bacterial DMSP metabolism in the surface microlayer of the eastern China marginal seas

The microbial cycling of dimethylsulfoniopropionate (DMSP) and the resulting gaseous catabolites dimethylsulfide (DMS) or methylmercaptan (MeSH) play key roles in the global sulfur cycle and potentially climate regulation. As the ocean-atmosphere boundary, the sea surface microlayer (SML) is important for the generation and emission of DMS and MeSH. However, understanding of the microbial DMSP metabolism remains limited in the SML. Here, we studied the spatiotemporal differences for DMS/DMSP, bacterial community structure and the key bacterial DMSP metabolic genes between SML and subsurface seawater (SSW) samples in the eastern China marginal seas (the East China Sea and Yellow Sea). In general, DMSP_d and DMSP_t concentrations, and the abundance of total, free-living and particle-associated bacteria were higher in SML than that in SSW. DMSP synthesis (~7.81-fold for dsyB, ~2.93-fold for mmtN) and degradation genes (~5.38-fold for dmdA, ~6.27-fold for dddP) detected in SML were more abundant compared with SSW samples. Free-living bacteria were the main DMSP producers and consumers in eastern Chinese marginal sea. Regionally, the bacterial community structure was distinct between the East China Sea and the Yellow Sea. The abundance of DMSP metabolic genes (dsyB, dmdA, and dddP) and genera in the East China Sea were higher than those of the Yellow Sea. Seasonally, DMSP/DMS level and DMSP metabolic genes and bacteria were more abundant in SML of the East China Sea in summer than in spring. Different from those in spring, Ruegeria was the dominant DMSP metabolic bacteria. In conclusion, the DMSP synthesis and degradation showed significant spatiotemporal differences in the SML of the eastern China marginal seas, and were consistently more active in the SML than in the SSW.

The Role of Extracellular Carbonic Anhydrase in Biogeochemical Cycling: Recent Advances and Climate Change Responses

Climate change has been predicted to influence the marine phytoplankton community and its carbon acquisition strategy. Extracellular carbonic anhydrase (eCA) is a zinc metalloenzyme that catalyses the relatively slow interconversion between HCO₃⁻ and CO₂. Early results indicated that sub-nanomolar levels of eCA at the sea surface were sufficient to enhance the oceanic uptake rate of CO₂ on a global scale by 15%, an addition of 0.37 Pg C year⁻¹. Despite its central role in the marine carbon cycle, only in recent years have new analytical techniques allowed the first quantifications of eCA and its activity in the oceans. This opens up new research areas in the field of marine biogeochemistry and climate change. Light and suitable pH conditions, as well as growth stage, are crucial factors in eCA expression. Previous studies showed that phytoplankton eCA activity and concentrations are affected by environmental stressors such as ocean acidification and UV radiation as well as changing light conditions. For this reason, eCA is suggested as a biochemical indicator in biomonitoring programmes and could be used for future response prediction studies in changing oceans. This review aims to identify the current knowledge and gaps where new research efforts should be focused to better determine the potential feedback of phytoplankton via eCA in the marine carbon cycle in changing oceans.

Global reduction of in situ CO2 transfer velocity by natural surfactants in the sea-surface microlayer

For decades, the effect of surfactants in the sea-surface microlayer (SML) on gas transfer velocity (k) has been recognized; however, it has not been quantified under natural conditions due to missing coherent data on in situ k of carbon dioxide (CO2) and characterization of the SML. Moreover, a sea-surface phenomenon of wave-dampening, known as slicks, has been observed frequently in the ocean and potentially reduces the transfer of climate-relevant gases between the ocean and atmosphere. Therefore, this study aims to quantify the effect of natural surfactant and slicks on the in situ k of CO2. A catamaran, Sea Surface Scanner (S3), was deployed to sample the SML and corresponding underlying water, and a drifting buoy with a floating chamber was deployed to measure the in situ k of CO2. We found a significant 23% reduction of k above surfactant concentrations of 200 µg Teq l-1, which were common in the SML except for the Western Pacific. We conclude that an error of approximately 20% in CO2 fluxes for the Western Pacific is induced by applying wind-based parametrization not developed in low surfactant regimes. Furthermore, we observed an additional 62% reduction in natural slicks, reducing global CO2 fluxes by 19% considering known frequency of slick coverage. From our observation, we identified surfactant concentrations with two different end-members which lead to an error in global CO2 flux estimation if ignored.

