ALBERT

Description: Long sediment cores recovered from the deep portions of Lake Titicaca are used to reconstruct the precipitation history of tropical South America for the past 25,000 years. Lake Titicaca was a deep, fresh, and continuously overflowing lake during the last glacial stage, from before 25,000 to 15,000 calibrated years before the present (cal yr B.P.), signifying that during the last glacial maximum (LGM), the Altiplano of Bolivia and Peru and much of the Amazon basin were wetter than today. The LGM in this part of the Andes is dated at 21,000 cal yr B.P., approximately coincident with the global LGM. Maximum aridity and lowest lake level occurred in the early and middle Holocene (8000 to 5500 cal yr B.P.) during a time of low summer insolation. Today, rising levels of Lake Titicaca and wet conditions in Amazonia are correlated with anomalously cold sea-surface temperatures in the northern equatorial Atlantic. Likewise, during the deglacial and Holocene periods, there were several millennial-scale wet phases on the Altiplano and in Amazonia that coincided with anomalously cold periods in the equatorial and high-latitude North Atlantic, such as the Younger Dryas.

Description: Massive microbial mats covering up to 4-meter-high carbonate buildups prosper at methane seeps in anoxic waters of the northwestern Black Sea shelf. Strong 13C depletions indicate an incorporation of methane carbon into carbonates, bulk biomass, and specific lipids. The mats mainly consist of densely aggregated archaea (phylogenetic ANME-1 cluster) and sulfate-reducing bacteria (Desulfosarcina/Desulfococcusgroup). If incubated in vitro, these mats perform anaerobic oxidation of methane coupled to sulfate reduction. Obviously, anaerobic microbial consortia can generate both carbonate precipitation and substantial biomass accumulation, which has implications for our understanding of carbon cycling during earlier periods of Earth's history.

Description: Burrowing activities of the only European fiddler crab, Uca tangeri, and its resulting influence on biotope in mudflats were investigated during 1989-1990 at Ria Formosa, Portugal. Individuals use the same burrow for ca 1 wk, then occupy another or dig a new one. Overall a burrow is inhabited for ca 3 mo by several individuals before it is abandoned. Vacated burrows decay within 2 to 3 wk. Burrow size and number vary with the season. Burrow density was highest in spring and early summer with ca 17 burrows m-2, and then decreased. Deepest burrows (up to 90 cm long) were found in winter, the shallowest (up to 40 cm long) in summer. Volume of the sediment moved by U. tangen varied monthly between 3000 and 6000 cm3 per m2 of mudflat. Water is only found in the lower third of the burrow. Burrow water contains less oxygen and more nitrate than the surrounding water of the Ria Formosa.

Description: Respiration and activity of eelpouts Zoarces viviparus L. were measured in an underwater respiration chamber in Kiel Bay (Germany) under short-term hypoxia. Respiration and swimming activity both declined almost continuously with decreasing oxygen saturation...

Description: Methane seeps in shallow waters in the northern Kattegat off the Danish coast form spectacular submarine landscapes - the 'bubbling reefs' - due to carbonate-cemented sandstone structures which are colonized by brightly coloured animals and plants. These structures may be 100 m2 in area and consist of pavements, complex formations of overlying slab-type layers, and pillars up to 4 m high. The carbonate cement (high-magnesium calcite, dolomite or aragonite) is 13C-depleted, indicating that it originated as a result of microbial methane oxidation. It is believed that the cementation occurred in the subsurface and that the rocks were exposed by subsequent erosion of the surrounding unconsolidated sediment. The formations are interspersed with gas vents that intermittently release gas, primarily methane, at up to 25 1 h-' The methane most likely originated from the microbial decomposition of plant material eposited during the Eemian and early Weichselian periods, i.e. l00 000 to 125 000 years B.P. Aerobic methane oxidation in the sediment was restricted Lo the upper 4 cm in muddy sand and to the upper 13 cm In coarse sand. Maximum aerobic methane oxidation rates ranged from 4.8 to 45.6 pm01 dm-3 d". The rock surfaces and epifauna around the seeps were also sites of methane-oxidizing activity. Integrated sulphate reduction rates for the upper 10 cm of muddy sand gave 4.2 to 26.6 mm01 m-2 d-' These rates are higher than those previously reported from similar water depths in the Kattegat but did not relate to the sediment methane content. Since gas venting occurs over several km2 of the sea floor in the Kattegat it is likely to make a significant local contribution to the cycling of elements in the sediment and the water column. The rocks support a diverse ecosystem ranging from bacteria to macroalgae and anthozoans. Many animals live within the rocks in holes bored by sponges, polychaetes and bivalves. Stable carbon isotope composition (6'") of tissues of invertebrates from the rocks were in the range -17 to -24 'A, indicating that methane-derived carbon makes little direct contribution to their nutrition. Within the sediments surrounding the seeps there is a poor metazoan fauna, in terms of abundance, diversity and biomass. This may be a result of toxicity due to hydrogen sulphide input from the gas.

