Organizational Unit:
School of Biological Sciences
School of Biological Sciences
Permanent Link
Research Organization Registry ID
Description
Previous Names
Parent Organization
Parent Organization
Organizational Unit
Includes Organization(s)
Organizational Unit
ArchiveSpace Name Record
Publication Search Results
Now showing
1 - 10 of 50
-
ItemNitrogen stable isotope dynamics in the central Baltic Sea: influence of deep-water renewal on the N-cycle(Georgia Institute of Technology, 1997-11-17) Voss, Maren ; Nausch, Günter ; Montoya, Joseph P.The vertical profiles of NO₃-, NH₄+, O₂, and H₂S as well as the isotopic composition of particulate nitrogen and NH4+ were sampled yearly over a 5 yr period in the Gotland Basin to follow biochemical changes in N-cycling resulting from an inflow of saltwater. The water column has a pronounced interface at 80 to 120 m depth which separates warm (13°C) brackish surface waters (salinity 7 psu) and the underlying cold winter water layer from more saline (9 to 11 psu) bottom waters originating from irregularly occurring inflow events of oxygenated, nitrate-rich North Sea water masses. Anoxic conditions usually exist in the deep stagnant waters, where nutrients only occur as ammonia, which reaches concentrations of up to 30 µmol l-1. In spring 1993 large amounts of nitrate- and oxygen-rich water were transported into the deep waters of the Gotland Basin, thus displacing the stagnant deep water body. With the inflow, oxygen and nitrate concentrations rose by 3 ml l-1 and more than 10 µmol l-1 respectively. During the following years the concentrations of oxygen in the near bottom layer decreased again. The isotope signature of the suspended particles in the layer below 120 m reflects these changes: in 1993 the mean stable nitrogen isotope value in the anoxic water was at 1.1o/oo. We assume bacterial incorporation of ammonia to be the mechanism producing isotopically light particles. A fractionation factor calculated for ammonia uptake of 11 ‰ supports this hypothesis. During the following years the particles in the oxygenated water column were around 8o/oo which is characteristic for microbially degraded material. The surface sediment of the central Gotland Sea has a low isotope signal of 3 to 4o/oo. These findings might have consequences for the interpretation of sediment δ15N data where low isotope contents are usually taken as an indicator of high nutrient concentrations in surface waters.
-
ItemFunctional Significance of the Sexual Dimorphism in the Cephalic Appendages of Euchaeta Rimana Bradford(Georgia Institute of Technology, 1997-09) Boxshall, Geoffrey A. ; Yen, Jeannette ; Strickler, Rudi J.In Euchaeta rimana the cephalic appendages become profoundly sexually dimorphic at the final molt. The enlarged maxillipeds and robust maxillae, which are related to the predatory nature of this pelagic marine copepod are fully functional on the fifth copepodid (CV) males and adult females. The presumed prey detection sensory apparatus, a paired 4-point setal array on the antennules, is located within the capture volume of the feeding current of the CVs and adult females. SEMs reveal specialized basal articulations of the straight and curved setae of this array, that allow setal rotations that streamline the antennule during escape movements. In the final molt of the CV male to the adult stage, the maxillae are reduced to vestiges and the maxillipeds to half the size of the female's. The loss of functional prey capture appendages is accompanied by the loss of the prominent antennulary setal array, giving support for their postulated function as prey sensors. Instead, the male gains 19 more aesthetascs primarily in the proximal region of the antennules, where fluid velocities of the scanning current are greatest. Detection of pheromones transported within the feeding/scanning current is the hypothesized function of the chemosensory system of the adult male copepod. The parallel changes in the structure of the antennule as well as of the adjacent cephalic appendages suggest that a homeobox-like gene control system could be coordinating these morphological changes.
