Kelps on demand: Closed-system protocols for culturing large bull kelp sporophytes for research and restoration.

Culturing kelps for commercial, conservation, and scientific purposes is becoming increasingly widespread. However, kelp aquaculture methods are typically designed for ocean-based farms, and these methods may not be applicable for smaller scale cultivation efforts common in research and restoration. Growing kelps in closed, recirculating culture systems may address many of these constraints, yet closed system approaches have remained largely undescribed. Extensive declines of the bull kelp (Nereocystis luetkeana), an ecologically important canopy species in the Northeast Pacific, have received widespread attention and prompted numerous research and conservation initiatives. Here, we detail two approaches for cultivating N. luetkeana sporophytes in closed recirculating systems. Nereocystis luetkeana were reared as attached thalli in custom seaweed growth flumes and also free-floating in tumble culture tanks. Careful control of stocking density, water motion, aeration, and nutrient levels allowed for rapid growth and normal morphogenesis of laboratory-grown kelp. Culture systems reached up to 3 kg · m-3 , and individual thalli attained lengths of up to 6 m before the trials were terminated. Our results demonstrate the potential of recirculating, closed culture systems to overcome limitations associated with traditional culture methods. Recirculating systems enable the precise control of culture conditions, improving biosecurity and facilitating cultivar development and other research. Kelps can be grown away from the ocean or outside their native ranges, and seasonal or annual species can be produced year-round without seasonal constraints.

Kelp and sea urchin settlement mediated by biotic interactions with benthic coralline algal species.

Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin-dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non-calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery.

Resolving some of the earliest names for Corallina species (Corallinales, Rhodophyta) in the North Pacific by sequencing type specimens and describing the cryptic C. hakodatensis sp. nov. and C. parva sp. nov.

Partial rbcL sequences from type specimens of three of the earliest described Corallina species showed that C. arbuscula (type locality: Unalaska Island, Alaska, USA) and C. pilulifera (type locality: Okhotsk Sea, Russia) are synonymous, with C. pilulifera as the taxonomically accepted name and that C. vancouveriensis (type locality: Botanical Beach, Vancouver Island, Canada) is a distinct species. To identify molecular species limits and clarify descriptions and distributions of C. pilulifera and C. vancouveriensis, we sequenced and analyzed portions of one mitochondrial and two plastid genes from historical and recent collections. The single-gene phylogenetic reconstructions support the recognition of both species as distinct, as well as two additional species, C. hakodatensis sp. nov. and C. parva sp. nov., which are sister to, and often morphologically indistinguishable from C. pilulifera and C. vancouveriensis, respectively. DNA sequence data currently illustrate that C. pilulifera is found in the cold northern Pacific waters from the Okhotsk Sea of Russia to Hokkaido, Japan, eastward across the Aleutian Islands to Knoll Head, Alaska, and as far south as Nanaimo, British Columbia. Corallina vancouveriensis is distributed as far west as Attu Island in the Aleutian Islands to Sitka, Alaska, and southeasterly at numerous sites from British Columbia to the north of Point Conception, California, USA. The cryptic species C. hakodatensis and C. parva occur sympatrically with their sister species but with narrower ranges. The complex phylogenetic relationships shown by the single gene trees recommend Corallina as a model genus to explore coralline algal biogeography, evolution, and patterns of speciation.

Functional characterization of a cellulose synthase, CtCESA1, from the marine red alga Calliarthron tuberculosum (Corallinales).

