The indirect role of nutrients in enhancing the invasion of Caulerpa racemosa var cylindracea

The effects of nutrients dynamics on biological invasions in marine habitats have not been fully investigated. The present study aimed to evaluate the role of nutrient pollution in determining the successful invasion of the introduced seaweed Caulerpa racemosa var. cylindracea in a Mediterranean subtidal rocky habitat. To this end, a manipulative field experiment was carried out by supplying the water column with nutrients for a one-year period. Afterwards C. racemosa was transplanted in both nutrient-enriched and control macroalgal assemblages. Results showed that the penetration and biomass of C. racemosa in the assemblages previously treated with nutrients were higher than in control ones, indicating that communities stressed by nutrient pollution are more vulnerable to invasion. The nutrient enrichment changed the structure of natural macroalgal assemblages, mainly by encouraging the growth of turf species. No significant effects were detected for the erect and prostrate layers and for the number of species, suggesting that the susceptibility of the community to invasion may depend more on the morpho-functional identity of macroalgae rather than on the diversity of assemblages. Changes in the availability of seawater nutrients may foster the spread of C. racemosa, both by enhancing its growth and eroding the natural resistance of macroalgal assemblages. Thus, the maintenance of good water quality may play an important role in containing both C. racemosa and other introduced seaweeds.     

Invasion of alien macroalgae in different Mediterranean habitats

The study evaluated different macroalgal invasions in the main Mediterranean coastal habitats on hard bottom. Biodiversity, species composition and structure of macroalgal assemblages were compared among non-invaded areas and areas invaded by the Chlorophyta Caulerpa racemosa var. cylindracea and by the turf-forming Rhodophyta Womersleyella setacea in three different habitats: shallow rocky bottom, deep rocky bottom and dead matte of the seagrass Posidonia oceanica. Results showed that alien macroalgae constituted a relevant component of benthic assemblages in invaded areas of the Mediterranean Sea. Assemblages invaded by Womersleyella setacea and Caulerpa racemosa showed lower values of diversity and large differences in the structure and species composition related to non-nvaded assemblages. The species that mostly suffered from invasion were erect species reproducing sexually; moreover, the dominance of W. setacea led to low abundance of native filamentous algae, while C. racemosa colonization seemed particularly threatening for encrusting algae. All the studied habitats appeared highly invasible by alien macroalgae, even if W. setacea appeared more invasive in deeper habitats, while colonization of C. racemosa seemed more serious in shallower habitats; the dead matte of P. oceanica represented a suitable substrate for the spread of both species. Differences among assemblages in different habitats were reduced in invaded areas.     

A comparative study: Assessment of the antioxidant system in the invasive green alga Caulerpa racemosa and some macrophytes from the Mediterranean

Reactive oxygen species (ROS) are of great importance in plant metabolism. However, uncontrolled activation of ROS might have deleterious effects in cells. Eleven Mediterranean countries are still under threat of an introduced taxon of Caulerpa racemosa var. cylindracea. In the present study, it has been aimed to compare the antioxidant status of this highly invasive alga with some Mediterranean macrophytes collected in the same habitat. For this purpose, such antioxidant enzyme activities as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and lipid peroxidation (LPO) as well, levels have been determined in C. racemosa, Cystoseira barbata C. Agardh, Padina pavonica (Linnaeus) Thivy and Enteromorpha sp. The highest SOD, CAT, GSH-Px enzyme activities and the lowest LPO level have been measured in the invasive C. racemosa. In conclusion, C. racemosa var. cylindracea seems more tolerant in warring with ROS than the Mediterranean species tested do. These results could partly explain the amazing success of C. racemosa var. cylindracea introduced in the Mediterranean.     