-
ItemSpacing mechanisms within light-induced copepod swarms(Georgia Institute of Technology, 1997-08-28) Leising, Andrew W. ; Yen, JeannetteMany species of copepods form dense aggregations, known as swarms. In the laboratory, we experimentally induced 5 different species of copepod to swarm in response to a point source of light. To map out the (x, y, z, t) positions of swarm members, 2 right-angle views of the 3-dimensional swarm were videotaped. Since images of individual copepods appear indistinguishable on the paired 2-dimensional projections, an algorithm was developed which matched the temporal changes of the vertical (z) positions of all images from the 2-dimensional projections of the 3-dimensional copepod movement to produce (x, y, z, t) positions of each individual. With the temporal/spatial positional data of swarm members, we tested the hypothesis that the fluid disturbance surrounding individual moving copepods, rather than the exoskeleton, maintains minimum separation distance. As the density of the swarm increased, the average nearest-neighbor distance NND decreased, as did the mean minimum NND (MNND). For 3 of the 5 species, the MNND was significantly greater than that predicted from a random distribution, and was greater than twice the antennule or prosome length. While occasional physical contact may occur, resulting in escapes or attempted mating, it appears that most swarm members remain outside the field of self-generated fluid motion in the boundary layers surrounding their neighbors.
-
ItemDeterminants of secondary structure of polypeptide chains: Interplay between short range and burial interactions(Georgia Institute of Technology, 1997-07-15) Kolinski, Andrzej ; Skolnick, JeffreyThe effect of tertiary interactions on the observed secondary structure found in the native conformation of globular proteins was examined in the context of a reduced protein model. Short-range interactions are controlled by knowledge based statistical potentials that reflect local conformational regularities seen in a database of three-dimensional protein structures. Long-range interactions are approximated by mean field, single residue based, centrosymmetric hydrophobic burial potentials. Even when pairwise specific long-range interactions are ignored, the inclusion of such burial preferences noticeably modifies the equilibrium chain conformations, and the observed secondary structure is closer to that seen in the folded state. For a test set of 10 proteins (belonging to various structural classes), the accuracy of secondary structure prediction is about 66% and increases by 9% with respect to a related model based on short-range interactions alone [Kolinski et al., J. Chem. Phys. 103, 4312 (1995)]. The increased accuracy is due to the interplay between the short-range conformational propensities and the burial and compactness requirements built into the present model. While the absolute level of accuracy assessed on a per residue basis is comparable to more standard techniques, in contrast to these approaches, the conformation of the chain now has a better defined geometric context. For example, the assumed spherical domain protein model that simulates the segregation of residues between the hydrophobic core and the hydrophilic surface allows for the prediction of surface loops/turns where the polypeptide chain changes its direction. The implications of having such self-consistent secondary structure predictions for the prediction of protein tertiary structure are briefly discussed.
-
ItemRibonuclease P RNA: Models of the 15/16 Bulge From Escherichia Coli and the P15 Stem Loop of Bacillus Subtilis(Georgia Institute of Technology, 1997-06) Easterwood, Thomas R. ; Harvey, Stephen C.The Escherichia coli ribonuclease P RNA 15/16 internal bulge loop and the Bacillus subtilis P15 stem loop are important substrate binding sites for the CCA-3' terminus of pre-tRNA. Models of E. coli 15/16 bulge loop and the B. subtilis P15 stem loop have been constructed using MC-SYM, a constraint satisfaction program. The models use covariation analysis data for suggesting initial base pairings, chemical probing, and protection/modification results to determine particular pairing orientations, and mutational experimental analysis data for tRNA-RNase P RNA contacts. The structures from E. coli and B. subtilis, although different _in secondary structure, have similar sequence and function. Using MC-SYM, we are able to illustrate how the 3' end of the pre-tRNA is able to interact with this segment of the catalytic RNase P RNA. In addition, we propose additional hydrogen bonding between A76 in the 3' terminus of the tRNA and the 15/16 region of E. coli and to the loop of B. subtilis.