In land plants and algae, cellulose is important for strengthening cell walls and preventing breakage due to physical forces. Though our understanding of cellulose production by cellulose synthases (CESAs) has seen significant advances for several land plant and bacterial species, functional characterization of this fundamental protein is absent in red algae. Here we identify CESA gene candidates in the calcifying red alga Calliarthron tuberculosum using sequence similarity-based approaches, and elucidate their phylogenetic relationship with other CESAs from diverse taxa. One gene candidate, CtCESA1, was closely related to other putative red algal CESA genes. To test if CtCESA1 encoded a true cellulose synthase, CtCESA1 protein was expressed and purified from insect and yeast expression systems. CtCESA1 showed glucan synthase activity in glucose tracer assays. CtCESA1 activity was relatively low when compared with plant and bacterial CESA activity. In an in vitro assay, a predicted N-terminal starch-binding domain from CtCESA1 bound red algal floridean starch extracts, representing a unique domain in red algal CESAs not present in CESAs from other lineages. When the CtCESA1 gene was introduced into Arabidopsis thaliana cesa mutants, the red algal CtCESA1 partially rescued the growth defects of the primary cell wall cesa6 mutant, but not cesa3 or secondary cell wall cesa7 mutants. A fluorescently tagged CtCESA1 localized to the plasma membrane in the Arabidopsis cesa6 mutant background. This study presents functional evidence validating the sequence annotation of red algal CESAs. The relatively low activity of CtCESA1, partial complementation in Arabidopsis, and presence of unique protein domains suggest that there are probably functional differences between the algal and land plant CESAs.

Less Than the Sum of Its Parts: Blade Clustering Reduces Drag in the Bull Kelp, Nereocystis luetkeana (Phaeophyceae).

Nereocystis luetkeana is a large, canopy-forming kelp that is commonly found in nearshore waters between Alaska and California. Despite regularly reaching lengths in excess of 30 m, this alga demonstrates a remarkable ability to endure hydrodynamically stressful environments without being dislodged by waves or currents. While morphological aspects of this kelp, including its long flexible stipe, have been shown to reduce drag, blade clustering has never been directly tested. In this study, we examined how the distinctive multi-bladed morphology of Nereocystis thalli limits drag in flow. We measured drag on whole kelps in a large recirculating flume and quantified how drag was affected by serial removal of blades. We then compared measured drag with predictions of "additive drag", which we defined as the sum of the drag that each blade experiences in isolation. We found that, on average, intact thalli experience only 37% of the predicted additive drag when subjected to a flow rate of 0.40-0.50 m · s-1 . Our results indicate that the subdivision of the photosynthetic area into multiple blades has a drag-reducing effect on Nereocystis thalli. We found drag increases less than proportionally with the cumulative area, meaning the contribution of individual blades to overall drag decreases with increasing blade number. That is, as thalli develop, each additional blade incurs a reduced hydrodynamic cost, perhaps lending insight into how thalli can grow so large.

Trophic control of cryptic coralline algal diversity.

Understanding how trophic dynamics drive variation in biodiversity is essential for predicting the outcomes of trophic downgrading across the world's ecosystems. However, assessing the biodiversity of morphologically cryptic lineages can be problematic, yet may be crucial to understanding ecological patterns. Shifts in keystone predation that favor increases in herbivore abundance tend to have negative consequences for the biodiversity of primary producers. However, in nearshore ecosystems, coralline algal cover increases when herbivory is intense, suggesting that corallines may uniquely benefit from trophic downgrading. Because many coralline algal species are morphologically cryptic and their diversity has been globally underestimated, increasing the resolution at which we distinguish species could dramatically alter our conclusions about the consequences of trophic dynamics for this group. In this study, we used DNA barcoding to compare the diversity and composition of cryptic coralline algal assemblages at sites that differ in urchin biomass and keystone predation by sea otters. We show that while coralline cover is greater in urchin-dominated sites (or "barrens"), which are subject to intense grazing, coralline assemblages in these urchin barrens are significantly less diverse than in kelp forests and are dominated by only 1 or 2 species. These findings clarify how food web structure relates to coralline community composition and reconcile patterns of total coralline cover with the widely documented pattern that keystone predation promotes biodiversity. Shifts in coralline diversity and distribution associated with transitions from kelp forests to urchin barrens could have ecosystem-level effects that would be missed by ignoring cryptic species' identities.