The Virioneuston: A Review on Viral⁻Bacterial Associations at Air⁻Water Interfaces

Vast biofilm-like habitats at air–water interfaces of marine and freshwater ecosystems harbor surface-dwelling microorganisms, which are commonly referred to as neuston. Viruses in the microlayer, i.e., the virioneuston, remain the most enigmatic biological entities in boundary surface layers due to their potential ecological impact on the microbial loop and major air–water exchange processes. To provide a broad picture of the viral–bacterial dynamics in surface microlayers, this review compiles insights on the challenges that viruses likely encounter at air–water interfaces. By considering viral abundance and morphology in surface microlayers, as well as dispersal and infection mechanisms as inferred from the relevant literature, this work highlights why studying the virioneuston in addition to the bacterioneuston is a worthwhile task. In this regard, major knowledge gaps and possible future research directions are discussed.

Blue pigmentation of neustonic copepods benefits exploitation of a prey-rich niche at the air-sea boundary

The sea-surface microlayer (SML) at the air-sea interface is a distinct, under-studied habitat compared to the subsurface and copepods, important components of ocean food webs, have developed key adaptations to exploit this niche. By using automated SML sampling, high-throughput sequencing and unmanned aerial vehicles, we report on the distribution and abundance of pontellid copepods in relation to the unique biophysicochemical signature of the SML. We found copepods in the SML even during high exposure to sun-derived ultraviolet radiation and their abundance was significantly correlated to increased algal biomass. We additionally investigated the significance of the pontellids’ blue pigmentation and found that the reflectance peak of the blue pigment matched the water-leaving spectral radiance of the ocean surface. This feature could reduce high visibility at the air-sea boundary and potentially provide camouflage of copepods from their predators.

Taxon-specific aerosolization of bacteria and viruses in an experimental ocean-atmosphere mesocosm

Ocean-derived, airborne microbes play important roles in Earth’s climate system and human health, yet little is known about factors controlling their transfer from the ocean to the atmosphere. Here, we study microbiomes of isolated sea spray aerosol (SSA) collected in a unique ocean–atmosphere facility and demonstrate taxon-specific aerosolization of bacteria and viruses. These trends are conserved within taxonomic orders and classes, and temporal variation in aerosolization is similarly shared by related taxa. We observe enhanced transfer into SSA of Actinobacteria, certain Gammaproteobacteria, and lipid-enveloped viruses; conversely, Flavobacteriia, some Alphaproteobacteria, and Caudovirales are generally under-represented in SSA. Viruses do not transfer to SSA as efficiently as bacteria. The enrichment of mycolic acid-coated Corynebacteriales and lipid-enveloped viruses (inferred from genomic comparisons) suggests that hydrophobic properties increase transport to the sea surface and SSA. Our results identify taxa relevant to atmospheric processes and a framework to further elucidate aerosolization mechanisms influencing microbial and viral transport pathways.

Factors controlling the transfer of microbes from the ocean to the atmosphere are unclear. Here, Michaud et al. study this process in an enclosed ocean-atmosphere facility, and show that the degree of aerosolization of bacteria and viruses is taxon-specific.

