Symbiotic Relations of Sediment-Agglutinating Nematodes and Bacteria in Detrital Habitats: The Enzyme-Sharing Concept

Abstract. A new concept (‘enzyme sharing’) concerning the interaction of marine nematodes and microbes in the degradation of sedimentary detritus is presented. Elements of this concept are (1) the notorious tendency of many aquatic nematodes to agglutinate detrital particles by mucus secretions, (2) new observations on the stimulation of microbial growth by nematodes in agar plates, and (3) literature data on limited endogenous proteolytic capacities of aquatic nematodes.
Observations on nematode‐microbe associations in agar plates prompted the conceptual synthesis. In agar medium without the addition of any nutrients a spectacular growth of bacteria was visible on the sinusoidal crawling trails of nematodes only 2 – 3 days after introduction of the worms (species of Adoncholaimus, Anoplostoma and Sabatieria). Juveniles of Anoplostoma that hatched in the agar cultures left their minute trails in the medium and these were rapidly occupied by bacteria. The nematodes repeatedly visited their bacterial trails, which persisted as a peculiar biotic structure for more than one year and survived the nematodes.
In sterile agar preparations containing the fluorogenic methylumbelliferyl‐β‐glucoside in the presence of the nematode Adoncholaimus, an enhanced fluorescence of the medium was visible, indicating β‐glucosidase activity. We therefore assume that oncholaimid nematodes discharge enzymes that alone, or in concert with microbial activities, contribute to the hydrolytic cleavage of refractory polysaccharides containing β‐glucosidic bonds such as agar components and cellulose. The sugars thus produced may then be taken up by the nematodes and concomitantly support the conspicuous growth of microbes. Since we did not observe any feeding of the nematodes on the associated microbes in agar plates, we question the nutritive potential of intact microbial cells for a number of nematodes abounding in detrital habitats, and call attention to the significance of ambient dissolved or adsorbed organic monomeric nutrients.
Consequently, we perceive the puzzling perpetual accretion of detrital organic particles to sediment agglutinations by nematodes as an adaptation for operating an ‘enzymatic reactor’ for the production of dissolved nutrients. We hypothesise a relationship of mutual commensalism of nematodes and heterotrophic microbes in detrital habitats and propose the term ‘enzyme sharing’ for this relationship. Both parties invest in a common enzyme pool that decomposes organic detritus for their nutrition. We present here evidence that nematodes contribute β‐glucosidase, which is involved in the cellulase system. Data from the literature suggest that microbial enzymatic processing of detrital proteins yields amino acids available to nematodes, which apparently have no efficient proteolytic enzyme system in their intestines.     

Food Preferences of Dominant Salt Marsh Herbivores and Detritivores

Abstract. Food preferences of six dominant salt marsh invertebrates were studied to identify detritivores and to assess differences in their diets. Animals fed on agar suspensions of dead and live foods in petri dishes with four compartments. Only two foods were included in each dish. Relative palatability was assessed by measuring the amount of suspension removed or by counting the number of feeding marks left on the surface of the suspensions.
Feeding marks reflected differences in mouth parts and feeding mechanisms of the six invertebrates. Melampus bidentatus, Orchestia grillus , and Philoscia vittata preferred dead litter over live tissues of marsh graminoids, blue-green algae, and sulfur bacteria. No preference for detritus from different graminoids was shown by M. bidentatus. Orchestia grillus fed preferentially on Spartina patens detritus. Philoscia vittata preferred detritus from S. alterniflora and S. patens. Blue-green algae and sulfur bacteria were preferred over detritus by Littorina saxatalis , but detritus was preferred over live graminoids. Littorina saxatalis fed preferentially on Juncus gerardi detritus. Blue-green algae, sulfur bacteria, and live graminoids were preferred over detritus by L. littorea. Talorchestia longicornis also preferred blue-green algae.
On the basis of their food preferences, Littorina saxatalis, Melampus bidentatus, Orchestia grillus , and Philoscia vittata were classified as detritivores. Feeding on detritus from different plant species could result in a partitioning of this food resource in the detritus-based food chains of the salt marsh ecosystem.     

Retailing local food in the Scottish-English borders: A supply chain perspective

Local food is championed as one alternative response to industrial systems of food production and supply. While advocacy for local food is high, there is a lack of empirical evidence about the actual shape and scale of such food supply chains, especially from a retail perspective. Using supply chain diagrams, this paper presents a summary of ‘new’ agro-food geographies for five different retail types—farm shops, butchers, caterers, specialist shops, supermarkets/department stores—that all source local food from suppliers in the Scottish-English borders. Presented as five separate ‘shopping trips’, the paper examines where, how and why retailers source local food. Results reveal the complex nature of local food systems, especially in terms of intra-sector competitive dynamics (with a notable tension between direct forms of retail and established (independent) retailers), links and overlaps with ‘normal’ food retail systems and elastic notions of the ‘local’. The paper also draws a key distinction between locally produced and locally supplied food products.     