The Class II KNOX genes KNAT3 and KNAT7 work cooperatively to influence deposition of secondary cell walls that provide mechanical support to Arabidopsis stems.

The transcription factor KNOTTED ARABIDOPSIS THALIANA7 (KNAT7) is a Class II KNOTTED1-like homeobox (KNOX2) gene that, in interfascicular fibres, acts as a negative regulator of secondary cell wall biosynthesis. In addition, knat7 loss-of-function mutants display an irregular xylem (irx) phenotype, suggesting a potential positive regulatory role in xylem vessel secondary cell wall deposition. Although our understanding of the role of KNAT7 is evolving, the function(s) of the closely related KNOX2 genes, KNAT3, KNAT4, and KNAT5, in secondary wall formation still remain unclear. We found that all four Arabidopsis KNOX2 genes were expressed in the inflorescence stems. However, only the knat3 knat7 double mutants showed a phenotype, displaying an enhanced irx phenotypes relative to the single mutants, as well as decreased interfascicular fibre cell wall thickness. Moreover, knat3 knat7 double mutants had reduced stem tensile and flexural strength compared with wild-type and single mutants. In contrast, KNAT3 overexpression resulted in thicker interfascicular fibre secondary cell walls in inflorescence stems, suggesting a potential positive regulation in interfascicular fibre secondary wall development. This work identifies KNAT3 as a potential transcriptional activator working together with KNAT7 to promote secondary cell wall biosynthesis in xylem vessels, while concurrently acting antagonistically with KNAT7 to influence secondary wall formation in interfascicular fibres.

Kelp-associated Microbiota are Structured by Host Anatomy1.

Seaweed-associated microbiota are essential for the health and resilience of nearshore ecosystems, marine biogeochemical cycling, and host health. Yet much remains unknown about the ecology of seaweed-microbe symbioses. In this study, we quantified fine-scale patterns of microbial community structure across distinct anatomical regions of the kelp Laminaria setchellii. These anatomical regions represent a gradient of tissue ages: perennial holdfasts can be several years old, whereas stipe epicortex and blades are younger annual structures. Within blades, new growth occurs at the base, while the blade tips may be several months old and undergoing senescence. We hypothesized that microbial communities will differ across anatomical regions (holdfast, stipe, blade base, and blade tip), such that younger tissues will harbor fewer microbes that are more consistent across replicate individuals. Our data support this hypothesis, with the composition of bacterial (16S rRNA gene) and microeukaryote (18S rRNA gene) communities showing significant differences across the four anatomical regions, with the surfaces of older tissues (holdfast and blade tips) harboring significantly greater microbial richness compared to the younger tissues of the meristematic region. Additional samples collected from the surfaces of new L. setchellii recruits (<1y old) also showed differences in microbial community structure across anatomical regions, which demonstrates that these microbial differences are established early. We also observed this pattern in two additional algal species, suggesting that microbial community structure across host anatomy may be a common feature of the seaweed microbiome.

Type specimen sequencing, multilocus analyses, and species delimitation methods recognize the cosmopolitan Corallina berteroi and establish the northern Japanese C. yendoi sp. nov. (Corallinaceae, Rhodophyta).

A partial rbcL sequence of the lectotype specimen of Corallina berteroi shows that it is the earliest available name for C. ferreyrae. Multilocus species delimitation analyses (ABGD, SPN, GMYC, bPTP, and BPP) using independent or concatenated COI, psbA, and rbcL sequences recognized one, two, or three species in this complex, but only with weak support for each species hypothesis. Conservatively, we recognize a single worldwide species in this complex of what appears to be multiple, evolving populations. Included in this species, besides C. ferreyrae, are C. caespitosa, the morphologically distinct C. melobesioides, and, based on a partial rbcL sequence of the holotype specimen, C. pinnatifolia. Corallina berteroi, not C. officinalis, is the cosmopolitan temperate species found thus far in the NE Atlantic, Mediterranean Sea, warm temperate NW Atlantic and NE Pacific, cold temperate SW Atlantic (Falkland Islands), cold and warm temperate SE Pacific, NW Pacific and southern Australia. Also proposed is C. yendoi sp. nov. from Hokkaido, Japan, which was recognized as distinct by 10 of the 13 species discrimination analyses, including the multilocus BPP.