Marine Aerosols and Clouds

The role of marine bioaerosols in cloud formation and climate is currently so uncertain that even the sign of the climate forcing is unclear. Marine aerosols form through direct emissions and through the conversion of gas-phase emissions to aerosols in the atmosphere. The composition and size of aerosols determine how effective they are in catalyzing the formation of water droplets and ice crystals in clouds by acting as cloud condensation nuclei and ice nucleating particles, respectively. Marine organic aerosols may be sourced both from recent regional phytoplankton blooms that add labile organic matter to the surface ocean and from long-term global processes, such as the upwelling of old refractory dissolved organic matter from the deep ocean. Understanding the formation of marine aerosols and their propensity to catalyze cloud formation processes are challenges that must be addressed given the major uncertainties associated with aerosols in climate models.

Seasonal variations of the sea surface microlayer at the Boknis Eck Times Series Station (Baltic Sea)

The sea surface microlayer (SML) is the uppermost layer of the water column that links the ocean and atmosphere. It accumulates a variety of biogenic surface-active and buoyant substances, including gelatinous material, such as transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP), potentially affecting air-sea exchange processes. Here, we studied the influence of the annual cycle of phytoplankton production on organic matter (OM) accumulation in the SML relative to the subsurface water (SSW). Sampling was performed monthly from April 2012 to November 2013 at the Boknis Eck Time Series Station (Baltic Sea). For SML sampling, we used the Garrett screen, while SSW samples were collected by Niskin bottles at 1 m depth. Samples were analyzed for carbohydrates, amino acids, TEP, CSP, chlorophyll a (SSW only) and bacterial abundance. Our data showed that the SML reflected the SSW during most parts of the year, with changes mainly responding to bloom formation and decay. OM composition during phytoplankton blooms clearly differed from periods of higher bacterial abundance. Of all components investigated, only the enrichment of total carbohydrates in the SML was inversely related to the wind speed indicating that wind-driven mixing also affected the accumulation of OM in the SML during our study.

Distribution of surfactants along the estuarine area of Selangor River, Malaysia

This study aims to determine the levels of methylene blue active substances (MBAS) and ethyl violet active substances (EVAS) as anionic surfactants and of disulphine blue active substances (DBAS) as cationic surfactants in the surface microlayer (SML) around an estuarine area using colorimetric methods. The results show that the concentrations of surfactants around the estuarine area were dominated by anionic surfactants (MBAS and EVAS) with average concentrations of 0.39 and 0.51 μmol L⁻¹, respectively. There were significant between-station differences in surfactant concentrations (p<0.05) with higher concentrations found at the stations near the sea. The concentration of surfactants was higher during the rainy season than the dry season due to the influence of runoff water. Further investigation using total organic carbon (TOC) and total organic nitrogen (TON) shows that there is a significant correlation (p<0.05) between both anionic and cationic surfactants and the TON concentration.

Analysis of antibiotic resistance in bacteria isolated from the surface microlayer and underlying water of an estuarine environment

We compared the prevalence of cultivable antibiotic-resistant bacteria and resistance genes in the surface microlayer (SML) and underlying waters (UW) of an estuary. Prevalence of resistant bacteria was determined in antibiotic-supplemented agar. Bacterial isolates from the UW (n=91) and SML (n=80), selected in media without antibiotic, were characterized concerning susceptibility against nine antibiotics. The presence of genes bla(TEM), bla(OXA-B), bla(SHV), bla(IMP), tet(A), tet(B), tet(E), tet(M), cat, sul1, sul2, sul3, aadA, IntI1, IntI2, and IntI3 was assessed by PCR. The variable regions of integrons were sequenced. Ampicillin- and streptomycin-resistant bacteria were significantly more prevalent in SML. Resistance levels among the bacterial collections were generally low, preventing detection of significant differences between SML and UW. The tet(E) gene was detected in two Aeromonas isolates and tet(M) was detected in a Pseudomonas isolate. Gene sul1 was amplified from three Aeromonas isolates. Prevalence of intI genes was 2.11%. Cassette arrays contained genes encoding resistance to aminoglycosides and chloramphenicol. A higher prevalence of antibiotic-resistant bacteria in the SML, although only detectable when bacteria were selected in antibiotic-supplemented agar, suggests that SML conditions select for antibiotic resistance. Results also showed that antibiotic resistance was uncommon among estuarine bacteria and the resistance mechanisms are probably predominantly intrinsic.