Withered Geography

This commentary is a response to an article by Jay R. Harman in the November 2003 issue of The Professional Geographer. I argue that Harman's claim that scholarly disciplines offer social “returns” in a competitive “market” obscures the fundamentally political nature of how social resources are allocated and how social needs are defined. Harman would have us subordinate scholarly research to agendas set elsewhere, by politicians and other powerful interests, but I argue that such a vision would turn geographers into mere technicians. A healthier role for the discipline is for geographers to seek ways of asserting intellectual leadership and of shaping social agendas along more humane and socially just lines.     

Food Deserts in the Prairies? Supermarket Accessibility and Neighborhood Need in Edmonton,Canada*

The U.S. and U.K. literatures have discussed “food deserts,” reflecting populated, typically urban, low‐income areas with limited access to full‐service supermarkets. Less is known about supermarket accessibility within Canadian cities. This article uses the minimum distance and coverage methods to determine supermarket accessibility within the city of Edmonton, Canada, with a focus on high‐need and inner‐city neighborhoods. The results show that for 1999 both of these areas generally had higher accessibility than the remainder of the city, but six high‐need neighborhoods had poor supermarket accessibility. We conclude by examining potential reasons for differences in supermarket accessibility between Canadian, U.S., and U.K. cities.     

RACIALIZED TOPOGRAPHIES: ALTITUDE AND RACE IN SOUTHERN CITIES*

ABSTRACT. This study examines altitudinal residential segregation by race in 146 cities in the U.S. South. It begins by embedding the topic in recent theorizations of the social construction of nature, the geography of race, and environmental justice. Second, it focuses on how housing markets, particularly in the South, tend to segregate minorities in low‐lying, flood‐prone, and amenity‐poor segments of urban areas. It tests empirically the hypothesis that blacks are disproportionately concentrated in lower‐altitude areas using gis to correlate race and elevation by digital elevation‐model block group within each city in 1990 and 2000. The statistical results confirm the suspected trend. A map of coefficients indicates strong positive associations in cities in the interior South‐where the hypothesis is confirmed‐and an inverse relationship near the coast, where whites dominate higher‐valued coastal properties. Selected city case studies demonstrate these relationships connecting the broad dynamics of racial segregation to the particularities of individual places.     

Managing sustainable farmed landscape through 'alternative' food networks: a case study from Italy

This paper focuses on a case study of an 'alternative' food network based in the Abruzzo National Park, Italy, to explore how ideas of sustainable farmland management can be expressed through broader understandings of developing networks of care concerned with local economies and societies, high-quality specialist food products, particular 'traditional' farming practices and livestock breeds, as well as the ecology of a farmed landscape. The scheme allows customers, internationally as well as in Italy, to 'adopt' a milking sheep on a large mountain farm. In return, adopters are sent food products from the farm. The adoption scheme is inter-twined with an agri-tourism project which provides accommodation, runs a restaurant and engages in educational activities. The scheme is the result of the individual initiative of its founder, and is associated with a strongly expressed ethical position concerning the value of sustaining valued local rural landscapes and lifestyles, and the importance of 'reconnecting' urban dwellers with rural areas, farming and 'quality' food production. Yet the localness of the scheme is sustained through wider national and international networks: volunteer and paid workers are drawn from several European countries, funding has been acquired from the EU LEADER programme, and internet and transport technologies are essential in connecting with and supplying an international customer base. The broader economy of care instanced in this case study draws attention to a need to develop strategies for sustainable farmland management constructed around wider programmes of social, economic and cultural, as well as environmental, concern.     

Can we sustain sustainable agriculture? Learning from small-scale producer-suppliers in Canada and the UK

There has been particular interest in 'alternative' food over the last 10 years, with many policymakers and researchers throughout the Minority World following a growing number of consumers and producers in supporting organic farming and a host of 'alternative' food networks. To date, there has been a tendency for theory and policy to emerge somewhat divorced from the grounded practices and experiences of producer-suppliers themselves within these networks. Urging a shift from 'alternativity' to 'sustainability' as a more critical and valuable tool to analyse food networks, this paper draws upon in-depth ethnographic research with small-scale producer-supplier case studies in south Wales and southern Ontario. In so doing it explores often overlooked voices and stories within sustainable food discourses. Focusing on the value of farmer-led understandings and responses, the paper highlights important implications for policymakers and consumers and outlines future research on sustainable food networks.