Gas Composition of Developing Pneumatocysts in Bull Kelp Nereocystis luetkeana (Phaeophyceae)1.

The subtidal kelp Nereocystis luetkeana (hereafter Nereocystis) maintains an upright stature by producing a single gas-filled float (pneumatocyst) that provides buoyancy. The ability of Nereocystis pneumatocysts to inflate with gas underwater is peculiar, and the gas composition of pneumatocysts has been the topic of several studies over the last 100 years. Past studies of pneumatocyst gases only examined large sporophytes, leaving open questions about the origins of these gases and how gas composition may change during development. In this study, we use developmental time as a means to understand the origin and physiological mechanisms that give rise to different gases within Nereocystis pneumatocysts. Total gas composition was measured across a range of pneumatocyst sizes (5-725 mL). Contrary to previous studies that documented pneumatocyst gas concentrations to be similar to air, this study found internal gas levels of CO, CO2 , and O2 to be 1.2 ± 0.8%, 0.6 ± 0.2%, and 59.9 ± 13.6%, respectively. Our data suggest that the composition of gases does not change as pneumatocysts grow and that the rate of each gas added is approximately proportional to changes in pneumatocyst volume. Therefore, cells constituting the pneumatocyst wall are likely producing more gas (per surface area) to fill pneumatocysts as they expand, maintaining proportional gas composition and corresponding internal pressure.

Morphological plasticity in the kelp Nereocystis luetkeana (Phaeophyceae) is sensitive to the magnitude, direction, and location of mechanical loading.

Nereocystis luetkeana is a canopy-forming kelp that exhibits morphological plasticity across hydrodynamic gradients, producing broad, undulate blades in slow flow and narrow, flattened blades in fast flow, enabling thalli to reduce drag while optimizing photosynthesis. While the functional significance of this phenomenon has been well studied, the developmental and physiological mechanisms that facilitate the plasticity remain poorly understood. In this study, we conducted three experiments to characterize how the (1) magnitude, (2) direction, and (3) location of plasticity-inducing mechanical stimuli affect the morphology of Nereocystis blades. We found that applying a gradient of tensile force caused blades to grow progressively longer, narrower, less ruffled, and heavier in a linear fashion, suggesting that Nereocystis is equally well adapted for all conditions within its hydrodynamic niche. We also found that applying tension transversely across blades caused the growth response to rotate 90°, indicating that there is no substantial separation between the sites of stimulus perception and response and suggesting that a long-distance signaling mechanism, such as a hormone, is unlikely to mediate this phenomenon. Meristoderm cells showed morphological changes that paralleled those of their respective blades in this experiment, implying that tissue-level morphology is influenced by cell growth. Finally, we found that plasticity was only induced when tension was applied directly to the growing tissue, reinforcing that long-distance signaling is probably not involved and possibly indicating that the mechanism on display generally requires an intercalary meristem to facilitate mechanoperception.

Elevated Temperature Affects Phenotypic Plasticity in the Bull Kelp (Nereocystis luetkeana, Phaeophyceae).