Blitzkrieg in a Marine Invasion: Caulerpa racemosa var. cylindracea (Bryopsidales, Chlorophyta) Reaches the Canary Islands (North-East Atlantic)

On the basis of morphological and genetic studies (rDNA ITS1, 5.8S, ITS2, and a 18S rDNA intron), we confirm here that Caulerpa racemosa var. cylindracea (Sonder) Verlaque, Huisman et Boudouresque, a southwestern Australian taxon recently introduced into the Mediterranean Sea also occurs in the Canary Islands. This is the first report of C. racemosa var. cylindracea in the Atlantic. It was observed for the first time in the Canary Archipelago in 1997–1998. The speed and regional scale of expansion (north Atlantic Ocean and the Mediterranean Sea) of this invasive species appear to be among the most dramatic ever recorded. The possible outcome of this introduction in the Atlantic is discussed.     

Phytotoxic activity of caulerpenyne from the Mediterranean invasive variety of Caulerpa racemosa: a potential allelochemical

Allelopathy has been postulated as a factor in the colonisation success of Caulerpa racemosa var. cylindracea, an introduced chlorophyte alga which has become invasive in the Mediterranean Sea. In order to reveal their possible phytotoxic activity, secondary metabolites were isolated from a population growing in the Gulf of Naples (Italy) and tested on leaf tissue of the native seagrass Cymodocea nodosa, which often co-occurs with Caulerpa in nature. The seaweed metabolites were identified by ¹H and ¹³C NMR analysis and leaf portions of C. nodosa were exposed to crude diethyl ether extract and purified compounds in the laboratory, under controlled conditions of irradiance and temperature for a period of 144 h. Eventual changes in the photosynthetic performance of the seagrass tissue following the treatment were tested by monitoring its optimal quantum yield with a pulse amplitude modulated (PAM) fluorometer. One of the identified compounds, caulerpenyne, showed a phytotoxic effect, while the other purified metabolites did not reveal any effect at the assayed concentrations. Thus, our results seem to suggest a possible allelopathic activity of caulerpenyne, which may play a role in the successful competition of the invasive C. racemosa var. cylindracea with native macrophytes, such as seagrasses.     

Nutrient availability in the sediment and the reciprocal effects between the native seagrass Cymodocea nodosa and the introduced rhizophytic alga Caulerpa taxifolia

Two reciprocal experiments testing for the effects of nutrient addition in the sediment and competitive interactions between the native seagrass Cymodocea nodosa (Ucria) Ascherson and the introduced alga Caulerpa taxifolia (Vahl) C. Agardh were performed. This study was conducted for 13 months (August 1995 until September 1996) in a bay on the south coast of Elba Island (Italy). Each experiment consisted of the manipulation of the level of nutrients (addition vs. control) and the manipulation of the neighbours (presence vs. removal). Response variables were blade density and size for one experiment and shoot density and leaf length of seagrass in the other. Results indicated that the presence of Caulerpa taxifolia did not affect significantly Cymodocea nodosa shoot density and the increased nutrient availability in the sediment did not alter this pattern. Neither the removal of the canopy of the seagrass nor the fertilization of the sediment has influenced significantly the density of the alga. Both species, where co-occurring, show larger size than where the neighbour is removed. Hence, results of this study suggest that the two species on the long term are likely to coexist and that the high nutrient supply of the sediment would not enhance the probability of success neither of the seagrass nor of the alga. Predictions made on the basis of short-term results, that high nutrient loads of the substratum would have represented an even more suitable condition for C. taxifolia to colonize C. nodosa beds and that on the long-term the alga has a high probability of success, did not occur.     