-
ItemInduction of seaweed chemical defenses by amphipod grazing(Georgia Institute of Technology, 1996-12) Cronin, Greg ; Hay, Mark E.Grazing by the generalist amphipod Ampithoe longimana induced increased concentrations of defensive secondary metabolites in the brown alga Dictyota menstrualis and made the seaweed less susceptible to further attack by the amphipod. Although A. longimana preferentially consumes D. menstrualis, its feeding rates can be reduced significantly by high concentrations of diterpenoid dictyols produced by the alga. In 1991, D. menstrualis from sites with high numbers of A. longimana had higher levels of grazing scars, higher concentrations of dictyols, and were less palatable to A. longimana than plants from sites with few amphipods. Among—site differences in palatability to amphipods did not correlate with plant differences in protein, nitrogen, or carbon content. Within a site, plants that had apparent amphipod grazing scars were significantly less palatable to A. longimana than neighboring undamaged plants. Controlled field experiments manipulating A. longimana densities supported the hypothesis that feeding by this amphipod induced elevated chemical defenses in the alga. Compared to undamaged control plants, amphipod—damaged plants had 19—34% more of three diterpenoid secondary metabolites and were 50% less palatable to amphipods. Soluble protein and thallus toughness were unaffected by amphipod grazing and thus could not have caused the differences in palatability. High—pressure liquid chromatography evaluation of adventitious branches growing from blade margins at sites of amphipod grazing scars showed that these branches had significantly elevated levels of two diterpenoids relative to normal blade apices or middles. Thus, the amphipod—induced resistance to further attack occurs through an increase in chemical defenses, and these defenses are, to some extent, localized within the plant thallus. Among—site differences in amphipod densities, grazing scars, seaweed defensive chemistry, and plant palatability that we documented in 1991 varied considerably during 1992 and 1993, suggesting that these interrelationships may be complex. In 1992, A. longimana densities did not differ between sites, and there were no between—site differences in palatability or concentrations of deterrent secondary metabolites. In 1993, however, A. longimana densities did differ between sites, but between—site differences were less dramatic than in 1991. Some secondary metabolites were slightly, but significantly, increased at the site with higher densities of A. longimana, but this had no effect on A. longimana feeding. It has been long recognized that marine herbivores are active participants in seaweed—herbivore interactions and can greatly influence the structure of benthic algal communities. Our findings suggest that seaweeds are not passive participants in these interactions, but can actively alter their susceptibility to herbivores in ecological time. Induced responses to herbivory help explain both spatial (i.e., within—thallus, within—site, and among—site) and temporal variation in the chemical defenses of D. menstrualis.
-
ItemAre tropical plants better defended? Palatability and defenses of temperate vs. tropical seaweeds(Georgia Institute of Technology, 1996-12) Bolser, Robin C. ; Hay, Mark E.Investigators working in both terrestrial and marine systems hypothesize that prey defenses are better developed in tropical than in temperate habitats because tropical consumers are more active and diverse. To investigate the hypothesis that chemical defenses are more potent in tropical than in temperate seaweeds, we conducted 38 feeding assays involving seven genera of algae. We offered tropical and temperate sea urchins the choice of a temperate alga from North Carolina or a closely related tropical alga from the Bahamas. Plants were freeze—dried, ground to a fine powder, embedded in agar, and presented in congeneric pairs (North Carolina species vs. Bahamian species), or same—species pairs, to the temperate urchin Arbacia punctulata and the tropical urchin Lytechinus variegatus. For the genera we tested (the red alga Amphiroa, the green alga Udotea, and the brown algae Dictyopteris, Dictyota, Lobophora, Padina, and Sargassum), we documented a general pattern in which the mean amount of North Carolina seaweeds eaten was approximately twice that of their Bahamian congeners. Dictyota was an exception to the overall pattern, in that mean consumption of temperate and tropical species was equivalent. In addition to showing a latitudinal difference in seaweed palatability, we found that some species exhibited significant, between—population variation in herbivore resistance on a local scale. The most notable example was Dictyota menstrualis from North Carolina. When collected from a shallow inshore site, this species was significantly more palatable than tropical species of Dictyota in four of six contrasts. When collected from a deeper offshore site, it was significantly less palatable than the tropical species in four of six contrasts. Thus, for this species, the between—population variance in palatability within a geographic region (North Carolina) bracketed the variance in palatability that we found among species and between regions. Feeding assays with both temperate and tropical urchins produced similar results, indicating that choices were made based on fundamental characteristics of the algae rather than on greater familiarity with local prey species. To see if chemical defenses could explain the differences in herbivore resistance that we noted using freeze—dried algae, we tested the effects of lipid—soluble and water—soluble extracts from each alga on urchin feeding by incorporating extracts into a standard palatable food. We also measured protein content and percent ash—free dry mass of the seaweeds in order to correlate urchin feeding preferences with these measures of food quality. Lipophilic chemical extracts explained urchin preferences in 9 of 15 (60%) North Carolina—Bahamas contrasts. Water—soluble extracts, protein content, and percent ash—free dry mass generally did not account for feeding patterns seen in the original assays. Our results support the general hypothesis that tropical seaweeds have stronger chemical defenses than temperate seaweeds. For some genera, however, smaller—scale local variability in herbivore resistance within a species can be as striking as overall latitudinal differences.
