Interspecific comparison of hydrodynamic performance and structural properties among intertidal macroalgae

Macroalgae use flexibility and reconfiguration, i.e. the alteration of shape, size and orientation as water velocity increases, to reduce the hydrodynamic forces imposed in the wave-swept rocky intertidal zone. Quantifying the effects of flexibility on hydrodynamic performance is difficult, however, because the mechanisms of reconfiguration vary with water velocity and the relationship between algal solid mechanics and hydrodynamic performance is poorly understood. In this study, the hydrodynamic performance, morphology and solid mechanics of 10 rocky shore macroalgal species were quantified to evaluate the influences of flexibility and morphology on reconfiguration. Hydrodynamic performance was measured in a flume by direct measurement of changes in size and shape during reconfiguration across a wide range of velocities, material stiffness was quantified with standard materials testing, and structural properties were calculated from material and morphological data. Hydrodynamic parameters varied significantly among species, indicating variation in the magnitude of reconfiguration and the velocities required for full reconfiguration. Structural properties also varied among species, and were correlated with hydrodynamic performance in some instances. The relationship between hydrodynamic and structural properties is velocity dependent, such that flexibility influences different aspects of reconfiguration at low and high velocities. Groups are identifiable among species based on hydrodynamic and structural properties, suggesting that these properties are useful for addressing functional-form hypotheses and the effects of hydrodynamic disturbance on macroalgal communities

The hydrodynamic effects of shape and size change during reconfiguration of a flexible macroalga

Rocky intertidal organisms experience large hydrodynamic forces due to high water velocities created by breaking waves. Flexible organisms, like macroalgae, often experience lower drag than rigid organisms because their shape and size change as velocity increases. This phenomenon, known as reconfiguration, has been previously quantified as Vogel\u27s E, a measure of the relationship between velocity and drag. While this method is very useful for comparing reconfiguration among organisms it does not address the mechanisms of reconfiguration, and its application to predicting drag is problematic. The purpose of this study was twofold: (1) to examine the mechanisms of reconfiguration by quantifying the change in shape and size of a macroalga in flow and (2) to build a mechanistic model of drag for reconfiguring organisms. Drag, frontal area and shape of the intertidal alga Chondrus crispus were measured simultaneously in a recirculating flume at water velocities from 0 to ∼2 m s–1. Reconfiguration was due to two separate mechanisms: whole-alga realignment (deflection of the stipe) at low velocities (\u3c0.2 m s–1) and compaction of the crown (reduction in frontal area and change in shape) at higher velocities. Change in frontal area contributed more to drag reduction than change in drag coefficient. Drag coefficient and frontal area both decrease exponentially with increasing water velocity, and a mechanistic model of drag was developed with explicit functions to describe these changes. The model not only provides mechanistic parameters with which to compare reconfiguration among individuals and species, but also allows for more reliable predictions of drag at high, ecologically relevant water velocities

Seasonal variation in mussel byssal thread mechanics

The blue mussel, Mytilus edulis, attaches itself to the substrate by producing a radially arranged complex of collagenous byssal threads. The strength of byssal attachment, or tenacity, has been shown to vary seasonally on Rhode Island shores, increasing twofold in spring in comparison with fall. It was previously assumed that this seasonality was due to increased thread production following periods of increased wave action; however, recent findings do not support this view. As an alternate hypothesis, this study evaluates the contribution of seasonal changes in the material properties of byssal threads to an annual cycle in mussel attachment strength. Tensile mechanical tests were performed seasonally, on both newly produced threads and on threads outplanted in the field for up to nine weeks. Threads produced in spring were over 60% stronger and 83% more extensible than threads produced in all other seasons. The mechanical integrity of byssal threads also deteriorated over time in spring and summer. These results suggest that reduced attachment strength in fall reflects the production of inferior quality threads following a period of increased decay. Here, we propose a new scheme where variation in byssal thread material properties, rather than quantity, explains the seasonal pattern in attachment strength observed on Rhode Island shores

An inland sea high nitrate-low chlorophyll (HNLC) region with naturally high pCO2