The sensitivity of kelps to elevated temperatures has been linked to recent declines in some kelp populations, with cascading impacts on marine communities. However, it remains unclear how thermal stress affects the ability of kelps to respond to other environmental factors, which could influence their vulnerability to climate change. We investigated the effect of thermal stress on the ability of the bull kelp Nereocystis luetkeana to acclimate to its surrounding hydrodynamic environment through tension-regulated plasticity in blade morphology. We first determined optimal and stressful temperatures for N. luetkeana by measuring growth over nine temperatures from 5°C to 22°C. We then exposed N. luetkeana blades to a factorial combination of temperature (13°C and 20°C) and tension (0.5 N and 2.0 N) simulating different flow conditions, and measured changes in blade length and width after 7 days. The temperature at which N. luetkeana exhibited maximum growth was estimated to be ~11.9°C, though growth was high over a relatively wide temperature range. When thermally stressed, N. luetkeana maintained morphological responses to simulated high flow, but were inhibited from acclimating to low flow, indicated by an inability of blades to widen. Our results suggest that N. luetkeana in sheltered habitats may be particularly vulnerable to climate warming, where an inability to adjust blade morphology to local hydrodynamic conditions could drive declines at sublethal levels of warming. As ecologically important foundation species, declines in sheltered kelp populations could result in major biodiversity loss and disrupt ecosystem function.

Cellulose-rich secondary walls in wave-swept red macroalgae fortify flexible tissues.

MAIN CONCLUSION: Cellulosic secondary walls evolved convergently in coralline red macroalgae, reinforcing tissues against wave-induced breakage, despite differences in cellulose abundance, microfibril orientation, and wall structure. Cellulose-enriched secondary cell walls are the hallmark of woody vascular plants, which develop thickened walls to support upright growth and resist toppling in terrestrial environments. Here we investigate the striking presence and convergent evolution of cellulosic secondary walls in coralline red algae, which reinforce thalli against forces applied by crashing waves. Despite ostensible similarities to secondary wall synthesis in land plants, we note several structural and mechanical differences. In coralline red algae, secondary walls contain three-times more cellulose (~ 22% w/w) than primary walls (~ 8% w/w), and their presence nearly doubles the total thickness of cell walls (~ 1.2 µm thick). Field emission scanning electron microscopy revealed that cellulose bundles are cylindrical and lack any predominant orientation in both primary and secondary walls. His-tagged recombinant carbohydrate-binding module differentiated crystalline and amorphous cellulose in planta, noting elevated levels of crystalline cellulose in secondary walls. With the addition of secondary cell walls, Calliarthron genicular tissues become significantly stronger and tougher, yet remain remarkably extensible, more than doubling in length before breaking under tension. Thus, the development of secondary walls contributes to the strong-yet-flexible genicular tissues that enable coralline red algae to survive along wave-battered coastlines throughout the NE Pacific. This study provides an important evolutionary perspective on the development and biomechanical significance of secondary cell walls in a non-model, non-vascular plant.

A comprehensive kelp phylogeny sheds light on the evolution of an ecosystem.

Reconstructing phylogenetic topologies and divergence times is essential for inferring the timing of radiations, the appearance of adaptations, and the historical biogeography of key lineages. In temperate marine ecosystems, kelps (Laminariales) drive productivity and form essential habitat but an incomplete understanding of their phylogeny has limited our ability to infer their evolutionary origins and the spatial and temporal patterns of their diversification. Here, we reconstruct the diversification of habitat-forming kelps using a global genus-level phylogeny inferred primarily from organellar genome datasets, and investigate the timing of kelp radiation. We resolve several important phylogenetic features, including relationships among the morphologically simple kelp families and the broader radiation of complex kelps, demonstrating that the initial radiation of the latter resulted from an increase in speciation rate around the Eocene-Oligocene boundary. This burst in speciation rate is consistent with a possible role of recent climatic cooling in triggering the kelp radiation and pre-dates the origin of benthic-foraging carnivores. Historical biogeographical reconstructions point to a northeast Pacific origin of complex kelps, with subsequent colonization of new habitats likely playing an important role in driving their ecological diversification. We infer that complex morphologies associated with modern kelp forests (e.g. branching, pneumatocysts) evolved several times over the past 15-20 MY, highlighting the importance of morphological convergence in establishing modern upright kelp forests. Our phylogenomic findings provide new insights into the geographical and ecological proliferation of kelps and provide a timeline along which feedbacks between kelps and their food-webs could have shaped the structure of temperate ecosystems.