-
ItemPalatability and chemical defenses of marine invertebrate larvae(Georgia Institute of Technology, 1996-11) Lindquist, Niels Lyle ; Hay, Mark E.Risk of larval mortality is a critical component of models and debates concerning the ecology and evolution of the differing reproductive characteristics exhibited by marine invertebrates. In these discussions, predation often is assumed to be a major source of larval mortality. Despite limited empirical support, most marine larvae are thought to be palatable and broadly susceptible to generalist predators. Previous studies of larval—planktivore interactions have focused primarily on larvae that typically feed, grow, and develop for weeks to months in the plankton. Such planktotrophic species commonly produce large numbers of small larvae that disperse over vast distances. In contrast, the nonfeeding lecithotrophic larvae from sessile invertebrates that brood are often large and conspicuous, lack morphological defenses, and have limited dispersal distances because they typically are competent to settle minutes to hours after spawning. Interactions between lecithotrophic larvae and consumers are not well studied. This has limited the ability of previous authors to test broad generalities about marine larvae. We show that brooded larvae of Caribbean sponges (11 species) and gorgonians (three species) as well as brooded larvae of temperate hydroids (two species) and a bryozoan are unpalatable to co—occurring fishes. In contrast, brooded larvae of temperate ascidians (three species), a temperate sponge, and Caribbean hard corals (three species) are readily consumed by fishes, as are larvae from four of six species of synchronous broadcast—spawning gorgonians from the Florida Keys. Frequencies of survivorship for larvae attacked and rejected by fishes were high and statistically indistinguishable from frequencies for unattacked control larvae. Frequency of metamorphosis (when it occurred) of rejected larvae never differed significantly from that of unattacked control larvae. Assays testing for larval vs. adult chemical defenses for five species with distasteful larvae showed that larvae of all five species were chemically distasteful to fishes, whereas only three of five adult extracts deterred fish feeding. A comparison of larval palatability among chemically rich taxa showed that brooded larvae were significantly more likely to be unpalatable (86% of the species tested) than larvae of broadcasters (33%), and that palatable larvae were rarely released during the day (23%) while unpalatable larvae usually were (89%). Additionally, the frequency of bright coloration was high (60%) for unpalatable larvae and low (0%) for palatable larvae, suggesting that unpalatable larvae often may be aposematically colored. Results of this broad survey cast doubt on the widely accepted notion that virtually all marine larvae are suitable prey for most generalized planktivores. Among species that do not chemically or physically protect larvae against fishes, selection appears to favor the release of larvae at night, or the production of smaller more numerous offspring that grow and develop at sea as a way of escaping consumer—rich benthic habitats. Because distasteful larvae are not similarly constrained, distasteful species should exhibit reproductive and larval characteristics selected more by the fitness—related consequences of larval development mode and dispersal distance than by the necessity of avoiding benthic predators. Production of large larvae and retention of offspring in parental habitats that have proved to be suitable for growth and reproduction have both been proposed as advantageous, but these advantages often were assumed to be offset by losses due to increased larval apparency to fishes. This assumed trade—off is not mandatory because larvae can be defended chemically. Distasteful larvae tend to be conspicuous, localized dispersers that can co—occur with benthic fishes, and yet not be consumed.