Plankton reach new heights in effort to avoid predators

The marine environment associated with the air-water interface (neuston) provides an important food source to pelagic organisms where subsurface prey is limited. However, studies on predator-prey interactions within this environment are lacking. Copepods are known to produce strong escape jumps in response to predators, but must contend with a low-Reynolds-number environment where viscous forces limit escape distance. All previous work on copepod interaction with predators has focused on a liquid environment. Here, we describe a novel anti-predator behaviour in two neustonic copepod species, where individuals frequently exit the water surface and travel many times their own body length through air to avoid predators. Using both field recordings with natural predators and high-speed laboratory recordings, we obtain detailed kinematics of this behaviour, and estimate energetic cost associated with this behaviour. We demonstrate that despite losing up to 88 per cent of their initial kinetic energy, copepods that break the water surface travel significantly further than those escaping underwater and successfully exit the perceptive field of the predator. This behaviour provides an effective defence mechanism against subsurface-feeding visual predators and the results provide insight into trophic interactions within the neustonic environment.

Enrichment and association of bacteria and particulates in salt marsh surface water

Elevated counts of bacteria were found during outgoing tides in surface microlayers ( approximately 300 mum) of Sippewissett salt marsh, Falmouth, Massachusetts, and Palo Alto salt marsh, Palo Alto, California. At both sampling sites, the degrees by which bacteria were concentrated into the surface microlayer were linearly dependent upon surface concentration of particulate material. A significant percentage of bacteria in the microlayer were found to be attached to particulate material, while bacterial populations in the subsurface water were largely planktonic. Proportions of the bacterial populations which could be grown on seawater nutrient agar were also greater in the microlayer than in the subsurface waters and were positively correlated with the fraction of bacteria attached to particulate matter. Data from these studies suggest that particulates in the microlayer waters of the salt marsh influenced the observed increase in both the readily grown and the total numbers of bacteria.

Bacterial predator-prey interaction at low prey density

A bacterial predator-prey interaction was studied using Bdellovibrio and bioluminescent prey bacteria. The attacking bdellovibrio causes decay of bioluminescence, which is correlated with bdellovibrio penetration into the prey. The behavior of the prey and predator populations over time was found to be well described by a Lotka-Volterra model. By using this model, the probability of bdellovibrio penetration after encountering a prey cell was found to be approximately 3.0%. The prey density required to give the bdellovibrios a 50% chance of survival was calculated to be at least 3.0 x 10 cells per ml, and the density required for population equilibria was calculated to be about 7 x 10 prey bacteria per ml. These values, not generally characteristic of natural habitats, suggest that the existence of Bdellovibrio in nature is limited to special ecological niches.

The bacterial community composition of the surface microlayer in a high mountain lake

The existence of bacterioneuston in aquatic ecosystems is well established, but little is known about its composition and dynamics, particularly in lakes. The bacterioneuston underlies extreme conditions at the air–water boundary, which may influence its dynamics in a different way compared with the bacterioplankton. In this study, we assessed quantitative changes in major bacterial groups of the surface microlayer (SML) (upper 900 μm) and the underlying water (ULW) (0.2–0.5 m depth) of an alpine lake during two consecutive ice-free seasons. Analysis of the bacterial community composition was done using catalyzed reporter deposition FISH with oligonucleotide probes. In addition, several physicochemical parameters were measured to characterize these two water layers. Dissolved organic carbon was consistently enriched in the SML and the dissolved organic matter pool presented clear signals of photodegradation and photobleaching. The water temperature was generally colder in the SML than in the subsurface. The bacterial community of the SML and the ULW was dominated by Betaproteobacteria and Actinobacteria. The bacterial community composition was associated with different combinations of physicochemical factors in these two layers, but temporal changes showed similar trends in both layers over the two seasons. Our results identify the SML of alpine lakes as a microhabitat where specific bacterial members such as of Betaproteobacteria seem to be efficient colonizers.