Sympatric kelp species share a large portion of their surface bacterial communities.

Kelp forest ecosystems are biodiversity hotspots, providing habitat for dense assemblages of marine organisms and nutrients for marine and terrestrial food webs. The surfaces of kelps support diverse microbial communities that facilitate the transfer of carbon from algal primary production to higher trophic levels. We quantified the diversity of bacteria on the surfaces of eight sympatric kelp species from four sites in British Columbia. Kelp-associated bacterial communities are significantly different from their environment, even though 86% of their bacterial taxa are shared with seawater and 97% are shared with rocky substrate. This differentiation is driven by differences in relative abundance of the bacterial taxa present. Similarly, a large portion of bacterial taxa (37%) is shared among all eight kelp species, yet differential abundance of bacterial taxa underlies differences in community structure among species. Kelp-associated bacterial diversity does not track host phylogeny; instead bacterial community composition is correlated with the life-history strategy of the host, with annual and perennial kelps supporting divergent bacterial communities. These data provide the first community-scale investigation of kelp forest-associated bacterial diversity. More broadly, this study provides insight into mechanisms that may structure bacterial communities among closely related sympatric host species.

Alternate life history phases of a common seaweed have distinct microbial surface communities.

Macroalgal life histories are complex, often involving the alternation of distinct free-living life history phases that differ in morphology, longevity and ploidy. The surfaces of marine macroalgae support diverse microbial biofilms, yet the degree of microbial variation between alternate phases is unknown. We quantified bacterial (16S rRNA gene) and microeukaryote (18S rRNA gene) communities on the surface of the common intertidal seaweed, Mastocarpus spp., which alternates between gametophyte (foliose, haploid) and sporophyte (encrusting, diploid) life history phases. A large portion (97%) of bacterial taxa on the surface Mastocarpus was also present in samples from the environment, indicating that macroalgal surface communities are largely assembled from the surrounding seawater. Still, changes in the relative abundance of bacterial taxa result in significantly different communities on alternate Mastocarpus life history phases, rocky substrate and seawater at all intertidal elevations. For microeukaryote assemblages, only high intertidal samples had significant differences between life history phases although sporophytes were not different from the rocky substrate at this elevation; gametophytes and sporophytes did not differ in microeukaryote communities in the mid and low zones. By sequencing three host genes, we identified three cryptic species of Mastocarpus in our data set, which co-occur in the mid-to-low intertidal zone. In these samples, M. alaskensis sporophytes harboured distinct bacterial communities compared to M. agardhii and M. intermedius sporophytes, which were not distinguishable. Conversely, microeukaryote communities did not differ among species.

Bending strategies of convergently evolved, articulated coralline algae.

The evolution of uncalcified genicula in upright calcified corallines has occurred at least three times independently, resulting in articulated corallines within Corallinoideae, Lithophylloideae, and Metagoniolithoideae. Genicula confer flexibility to otherwise rigid thalli, and the localization of bending at discrete intervals amplifies bending stress in genicular tissue. Genicular morphology must, therefore, be balanced between maintaining flexibility while mitigating or resisting stress. Genicula in the three articulated lineages differ in both cellular construction and development, which may result in different constraints on morphology. By studying the interaction between flexibility and morphological variation in multiple species, we investigate whether representatives of convergently evolving clades follow similar strategies to generate mechanically successful articulated fronds. By using computational models to explore different bending strategies, we show that there are multiple ways to generate flexibility in upright corallines but not all morphological strategies are mechanically equivalent. Corallinoids have many joints, lithophylloids have pliant joints, and metagoniolithoids have longer joints-while these strategies can lead to comparable thallus flexibility, they also lead to different levels of stress amplification in bending. Moreover, genicula at greatest risk of stress amplification are typically the strongest, universally mitigating the trade-off between flexibility and stress reduction.