Dinucleotide microsatellite markers in the genus <Emphasis Type="Italic">Caulerpa</Emphasis>

Caulerpa spp. are clonal green marine algae which often act as invasive species when growing outside their native biogeographical borders. Over the two past decades, Caulerpa taxifolia has spread along the Mediterranean coast, presently occurring at 70 sites and covering nearly 3,000 ha of subtidal area. New genetic markers (microsatellites) have been developed to assess clonal structure and genetic diversity of recently established populations of the invasive species C. taxifolia and Caulerpa racemosa in comparison with populations of the native Caulerpa prolifera in the Mediterranean. Our results show that nine polymorphic markers have been developed for C. prolifera, seven for C. taxifolia, and three for C. racemosa. Genetic diversity in Caulerpa was assessed in two geographical scales: one at a population scale where 40 thalli units were collected from C. prolifera in Cala d’Or, Mallorca, Spain, and another at a species scale, where 30 sample units were analyzed for C. prolifera, 24 for C. taxifolia, and 24 for C. racemosa from different sites in the Mediterranean, Atlantic, and Pacific Ocean. Number of alleles, expected heterozygosity, and marker amplification success are provided in each case.     

Genetic evidence for the identity of Caulerpa racemosa (Forsskål) J. Agardh (Caulerpales,Chlorophyta) in the Adriatic Sea

Three taxa of Caulerpa racemosa that differ morphologically and genetically have been reported in the Mediterranean Sea. The ‘invasive variety’ was recorded for the first time in the early 1990s in Libya. In less than 10 years, it was found in almost all parts of the Mediterranean. The first record of C. racemosa in the Croatian part of the Adriatic Sea was in 2000 and as many as 35 different localities were established by the end of 2004. In terms of morphology, the specimens from the Adriatic Sea resemble the ‘invasive variety’. To confirm this, we analysed populations from two different localities (the island of Mljet and the peninsula of Pelje?ac) using the ITS1-5.8S-ITS2 region of 18S-5.8S-26S rDNA unit as a molecular marker. The ITS1-5.8S-ITS2 region from nine individual plants was amplified by PCR, cloned in a pCR 2.1 vector and sequenced. Phylogenetic comparison of sequences from specimens found in the Adriatic Sea with specimens of the same and similar taxa, found both inside and outside the Mediterranean, provided genetic evidence that C. racemosa populating the Adriatic Sea corresponds to the Mediterranean C. racemosa var. cylindracea (Sonder) Verlaque, Huisman?&?Boudouresque, i.e. to the ‘invasive variety’.     

Caprellid assemblages (Crustacea: Amphipoda) in shallow waters invaded by <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> from southeastern Spain

Growth of the invasive algae Caulerpa racemosa var. cylindracea in shallow habitats may influence the faunal assemblage composition. We studied its effects on caprellid assemblages associated with shallow-water habitats of hard and soft bottoms from the SE Iberian Peninsula (native rocky-bottom algae, C. racemosa from hard and soft bottoms, and Caulerpa prolifera, Cymodocea nodosa and Posidonia oceanica from soft bottoms). Samples were taken in two different sampling periods (September 2004 and March 2005). A total of seven caprellid species were identified, with important differences in their distribution in different habitats. Total abundance of caprellids was very high in March on native algae on hard bottoms, and on C. racemosa on both soft and hard bottoms. On both hard and soft bottoms, abundances of Caprella hirsuta recorded from C. racemosa were low. On the other hand, a higher abundance of other species, namely C. acanthifera, C. santosrosai, Phtisica marina and Pseudoprotella phasma, was recorded from C. racemosa. The results indicate that C. racemosa may have a positive influence on some caprellid species, while seasonal changes are also evident. It is concluded that introduced C. racemosa may serve as a new habitat, promoting and maintaining caprellid populations in shallow Mediterranean habitats.     

Mediterranean Species of Caulerpa Are Polyploid with Smaller Genomes in the Invasive Ones