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography 60 (2015): 957–966, doi:10.1002/lno.10062.We present a time series of data for temperature, salinity, nitrate, and carbonate chemistry from September 2011 to July 2013 at the University of Washington's Friday Harbor Laboratories. Samples were collected at the Friday Harbor dock and pump house. Seawater conditions at Friday Harbor were high nitrate-low chlorophyll, with average nitrate and pCO2 concentrations of ∼ 25 ± 5 μmol L−1 and ∼ 700 ± 103 μatm (pH 7.80 ± 0.06). Transient decreases in surface water nitrate and pCO2 corresponded with the timing of a spring bloom (April through June). The high nitrate and pCO2 originate from the high values for these parameters in the source waters to the Salish Sea from the California Undercurrent (CU). These properties are due to natural aerobic respiration in the region where the CU originates, which is the oxygen minimum zone in the eastern tropical North Pacific. Alkalinity varies little so the increase in pCO2 is due to inputs of dissolved inorganic carbon (DIC). This increase in DIC can come from both natural aerobic respiration within the ocean and input of anthropogenic CO2 from the atmosphere when the water was last at the sea surface. We calculated that the anthropogenic “ocean acidification” contribution to DIC in the source waters of the CU was 36 μmol L−1. This contribution ranged from 13% to 22% of the total increase in DIC, depending on which stoichiometry was used for C/O2 ratio (Redfield vs. Hedges). The remaining increase in DIC was due to natural aerobic respiration.We thank The Educational Foundation of America (EFA) and National Science Foundation Field Station Marine Lab Program (FSML) (NSF DBI 0829486) for essential initial funding to JWM to develop the Ocean Acidification Experimental Lab (OAEL). Additional support was provided by NSF award EF1041213 to E. Carrington Ken Sebens for encouragement to involve students in this research as part of a FHL mini-apprenticeship course

Ocean acidification bends the Mermaid’s Wineglass

Ocean acidification lowers the saturation state of calcium carbonate, decreasing net calcification and compromising the skeletons of organisms such as corals, molluscs and algae. These calcified structures can protect organisms from predation and improve access to light, nutrients and dispersive currents. While some species (such as urchins, corals and mussels) survive with decreased calcification, they can suffer from inferior mechanical performance. Here, we used cantilever beam theory to test the hypothesis that decreased calcification would impair the mechanical performance of the green alga Acetabularia acetabulum along a CO2 gradient created by volcanic seeps off Vulcano, Italy. Calcification and mechanical properties declined as calcium carbonate saturation fell; algae at 2283 matm CO2 were 32% less calcified, 40% less stiff and 40% droopier. Moreover, calcification was not a linear proxy for mechanical performance; stem stiffness decreased exponentially with reduced calcification. Although calcifying organisms can tolerate high CO2 conditions, even subtle changes in calcification can cause dramatic changes in skeletal performance, which may in turn affect key biotic and abiotic interactions

Interactive effects of temperature and nitrogen on the physiology of kelps (Nereocystis luetkeana and Saccharina latissima)

Kelp forest declines have been linked to warming ocean temperatures worldwide. Ocean warming rarely occurs in isolation, so multiple stressor studies are necessary to understand the physiological responses of kelp to climate change. The canopy-forming bull kelp, Nereocystis luetkeana, is going locally extinct in areas of the Salish Sea that are seasonally warm and nutrient poor, while the understory kelp, Saccharina latissima, persists at those sites. Further, nitrogen availability can alter physiological responses of kelps to temperature stress, including alleviating warming stress. We compared the physiological responses of kelp sporophytes to high temperature stress and nitrogen limitation between two populations of N. luetkeana with different environmental histories (warm and nutrient poor vs. cold and nutrient rich) and between two species, N. luetkeana and S. latissima. Using laboratory mesocosms, we tested the interactive effects of short term (8-9 day) exposure of kelp blades to different temperatures: low (9, 13°C), moderate (15, 16°C), and warm (21°C) at two different nitrogen concentrations: low (1-3 μM) vs. high (>10 μM). We examined a wide array of physiological responses: blade growth, photosynthesis, respiration, photosynthetic yield, nutrient uptake, and tissue C:N. Both kelp species responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of both species showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that N. luetkeana and S. latissima are susceptible to thermal stress over short time periods. Populations of N. luetkeana from warm, nutrient poor and cool, nutrient rich areas were equally susceptible to the effects of ocean warming. Our results suggest that nutrient additions may actually reduce kelp performance at supra-optimal temperatures, and a thorough understanding of kelp responses to coastal temperature and nutrient dynamics is needed to guide conservation and restoration actions

Ecological genetics in the North Atlantic: environmental gradients and adaptation at specific loci

The North Atlantic intertidal community provides a rich set of organismal and environmental material for the study of ecological genetics. Clearly defined environmental gradients exist at multiple spatial scales: there are broad latitudinal trends in temperature, meso-scale changes in salinity along estuaries, and smaller scale gradients in desiccation and temperature spanning the intertidal range. The geology and geography of the American and European coasts provide natural replication of these gradients, allowing for population genetic analyses of parallel adaptation to environmental stress and heterogeneity. Statistical methods have been developed that provide genomic neutrality tests of population differentiation and aid in the process of candidate gene identification. In this paper, we review studies of marine organisms that illustrate associations between an environmental gradient and specific genetic markers. Such highly differentiated markers become candidate genes for adaptation to the environmental factors in question, but the functional significance of genetic variants must be comprehensively evaluated. We present a set of predictions about locus-specific selection across latitudinal, estuarine, and intertidal gradients that are likely to exist in the North Atlantic. We further present new data and analyses that support and contradict these simple selection models. Some taxa show pronounced clinal variation at certain loci against a background of mild clinal variation at many loci. These cases illustrate the procedures necessary for distinguishing selection driven by internal genomic vs. external environmental factors. We suggest that the North Atlantic intertidal community provides a model system for identifying genes that matter in ecology due to the clarity of the environmental stresses and an extensive experimental literature on ecological function. While these organisms are typically poor genetic and genomic models, advances in comparative genomics have provided access to molecular tools that can now be applied to taxa with well-defined ecologies. As many of the organisms we discuss have tight physiological limits driven by climatic factors, this synthesis of molecular population genetics with marine ecology could provide a sensitive means of assessing evolutionary responses to climate change