Characterization of Streptomyces spp. isolated from the sea surface microlayer in the Trondheim Fjord, Norway

The water surface microlayer is still poorly explored, although it has been shown to contain a high density of metabolically active bacteria, often called bacterioneuston. Actinomycetes from the surface microlayer in the Trondheim fjord, Norway, have been isolated and characterized. A total of 217 isolates from two separate samples morphologically resembling the genus Streptomyces have been further investigated in this study. Antimicrobial assays showed that about 80% of the isolates exhibited antagonistic activity against non-filamentous fungus, Gram-negative, and Gram-positive bacteria. Based on the macroscopic analyses and inhibition patterns from the antimicrobial assays, the sub-grouping of isolates was performed. Partial 16S rDNAs from the candidates from each subgroup were sequenced and phylogenetic analysis performed. 7 isolates with identical 16S rDNA sequences were further studied for the presence of PKS type I genes. Sequencing and phylogenetic analysis of the PKS gene fragments revealed that horizontal gene transfer between closely related species might have taken place. Identification of unique PKS genes in these isolates implies that de-replication can not be performed based solely on the 16S rDNA sequences. The results obtained in this study suggest that streptomycetes from the neuston population may be an interesting source for discovery of new antimicrobial agents.

Resistance of marine bacterioneuston to solar radiation

A total of 90 bacterial strains were isolated from the sea surface microlayer (i.e., bacterioneuston) and underlying waters (i.e., bacterioplankton) from two sites of the northwestern Mediterranean Sea. The strains were identified by sequence analysis, and growth recovery was investigated after exposure to simulated solar radiation. Bacterioneuston and bacterioplankton isolates were subjected to six different exposure times, ranging from 0.5 to 7 h of simulated noontime solar radiation. Following exposure, the growth of each isolate was monitored, and different classes of resistance were determined according to the growth pattern. Large interspecific differences among the 90 marine isolates were observed. Medium and highly resistant strains accounted for 41% and 22% of the isolates, respectively, and only 16% were sensitive strains. Resistance to solar radiation was equally distributed within the bacterioneuston and bacterioplankton. Relative contributions to the highly resistant class were 43% for gamma-proteobacteria and 14% and 8% for alpha-proteobacteria and the Cytophaga/Flavobacterium/Bacteroides (CFB) group, respectively. Within the gamma-proteobacteria, the Pseudoalteromonas and Alteromonas genera appeared to be highly resistant to solar radiation. The majority of the CFB group (76%) had medium resistance. Our study further provides evidence that pigmented bacteria are not more resistant to solar radiation than nonpigmented bacteria.

Bacterial diversity in the bacterioneuston (sea surface microlayer): the bacterioneuston through the looking glass

The bacterioneuston is defined as the community of bacteria present within the neuston or sea surface microlayer. Bacteria within this layer were sampled using a membrane filter technique and bacterial diversity was compared with that in the underlying pelagic coastal seawater using molecular ecological techniques. 16S rRNA gene libraries of approximately 500 clones were constructed from both bacterioneuston and the pelagic water samples and representative clones from each library were sequenced for comparison of bacterial diversity. The bacterioneuston was found to have a significantly lower bacterial diversity than the pelagic seawater, with only nine clone types (ecotaxa) as opposed to 46 ecotaxa in the pelagic seawater library. Surprisingly, the bacterioneuston clone library was dominated by 16S rRNA gene sequences affiliated to two groups of organisms, Vibrio spp. which accounted for over 68% of clones and Pseudoalteromonas spp. accounting for 21% of the library. The dominance of these two 16S rRNA gene sequence types within the bacterioneuston clone library was confirmed in a subsequent gene probing experiment. 16S rRNA gene probes specific for these groups of bacteria were designed and used to probe new libraries of 1000 clones from both the bacterioneuston and pelagic seawater DNA samples. This revealed that 57% of clones from the bacterioneuston library hybridized to a Vibrio sp.-specific 16S rRNA gene probe and 32% hybridized to a Pseudoalteromonas sp.-specific 16S rRNA gene probe. In contrast, the pelagic seawater library resulted in only 13% and 8% of 16S rRNA gene clones hybridizing to the Vibrio sp. and Pseudoalteromonas sp. probes respectively. Results from this study suggest that the bacterioneuston contains a distinct population of bacteria and warrants further detailed study at the molecular level.