Caulerpa species are marine green algae, which often act as invasive species with rapid clonal proliferation when growing outside their native biogeographical borders. Despite many publications on the genetics and ecology of Caulerpa species, their life history and ploidy levels are still to be resolved and are the subject of large controversy. While some authors claimed that the thallus found in nature has a haplodiplobiontic life cycle with heteromorphic alternation of generations, other authors claimed a diploid or haploid life cycle with only one generation involved. DAPI-staining with image analysis and microspectrophotometry were used to estimate relative nuclear DNA contents in three species of Caulerpa from the Mediterranean, at individual, population and species levels. Results show that ploidy levels and genome size vary in these three Caulerpa species, with a reduction in genome size for the invasive ones. Caulerpa species in the Mediterranean are polyploids in different life history phases; all sampled C. taxifolia and C. racemosa var. cylindracea were in haplophasic phase, but in C. prolifera, the native species, individuals were found in both diplophasic and haplophasic phases. Different levels of endopolyploidy were found in both C. prolifera and C. racemosa var. cylindracea. Life history is elucidated for the Mediterranean C. prolifera and it is hypothesized that haplophasic dominance in C. racemosa var. cylindracea and C. taxifolia is a beneficial trait for their invasive strategies.     

Feeding strategy of the sacoglossan opisthobranch <Emphasis Type="Italic">Oxynoe olivacea</Emphasis> on the tropical green alga <Emphasis Type="Italic">Caulerpa taxifolia</Emphasis>

The feeding behaviour of the shelled sacoglossan Oxynoe olivacea was investigated to better understand the role and importance of this species in influencing encroachments of the alien alga Caulerpa taxifolia in the Mediterranean sea. We tested whether this slug preferred, as preliminary field observations suggested, an aggregative feeding behaviour and which part of the algal thallus, phylloid vs rhizoid, it preferred. Results showed that O. olivacea fed in groups and actively selected phylloid. This outcome poses important questions regarding the possibility that this species, fragmenting the alga thallus, could enhance dispersion and regeneration of C. taxifolia.     

Impacts of environmental stress on resistance and resilience of algal‐associated bacterial communities

Algal‐associated bacteria are fundamental to the ecological success of marine green macroalgae such as Caulerpa. The resistance and resilience of algal‐associated microbiota to environmental stress can promote algal health and genetic adaptation to changing environments. The composition of bacterial communities has been shown to be unique to algal morphological niches. Therefore, the level of response to various environmental perturbations may in fact be different for each niche‐specific community. Factorial in situ experiments were set up to investigate the effect of nutrient enrichment and temperature stress on the bacterial communities associated with Caulerpa cylindracea. Bacteria were characterized using the 16S rRNA gene, and the community compositions were compared between different parts of the algal thallus (endo‐, epi‐, and rhizomicrobiome). Resistance and resilience were calculated to further understand the changes of microbial composition in response to perturbations. The results of this study provide evidence that nutrient enrichment has a significant influence on the taxonomic and functional structure of the epimicrobiota, with a low community resistance index observed for both. Temperature and nutrient stress had a significant effect on the rhizomicrobiota taxonomic composition, exhibiting the lowest overall resistance to change. The functional performance of the rhizomicrobiota had low resilience to the combination of stressors, indicating potential additive effects. Interestingly, the endomicrobiota had the highest overall resistance, yet the lowest overall resilience to environmental stress. This further contributes to our understanding of algal microbiome dynamics in response to environmental changes.     

Invasive alga <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> makes a strong impact on the Mediterranean sponge <Emphasis Type="Italic">Sarcotragus spinosulus</Emphasis>

Here we present the first observation of the impact of the invasive Caulerpa racemosa var. cylindracea on native photophilic sponge species in the Adriatic Sea, with special focus on Sarcotragus spinosulus. Caulerpa racemosa var. cylindracea is able to completely overgrow the sponge, developing an exceptionally thick canopy with a maximum measured density of 1,887 m of stolons m⁻² and 40,561 fronds m⁻². Necrosis of the sponge surface was significantly correlated with the algal dry biomass, frond number and stolon length. Dense algal canopy, penetration of the algal stolon and rhizoids into the sponge oscula and covering of the ostiae probably diminishes the seawater circulation through the sponge and consequently results in its smothering and even death. We suggest that chemotropism is the reason why C. racemosa penetrates the sponge oscula and establishes such dense canopy on the sponge.     

