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Native and introduced brown seaweeds, coexistence is possible

What happens within an marine ecosystem when a new species is introduced and proliferates? The comparison of three brown seaweeds from the family of Sargasso shows that introduced and native species co-exist owing to their different ecological and chemical strategies.

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Species found in marine ecosystems are either native or introduced, according to whether they naturally developed and evolved in a given habitat of the area, of were brought from a different geographic area and then found conditions suitable to settle and develop. The increasing number of introduced marine species is a consequence of human activities like shipping, fish or shellfish farming, or yachting. An introduced species is termed invasive when its populations grow in very large proportions, causing a regression of native species and a significant change in the structure of communities.

This is the case of some seaweeds, one of which is Sargassum muticum, a brown alga native from Japan and observed in Europe from 1973. In Brittany, it now coexists with two native species of the same family, Sargassaceae. How native and invasive species live together depends on the environmental factors (light, temperature, nutrient availability,…) and their respective interactions with other species within the community (predators, pathogens).

Trois Sagassaceae

The three algae studied (Photos : K. Le Lann)

From a metabolic standpoint, phenolic compounds play a role in the process of adaptation of seaweeds to environments factors, like the defence against grazers and epiphytes (other algae growing on their surface) or the protection against ultraviolet radiations. This paper deals with the spatial and temporal variations in the content of phenolic compounds of the invasive species Sargassum muticum. The study also compares the biological and chemical characteristics of this species and the two native ones. A difference in phenolic compounds content between them would be an important parameter for the success of the introduced species in its new habitat.

Seaweeds were sampled monthly for a 18-month period, at low tide, in a sandy sheltered site and a rocky one exposed to wave action; at each sampling date, number and length of individuals, and number of mature ones were recorded. Environmental parameters were also measured to study their potential influence on the phenolic composition of algae.

Structural differences were found between species and sites. The nature of the bottom and the currents both influence the distribution of two out of the three species, among which the invasive Sargassum muticum. These parameters also influence the length of its individuals, as larger ones are found in the sheltered site than in the exposed one. The main difference found between native and invasive species is the delay between their reproductive cycles: S. muticum reaches its peak of sexual maturity in summer whereas the two native species reaches theirs in winter. The amount of phenolic compounds fluctuates according to the same seasonal cycle, with an increase before the reproductive period.

Phenolic compounds do not vary only in their total quantity: their nature also varies through time, particularly as far as the size composition of these molecules is concerned. In S. muticum, the proportion of small compounds decreases in summer in favour of larger ones, which could contribute to the protection against UV radiations. As this species has its reproduction peak in summer, these medium-sized compounds could also contribute to the protection against predators and pathogens during this critical phase of its life cycle. In B. bifurcata, medium-sized compounds increase in summer on one site (photoprotection) and in winter on the other one, during the reproduction. In C. baccata, they increase only in winter, here again during reproduction.

As the production of medium-sized phenolic compounds if closely related to the critical phases of the life cycle (UV exposition and reproduction), these results show the important role they play in the chemical defence of seaweeds. In addition, the differences observed between the three species confirm that they have different ecological and chemical strategies in spite of similar environmental conditions (same season, same site). This could help explain how the invasive species settled and maintained its populations while coexisting in the same habitat with two native species.

When introduced, a species may be invasive and imposed a constraint on space and resources for native species. But an introduction does not necessarily entail the local extinction of native species. This study on seaweeds shows that if native species have the ability to modify their behaviour and their ecological strategies, species of both origins can coexist in the same habitat.

 

The paper

K. Le Lann, S. Connan, V. Stiger-Pouvreau, 2012. Phenology, TPC and size-fractioning phenolics variability in temperate Sargassaceae (Phaeophycae, Fucales) from Western Brittany: Native versus introduces species. Marine Environmental Research, 80, 1-11.
See the first page

The authors

This work was conducted by researchers from LEMAR laboratory of IUEM.

 

The journal

Published by the international science editor Elsevier, Marine Environmental Research is a leading journal on marine environmental research. It serves as a forum for new information on biology, chemistry, and toxicology and syntheses that advance understanding of marine environmental processes. It covers at national, regional and international scales a variety of fields like environmental changes of natural marine systems, contaminants, biogeochemistry of natural and anthropogenic substances or modelling of marine processes.

 

Contacts

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