Description: During the Bremerhaven Workshop in the southern North Sea, REMOTS sediment profile lmages (SPI) were recorded in order to supplement the benthic component of the workshop and other environmentally relevant parameters investigated along a spatial gradient at an abandoned exploratory drilling site off the Dutch coast. The sampling stations were in accordance with the other studies, but due to bad weather conditions only a small proportion of the intended samples were taken. The profile data were supplemented by video recordings of the sediment surface features taken on a second cruise. The results presented here have important implications for the interpretation of other benthic and sediment samples, and may help to interpret some enigmat~cw orkshop data.

Description: In vertebrates, genes of the major histocompatibility complex (MHC), with their pronounced polymorphism, potentially represent outstanding examples for the selective advantages of genetic diversity (1). Theoretical models predicted that, within an individual, MHC alleles can be subjected to two opposing selective forces, resulting in an optimal number of genes at intermediate individual MHC diversity (2, 3). Diversifying selection increases heterozygosity and enables wider recognition of pathogens (4). This process is opposed by the need to delete T cells that react with self peptide–MHC combinations (5) from the repertoire, which has been proposed as a possible mechanism constraining expansion of MHC genes. Because too high MHC diversity might delimit T cell diversity, it might also impose limitations on the efficiency of pathogen recognition. However, empirical evidence demonstrating fitness benefits in terms of parasite resistance caused by this type of optimal MHC diversity has been lacking. Therefore, we tested whether three-spined sticklebacks (Gasterosteus aculeatus L.) carrying an intermediate level of individual MHC diversity also displayed the strongest level of resistance against parasite infection. Sticklebacks are particularly suited to test MHC optimality, because MHC class II genotypes can differ markedly in the number of MHC class IIB alleles (6). We caught fish from an outbred population and used these to breed six sibships of immunologically naïve fish (i.e., they had no previous contact to parasites). Immunogenetic diversity ranged from three to nine MHC class IIB alleles found in reverse-transcribed messenger RNA (mRNA) [see (6) for details on genotyping]. The MHC genotypes within these sibships segregated above and below the hypothesized optimal number of ∼5 MHC class IIB alleles, which had previously been estimated in an epidemiological field survey (7). In individual infection treatments, fish from all sibships were simultaneously exposed to three of the most abundant parasite species identified in the field (Fig. 1A) (8). After two rounds of infection, separated by an interval of 8 weeks, we found a significant minimal mean infection rate at an intermediate number of individual MHC class IIB variants [i.e., 5.82 expressed alleles (Fig. 1B)]. This result was also confirmed when sibships were considered separately [i.e., 4.96 alleles (Fig. 1C)] (9). The strong pattern only appeared when infection with all three parasites was accounted for simultaneously. This may not be surprising, because single alleles are expected to correlate with single diseases and multiple alleles can contribute to resistance against several infectious agents (2).

Description: In vertebrates, genes of the major histocompatibility complex (MHC), with their pronounced polymorphism, potentially represent outstanding examples for the selective advantages of genetic diversity (1). Theoretical models predicted that, within an individual, MHC alleles can be subjected to two opposing selective forces, resulting in an optimal number of genes at intermediate individual MHC diversity (2, 3). Diversifying selection increases heterozygosity and enables wider recognition of pathogens (4). This process is opposed by the need to delete T cells that react with self peptide–MHC combinations (5) from the repertoire, which has been proposed as a possible mechanism constraining expansion of MHC genes. Because too high MHC diversity might delimit T cell diversity, it might also impose limitations on the efficiency of pathogen recognition. However, empirical evidence demonstrating fitness benefits in terms of parasite resistance caused by this type of optimal MHC diversity has been lacking. Therefore, we tested whether three-spined sticklebacks (Gasterosteus aculeatus L.) carrying an intermediate level of individual MHC diversity also displayed the strongest level of resistance against parasite infection. Sticklebacks are particularly suited to test MHC optimality, because MHC class II genotypes can differ markedly in the number of MHC class IIB alleles (6). We caught fish from an outbred population and used these to breed six sibships of immunologically naïve fish (i.e., they had no previous contact to parasites). Immunogenetic diversity ranged from three to nine MHC class IIB alleles found in reverse-transcribed messenger RNA (mRNA) [see (6) for details on genotyping]. The MHC genotypes within these sibships segregated above and below the hypothesized optimal number of ∼5 MHC class IIB alleles, which had previously been estimated in an epidemiological field survey (7). In individual infection treatments, fish from all sibships were simultaneously exposed to three of the most abundant parasite species identified in the field (Fig. 1A) (8). After two rounds of infection, separated by an interval of 8 weeks, we found a significant minimal mean infection rate at an intermediate number of individual MHC class IIB variants [i.e., 5.82 expressed alleles (Fig. 1B)]. This result was also confirmed when sibships were considered separately [i.e., 4.96 alleles (Fig. 1C)] (9). The strong pattern only appeared when infection with all three parasites was accounted for simultaneously. This may not be surprising, because single alleles are expected to correlate with single diseases and multiple alleles can contribute to resistance against several infectious agents (2).