Fragment quality and sediment organic loading regulate the survival of an invasive,clonal seaweed

Although propagule pressure is recognized as an important determinant of invasion dynamics, the role of propagule quality (i.e. the physical condition of a propagule) has received little attention. In particular, how the performance of vegetative propagules differing in quality varies across heterogeneous landscapes is yet to be explored. Caulerpa cylindracea is a clonal, invasive seaweed, widely distributed in the Mediterranean. By means of a laboratory experiment, we investigated how variation in the quality of seaweed fragments (intact vs. frond-removal vs. rhizoid-removal) influenced their survival on control versus sediments enriched with detritus from the native seagrass, Posidonia oceanica. The survival of seaweed fragments was low on non-enriched sediments, irrespective of their characteristics. On enriched sediments, survival was high in control and rhizoid-removal fragments, but low in frond-removal fragments. Our study shows that both fragment quality and sediment characteristics influence the survival of C. cylindracea propagules and, hence, long-term spreading dynamics of this seaweed. More generally, it brings novel evidence showing that the effects of propagule quality on invasion success are context-dependent.     

Tracking the origin of the invading Caulerpa (Caulerpales, Chlorophyta) with Geographic Profiling, a criminological technique for a killer alga

The application of the Geographic Profiling technique (with the “Rossmo formula”) proved to be effective in assessing the spreading origin of invading species of Caulerpa in the Mediterranean. Geoprofiling is a technique more frequently used in criminology. We applied this method to an algal invasion for the first time. The method was calibrated with the distribution data of Caulerpa taxifolia, whose spreading in the Mediterranean Sea started from the aquarium of Monaco. This is the first time that Geographic Profiling is calibrated on a data set of sites of presence of a biological invader, of which the spreading origin is known. The application on Caulerpa racemosa var. cylindracea showed that the probable spreading center of the alga should be located in Western Sicily for the Mediterranean and in the southern part of Tenerife for the Canary Islands (Las Galletas, a fishing village). The spreading centers correspond to areas with ports, indicating that the spreading of this alga should be related to (fishing) boats. This result confirmed the opinion of other authors. The parameters used in the Rossmo Formula, obtained through calibration with the known origin of C. taxifolia invasion for identifying the origin of the spread of C. racemosa var. cylindracea may be extended to other algae with similar propagation mode and similar habitat requirements.     

Antioxidant response and caulerpenyne production of the alien Caulerpa taxifolia (Vahl) epiphytized by the invasive algae Lophocladia lallemandii (Montagne)

Invasive algae alter the structure and function of ecosystems. The red algae Lophocladia lallemandii grows in the West Mediterranean epiphyting the green algae Caulerpa taxifolia, another invasive alga. Our aim was to determine whether the invasive algae Lophocladia, recently introduced in the West Mediterranean, induces oxidative stress and an antioxidant response in Caulerpa taxifolia. We measured the caulerpenyne production, the activities of antioxidant enzymes and the levels of markers of lipid peroxidation in Caulerpa taxifolia. Caulerpenyne concentration was significantly higher in Caulerpa taxifolia epiphytized by Lophocladia. End-products of lipid peroxidation -malondialdehyde (MDA) and 4-hydroxinonenal (4-HNE)- were significantly increased in Caulerpa taxifolia samples from the station with Lophocladia. Antioxidant enzyme activities -catalase and superoxide dismutase (SOD)-, as well as H₂O₂ production significantly increased in the Lophocladia station compared to the control station. The activities of catalase and SOD determined in Lophocladia lallemandii showed no significant differences between Lophocladia living alone and Lophocladia epiphytating Caulerpa taxifolia. Caulerpa taxifolia epiphytized by Lophocladia lallemandii responded by increasing the caulerpenyne and H₂O₂ production and the antioxidant enzymes activities as a defensive mechanism against the new invasive algae.     