Under pressure: biomechanical limitations of developing pneumatocysts in the bull kelp (Nereocystis luetkeana, Phaeophyceae).

Maintaining buoyancy with gas-filled floats (pneumatocysts) is essential for some subtidal kelps to achieve an upright stature and compete for light . However, as these kelps grow up through the water column, pneumatocysts are exposed to substantial changes in hydrostatic pressure, which could cause complications as internal gases may expand or contract, potentially causing them to rupture, flood, and lose buoyancy. In this study, we investigate how pneumatocysts of Nereocystis luetkeana resist biomechanical stress and maintain buoyancy as they develop across a hydrostatic gradient. We measured internal pressure, material properties, and pneumatocyst geometry across a range of thallus sizes and collection depths to identify strategies used to resist pressure-induced mechanical failure. Contrary to expectations, all pneumatocysts had internal pressures less than atmospheric pressure, ensuring that thalli are always exposed to a positive pressure gradient and compressional loads, indicating that they are more likely to buckle than rupture at all depths. Small pneumatocysts collected from depths between 1 and 9 m (inner radius = 0.4-1.0 cm) were demonstrated to have elevated wall stresses under high compressive loads and are at greatest risk of buckling. Although small kelps do not adjust pneumatocyst material properties or geometry to reduce wall stress as they grow, they are ~3.4 times stronger than they need to be to resist hydrostatic buckling. When tested, pneumatocysts buckled around 35 m depth, which agrees with previous measures of lower limits due to light attenuation, suggesting that hydrostatic pressure may also define the lower limit of Nereocystis in the field.

Evolutionary reversals in Bossiella (Corallinales, Rhodophyta): first report of a coralline genus with both geniculate and nongeniculate species.

This is the first report of a coralline genus with both geniculate (upright fronds with non-calcified joints) and nongeniculate species that has been verified by DNA sequence data. Two nongeniculate (crustose) species of Bossiella are recognized, B. mayae sp. nov. and B. exarticulata sp. nov. DNA sequencing of the lectotype specimen of Pseudolithophyllum whidbeyense revealed that this name had been misapplied and instead belongs to an undescribed coralline species in the Hapalidiales. Phylogenetic analyses of concatenated DNA sequences (psbA, rbcL, COI-5P) indicate that B. mayae and B. exarticulata represent phenotypic reversals from the geniculate character state back to the nongeniculate character state. Secondary loss of genicula has occurred three times in the subfamily Corallinoideae, once to generate the entirely nongeniculate genus Crusticorallina and twice in the now morphologically heterotypic Bossiella. Since phenotypic reversals have occurred several times during the evolution of coralline algae, we speculate about the putative mechanism and adaptive significance of this phenomenon.

Macroalgal spore dysfunction: ocean acidification delays and weakens adhesion.

Early life stages of marine organisms are predicted to be vulnerable to ocean acidification. For macroalgae, reproduction and population persistence rely on spores to settle, adhere and continue the algal life cycle, yet the effect of ocean acidification on this critical life stage has been largely overlooked. We explicitly tested the biomechanical impact of reduced pH on early spore adhesion. We developed a shear flume to examine the effect of reduced pH on spore attachment time and strength in two intertidal rhodophyte macroalgae, one calcified (Corallina vancouveriensis) and one noncalcified (Polyostea robusta). Reduced pH delayed spore attachment of both species by 40%-52% and weakened attachment strength in C. vancouveriensis, causing spores to dislodge at lower flow-induced shear forces, but had no effect on the attachment strength of P. robusta. Results are consistent with our prediction that reduced pH disrupts proper curing and gel formation of spore adhesives (anionic polysaccharides and glycoproteins) via protonation and cation displacement, although experimental verification is needed. Our results demonstrate that ocean acidification negatively, and differentially, impacts spore adhesion in two macroalgae. If results hold in field conditions, reduced ocean pH has the potential to impact macroalgal communities via spore dysfunction, regardless of the physiological tolerance of mature thalli.