Discordant Impact of HLA on Viral Replicative Capacity and Disease Progression in Pediatric and Adult HIV Infection

HLA class I polymorphism has a major influence on adult HIV disease progression. An important mechanism mediating this effect is the impact on viral replicative capacity (VRC) of the escape mutations selected in response to HLA-restricted CD8+ T-cell responses. Factors that contribute to slow progression in pediatric HIV infection are less well understood. We here investigate the relationship between VRC and disease progression in pediatric infection, and the effect of HLA on VRC and on disease outcome in adult and pediatric infection. Studying a South African cohort of >350 ART-naïve, HIV-infected children and their mothers, we first observed that pediatric disease progression is significantly correlated with VRC. As expected, VRCs in mother-child pairs were strongly correlated (p = 0.004). The impact of the protective HLA alleles, HLA-B*57, HLA-B*58:01 and HLA-B*81:01, resulted in significantly lower VRCs in adults (p<0.0001), but not in children. Similarly, in adults, but not in children, VRCs were significantly higher in subjects expressing the disease-susceptible alleles HLA-B*18:01/45:01/58:02 (p = 0.007). Irrespective of the subject, VRCs were strongly correlated with the number of Gag CD8+ T-cell escape mutants driven by HLA-B*57/58:01/81:01 present in each virus (p = 0.0002). In contrast to the impact of VRC common to progression in adults and children, the HLA effects on disease outcome, that are substantial in adults, are small and statistically insignificant in infected children. These data further highlight the important role that VRC plays both in adult and pediatric progression, and demonstrate that HLA-independent factors, yet to be fully defined, are predominantly responsible for pediatric non-progression

Mechanical design of mussell byssus: Load cycle and strain rate dependence

The ability to produce a strong byssal attachment is one key to the competitive dominance of mussels on many rocky shores. The byssus is composed of numerous extracellular collagenous threads, which in turn can be divided into proximal and distal regions that are distinct in ultrastructure and chemical composition. Our current understanding of the mechanical design of mussel byssus is largely based on quasi-static testing, where a fiber is slowly extended to failure. Mussels in nature, however, inhabit a dynamic environment where repetitive loads can be applied on short time scales. This study evaluates the mechanical properties of the threads of Mytilus californianus subjected to repeated subcritical loads and a range of strain rates. A subset of these mechanical tests was also performed on the threads of three other mytilid species. Results indicate that subcritical loading alters the mechanical properties of a thread in a manner that is dependent on the extension applied, and that thread stiffness and damping increase with increasing strain rate. Overall, this study provides insight into the mechanical design of a byssus that is subjected to dynamic loading

Byssus secrection of Mytilus galloprovincialis: Effect of site at macro and micro-geographical scales within Ría of Vigo (NW Spain)

The effect of the abiotic environment on byssus tenacity and associated features was investigated for Mytilus galloprovincialis in the Ría de Vigo (NW Spain). The effect of site was examined at macro-geographical (outer exposed Cabo Estay vs. inner sheltered San Simón Ensenada sites) and micro-geographical (intertidal vs. subtidal locations) scales. Site significantly influenced byssus tenacity, shape and byssus thread diameter, whereas location did not. Qualitative analysis of the byssus corroborated the importance of site; mussels inhabiting the rougher outer Ría secreted stronger and stiffer threads regardless of location and had a higher potential to form cross-links or metal chelation in the byssal collagen to gain structural integrity when needed. When mussels were transplanted between exposed and sheltered sites, asymmetrical changes were observed in tenacity, endogenous indices, byssus morphology and mechanical properties after 3 mo. Individuals transferred from the sheltered to the exposed site shifted all variables, suggesting that mussels have a plastic response to rougher environments by increasing byssus size and mechanical integrity. In contrast, mussels transplanted from the exposed to the sheltered site shifted tenacity, endogenous indices and thread length but not thread diameter nor mechanical properties. In summary, we report the highly dynamic nature of the mussel ability to modify byssus tenacity when subjected to abrupt environmental changes. Mussels have the potential to change byssus diameter and mechanical properties to increase strength in stressful abiotic conditions, and can re-allocate energy for vital structures such as gonadal and soft tissue growth in more benign environmentsThis study was partly funded by “National Science Foundation EF1041213 to E. 600 Carrington. JMF Babarro acknowledges the funding of the projects AGL2006-06986/ACU and 601 AGL2010-16464 (Ministerio de Ciencia e Innovación, Spanish Government). 602 603Peer reviewe