An investigation on the antioxidant status of the invasive alga Caulerpa racemosa var. cylindracea (Sonder) Verlaque, Huisman, et Boudouresque (Caulerpales, Chlorophyta)

Caulerpa racemosa var. cylindracea is a potential invader of the Mediterranean Sea and 11 Mediterranean countries are under threat from this alga. In the present study, in order to investigate seasonal changes in the antioxidant status of C. racemosa var. cylindracea, antioxidant enzyme activities such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and lipid peroxidation (LPO) levels were determined in C. racemosa var. cylindracea samples, which were collected in May 2003, September 2003, December 2003, and March 2004 from Turkish coastlines. SOD and CAT activities showed similar trends. These enzymes increased from May to December, then sharp decreases were observed. On the other hand, GSH-Px enzyme activities showed a fluctuation between May 2003 and March 2004. In contrast to increases in SOD and CAT activities up to December 2003, LPO level decreased in this period. No significant correlation was observed between antioxidant status and solar radiation. In conclusion, the antioxidant status of C. racemosa var. cylindracea is strictly not affected by both solar radiation and seawater temperature; however, the growth of epiphytes on fronds may change antioxidant status. Further investigations are strongly warranted to understand the contributions of non-enzyme-based antioxidants such as glutathione, vitamin E, and vitamin C.     

Effects of <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> invasion on soft-bottom assemblages in the Western Mediterranean Sea

The present study aimed at evaluating the effects of Caulerpa racemosa var. cylindracea spread on Mediterranean soft-bottom assemblages. The benthic assemblages colonized by C. racemosa were compared with non invaded assemblages at multiple spatial scales. In addition, a manipulative experiment has been conducted over a one-year period in order to compare the structure of native assemblages invaded by the alga with others where the alga has been manually removed and others that were not invaded. Results of both studies showed that Mediterranean soft-bottom assemblages invaded by C. racemosa differed from non invaded ones in terms of species composition, abundance and patterns of spatial variability. Moreover, in areas cleaned out from the alga, the benthic assemblages begin to recover their structure and after one year they get more similar to the assemblages observed in non invaded areas. The present paper, taking into account different aspects of C. racemosa invasion in Mediterranean soft bottoms, highlighted that the observed increase in alpha diversity did not correspond to an increase in the overall diversity of the studied system. Infact, the loss of beta diversity in invaded sites, together with the spread of species typical of vegetated habitats, may contribute to a homogenization of the Mediterranean coastal system. Furthermore, a possible role of ecosystem engineer for C. racemosa can be hypothesized.     

Stress tolerance of the invasive macroalgae <Emphasis Type="Italic">Codium fragile</Emphasis> and <Emphasis Type="Italic">Gracilaria vermiculophylla</Emphasis> in a soft-bottom turbid lagoon

Invasive species are often hypothesized to have superior performance traits. We compared stress tolerance (as change in biomass) of the invasive macroalgae Codium fragile ssp. tomentosoides and Gracilaria vermiculophylla to the native macroalgae Fucus vesiculosus, Agardhiella subulata, Hypnea musciformis and Ulva curvata in Hog Island Bay, a shallow lagoon in Virginia, USA. We hypothesized that the success of the two aliens is due to their high tolerances of turbidity, sedimentation, desiccation, grazing and nutrient enrichment. Like many lagoons, Hog Island Bay is characterized by extensive intertidal mudflats, high turbidity and sedimentation, and high densities of omnivorous mud snails. Nutrient enrichment may also become a problem as land use practices in adjacent watersheds change. Contrary to our hypothesis, C. fragile was less resistant to sedimentation, desiccation and grazing than other algae and had low growth at all light and nutrient levels. This suggests that any superior performance of this invasive species compared to native algae is probably limited to microhabitats where stress is minimal and where bivalve shells facilitate recruitment and long-term persistence. In contrast, G. vermiculophylla was resistant to desiccation, burial and grazing, and was not negatively influenced by either high or low light or nutrient levels. These traits reflect the current success of G. vermiculophylla in already invaded lagoons and estuaries, and indicates that it will likely continue its spread in European and North American turbid and tidal soft-sediment systems.