Epibiotic microorganisms on copepods and other marine crustaceans

Although the occurrence of microbial (algal, protozoan, bacterial, and fungal) epibionts on marine crustaceans and other invertebrates has been documented repeatedly, the ecological context and significance of these relationships generally are not well understood. Recently, several studies have examined the population and community ecology of algal and protozoan epibionts on freshwater crustaceans. Even so, the study of microbial epibionts in aquatic environments is still in its infancy. In this review, we summarize associations of microalgae, protozoans, and bacteria with marine crustaceans, especially copepods. We note differences and commonalities across epibiont taxa, consider host-epibiont cycling of nutrients, generate hypotheses relevant to the ecology of the host and the epibiont, and suggest future research opportunities.

Phenanthrene degradation by estuarine surface microlayer and bulk water microbial populations

Paired surface microlayer and bulk water samples from five sites in the Great Bay Estuary, New Hampshire, were examined with regard to numbers of bacteria,(14)C-phenanthrene biodegradation potentials, and organic and inorganic chemical characteristics. Microlayer samples were generally enriched in nutrients (N and P), dissolved organic matter, and culturable heterotrophic bacteria compared with their corresponding bulk waters. Microlayer samples from marina environments were also enriched in aromatic hydrocarbons, as determined by UV spectrophotometric and fluorometric analyses, and demonstrated substantial phenanthrene biodegradation activity in the assay employed. Biodegradation activity of marina bulk water samples ranged from nil to levels exceeding those exhibited by microlayer samples. No diminution of biodegradation activity was observed after filtration (1.2 μm effective retention) of microlayer water, indicating that the responsible organisms were not particle-associated. Phenanthrene-degrading bacteria, enumerated by counting clearing zones in a crystalline phenanthrene overlay after colony development on a phenanthrene/toluene agar (PTA) medium, were superior to epifluorescence direct counts or standard plate counts on PTA or estuarine nutrient agar in predicting(14)C-phenanthrene biodegradative activity.

Negative staining of freshwater bacterioneuston sampled directly with electron microscope specimen support grids

A technique for observation of surface microlayer bacteria (bacterioneuston) is described, utilizing direct sampling of the air-water interface with carbon-stabilized electron microscope specimen support grids, followed by negative staining and transmission electron microscopy. The method resulted in excellent preservation of forms of microcolonial association, regular surface arrays, surface appendages, and prosthecae in the bacterioneuston of a freshwater pond.

Muramic Acid Measurements for Bacterial Investigations in Marine Environments by High-Pressure Liquid Chromatography

Muramic acid, a constituent of procaryotic cell walls, was assayed by high-pressure liquid chromatography in samples from several marine environments (water column, surface microlayer, and sediment) and a bacterial culture. It is used as a microbial biomass indicator. The method gave a good separation of muramic acid from interfering compounds with satisfactory reproducibility. A pseudomonad culture had a muramic acid content of 4.7 � 10 −10 to 5.3 � 10 −10 μg per cell during growth. In natural water samples, highly significant relationships were found between muramic acid concentrations and bacterial numbers for populations of 10 8 to 10 11 cells per liter. The muramic acid content in natural marine water decreased from 5.3 � 10 −10 to 1.6 � 10 −10 μg per cell with increasing depth. In coastal sediments exposed to sewage pollution, concentrations of muramic acid, ATP, organic carbon, and total amino acids displayed a parallel decrease with increasing distance from the sewage outlet. Advantages of muramic acid measurement by high-pressure liquid chromatography are its high sensitivity and reduction of preparation steps, allowing a short time analysis.