-
ItemDefensive synergisms?: Reply to Pennings(Georgia Institute of Technology, 1996-09) Hay, Mark E.
-
ItemCoral-seaweed-grazer-nutrient interactions on temperate reefs(Georgia Institute of Technology, 1996-08) Miller, Margaret W. ; Hay, Mark E.Why corals dominate tropical reefs but are rare or absent on temperate reefs is one of the more intriguing global—scale questions in marine ecology. Restriction of corals from temperate reefs has been suggested to be due to low temperature, competition with seaweeds, and synergistic interactions of physical and biological factors. However, most studies addressing these hypotheses have been non—experimental and conducted on tropical corals and reefs near the extremes of their distribution rather than in temperate habitats using corals that are physiologically suited to temperate environments. Some corals occur only in temperate regions, but their ecology is largely unstudied. In an attempt to understand how temperate corals function at latitudes where corals are relatively rare and where reefs tend to be dominated by seaweeds, we examined responses of the temperate coral Oculina arbuscula to competition with seaweeds on reefs in North Carolina (USA). We asked how competitive interactions were affected by levels of grazing, nutrients, or interactions of these factors. Sampling of 12 reef habitats ranging from 1 m deep, inshore sites to 25 m deep, offshore sites demonstrated a strong negative relationship between percentage cover of seaweeds and density of Oculina colonies. Seaweeds dominated well—lit habitats (shallow inshore reefs or deep offshore reefs with clear waters), while coral was significantly more abundant in low—light habitats where seaweeds were rare (deep inshore habitats or nearshore reefs with turbid waters). This general among—site pattern also occurred within an individual site as one moved from shallow, well—lit waters to deeper, darker waters. Corals transplanted into seaweed—dominated areas grew well if seaweed canopies were removed but grew poorly, or not at all, if canopies were left intact. Seaweeds also significantly inhibited recruitment of O. arbuscula. At a turbid inshore reef, recruitment was high at a depth of 6 m where seaweeds were rare, but very low at a depth of 1 m where seaweeds were abundant. Removal of seaweeds from shallow plots increased recruitment about 12—fold to levels that did not differ significantly from those at 6 m. In grazer—exclusion cages on offshore reefs, Oculina recruitment varied significantly between sites and years. Almost no recruitment was observed at a well—lit, plant—dominated site while recruitment was higher, to very high, at a turbid site with few seaweeds. At this turbid site, recruitment was facilitated by grazing but could not be related to grazer effects on seaweeds. Facilitation was apparently due to consumers removing barnacles, which dominated this low—light site if grazers were excluded. Cage exclusion of larger herbivores at two offshore sites (primarily fishes) and one inshore site (fishes and urchins) had no significant effect on coral growth. At the well—lit inshore site, herbivores had a large effect on the species composition of the seaweed community, but little effect on the total abundance of seaweeds. Large grazers caused palatable red seaweeds to be replaced by similar amounts of unpalatable brown seaweeds. Therefore, herbivory alone had little impact on total seaweed abundance and the levels of seaweed competition affecting co—occurring O. arbuscula. In contrast, when we conducted a factorial experiment manipulating both herbivory and nutrient levels, exclusion cages significantly reduced coral growth. In this experiment, nutrient addition had no effect on brown seaweeds but significantly increased the percentage cover of red seaweeds in exclusion cages. Nutrient addition also tended to suppress coral growth in herbivore—exclusion cages where red seaweeds were stimulated, but to increase coral growth in open cages where herbivores had removed the red seaweeds. Thus, nutrients and herbivory may have acted synergistically to affect seaweeds, and hence corals, on this temperate reef. Different groups of algae (red vs. brown) experienced differential degrees of nutrient limitation and exerted differential competitive effects on corals at this site. Our findings show that competition with seaweeds plays a large role in excluding Oculina from well—lit temperate reefs, and support the hypothesis that seaweed competition may interact with latitudinal changes in physical parameters to limit coral recruitment, growth, and accumulation at high latitudes, thus suppressing the potential for reef development.