Size of Suspended Bacterial Cells and Association of Heterotrophic Activity with Size Fractions of Particles in Estuarine and Coastal Waters

The size of bacteria and the size distribution of heterotrophic activity were examined in estuarine, neritic, and coastal waters. The data indicated the small size of suspended marine bacteria and the predominance of free-living cells in numerical abundance and in the incorporation of dissolved amino acids. The average per-cell volume of suspended marine bacteria in all environments was less than 0.1 μm 3 . Cell volume ranged from 0.072 to 0.096 μm 3 at salinities of 0 to 34.3‰ in the Newport River estuary, N.C., and from 0.078 to 0.096 μm 3 in diverse areas of the Gulf of Mexico. Thus, the free-living bacteria were too small to be susceptible to predation by copepods. In the Newport River estuary, ca. 93 to 99% of the total number of cells and 75 to 97% of incorporated tritium (from 3 H-labeled mixed amino acids) retained by a 0.2-μm-pore-size filter passed through a 3.0-μm-pore-size filter. Although the amino acid turnover rate per cell was higher for the bacteria in the >3.0-μm size fraction than in the <3.0-μm size fraction, the small number of bacteria associated with the >3.0-μm size particles resulted in the low relative contribution of attached bacteria to total heterotrophic activity in the estuary. For coastal and neritic samples, collected off the coast of Georgia and northeast Florida and in the plume of the Mississippi River, 56 to 98% of incorporated label passed through a 3.0-μm-pore-size filter. The greatest activity in the >3.0-μm fraction in the Georgia Bight was at nearshore stations and in the bottom samples. Our data were consistent with the hypothesis that resuspension of bottom material is an important factor in influencing the proportion of heterotrophic activity attributable to particle-associated bacteria.

Response of marine bacterioplankton to differential filtration and confinement

The bacterioplankton community of confined seawater at 25 degrees C changed significantly within 16 h of collection. Confinement increased CFU, total cell number (by epifluorescence microscopy), and average cell volume of bacterioplankton and increased the turnover rate of amino acids in seawater sampled at Frying Pan Shoals, N.C. The bacterioplankton community was characterized by two components: differential doubling times during confinement shifted dominance from bacteria which were nonculturable to bacteria which were culturable on a complex nutrient medium. Culturable cells (especially those of the genera Pseudomonas, Alcaligenes, and Acinetobacter) increased from 0.08% of the total cell number in the seawater immediately after collection to 13% at 16 h and 41% at 32 h of confinement. Differential filtration before confinement indicated that particles passing through a 3.9-microns-, but retained by a 0.2-micron-, pore-size Nuclepore filter may be a major source of primary amines to the confined population. The 3.0-microns filtration increased growth rate and ultimate numbers of culturable cells through the removal of bacterial predators or the release of primary amines from cells damaged during filtration or both.

Metabolic and ultrastructural response to glucose of two eurytrophic bacteria isolated from seawater at different enriching concentrations

Two marine bacteria, an Acinetobacter sp. (strain GO1) and a vibrio sp. (strain G1), were isolated by extinction dilution and maintained in natural seawater supplemented with nitrogen, phosphorus, and glucose at 0.01 and 10 mg of glucose carbon per liter above ambient monosaccharide concentrations, respectively. After 3 days in unsupplemented natural seawater, growth in batch culture with glucose supplements was determined by changes in cell numbers and glucose concentration. The exponential growth of the Acinetobacter strain with added glucose was indistinguishable from that in natural seawater alone, whereas that of the Vibrio strain was more rapid in the presence of glucose supplements, suggesting that the Acinetobacter strain preferred the natural organic matter in seawater as a carbon source. The ultrastructure for both isolates was unaffected by glucose supplements during exponential growth, but there were marked changes in stationary-phase cells. The Vibrio strain formed polyphosphate at 10 mg of glucose carbon per liter, whereas poly-beta-hydroxybutyrate formation occurred at 100 mg and became excessive at 1,000 mg, disrupting the cells. In contrast, the Acinetobacter strain elongated at 100 and 1,000 mg of glucose carbon per liter but failed to show poly-beta-hydroxybutyrate formation. The diversity of responses shown here would not have been detected with a single concentration of substrate, often used in the literature to characterize both pure and natural populations of marine bacteria.

Bacterial activity at the air/water interface

By using substrate molecules of varying degrees of surface activity, we were able to measure some features of bacterial activity in the surface microlayers (SM) and in the subsurface (bulk) water. The fraction of active cells was determined by a combined microautoradiography-epifluorescence (ME) method. Measurements were made of(14)CO2 evolution to determine the rate of respiration. Results from in situ measurements showed no significant difference between fraction of active cells in the SM and in the bulk. This may be due to an exchange of bacteria between SM and bulk. This exchange was assessed by spreading a film of(3)H-palmitic acid on the surface and, after incubation, measuring the amount of labeled cells at the surface and in the bulk. Test bacteria showing a high accumulation at the surface also showed a low exchange between the 2 strata. When low concentrations of added(14)C-protein were used, the respiration measurements showed a lower value for bulk than for interface localized protein. At higher concentrations, the evolved(14)CO2 was the same whether the protein was mixed in the bulk or spread at the surface. When 2.4-12 ng·cm(-2) of(14)C-palmitic acid was spread on the surface, there was a linear relation between turnover time and amount of added substrate. At higher substrate concentrations there was a deviation from the straight line. Results are discussed in terms of the unique habitat found at an interface.

Heterotrophic Bacteria and Bacterivorous Protozoa in Oceanic Macroaggregates

Oceanic macroaggregates (marine snow and Rhizosolenia mats) sampled from the Sargasso Sea are associated with bacterial and protozoan populations up to four orders of magnitude greater than those present in samples from the surrounding water. Filamentous, curved, and spiral bacteria constituted a higher proportion of the bacteria associated with the particles than were found among bacteria in the surrounding water. Protozoan populations were dominated numerically by heterotrophic microflagellates, but ciliates and amoebas were also observed. Macroaggregates are highly enriched heterotrophic microenvironments in the oceans and may be significant for the cycling of particulate organic matter in planktonic food chains.

Protist biogeography

Autecology (cellular organelles and secretions, encystment and dispersal abilities), environmental conditions (physiological tolerances and interaction with other organisms), and evolutionary history contribute to protist biogeography, which manifests ecological and historical aspects. Ecological biogeography is seen in the influence of geochemistry on the distribution of fresh-water phytoflagellates and algae, and of moisture and vegetation type on soil-litter protists. A temporal feature is often present because many protists encyst and respond only to certain ranges of temperature and organic content. Historical biogeography has occurred by radiative host evolution on symbiotic protozoa (e.g., termite flagellates and rumen ciliates), and by the isolating effects of water currents, temperature, and density gradients on oceanic protists (coccoliths, dinoflagellates, foraminifera, radiolaria, and tintinnines). These two aspects combine in protists living on animal surfaces. Humans affect protist biogeography by increasing parasite ranges through human migrations and by water pollution. They can diminish these situations by disease control and exploiting appropriate ciliates in sewage disposal. Many free-living protozoa appear to be cosmopolitan, but mating types and isoenzyme studies suggest that speciation with its geographic connotations may be more widespread than presently appreciated.