It Survived a Freeze, but can Ascophyllum nodosum take the Heat?

Ascophyllum nodosum is a fucoid intertidal seaweed found along North Atlantic coastlines. Its northerly range extends from Arctic Canada, Norway, Greenland, and Iceland down to Portugal and Long Island where it has an abrupt southern limit at the 42° N latitude. A. nodosum is a dominant rocky intertidal species and provides habitat and canopy for other intertidal organisms. Because of its role as a foundational species, change in the range and abundance of A. nodosum has substantial effects on entire intertidal communities (Olsen et al. 2010).

The distribution of present day intertidal species was affected by the last glacial maximum (LGM) that occurred 20,000 years ago.  The advancing ice pushed many northern algae and invertebrate species to southern refugia. Refugia do not always exist at the southern most edge, but most expansion after the glacial retreat has been from the southern edge to the north. As a result, post-glacial populations have been genetically characterized by “southern richness and northern purity” (Hewitt 1999). At the front of the dispersal, the northern populations typically exhibit low genetic diversity. Southern populations, however, usually have increased diversity due to the fusion of once isolated subpopulations. Olsen et al.  (2010) tested this hypothesis that links genetic variation and population structure to climactic periods on A. nodosum, using six microsatellites and an mtDNA intergenic spacer as markers.

The predicted haplotype diversity is not exhibited by A. nodosum across the 28 locations (see Fig 1 below). The coasts have retained a genetic signature that indicates isolation that predated the LGM. Population divergence between the North American and European halpotypes is estimated to be between 1.8 million years ago and 0.08 million years ago, indicating separate populations existed on both sides of the Atlantic long before the LGM. Private alleles also confirm its existence on both Atlantic coasts prior to the LGM.

Figure 1. A. nodosum haplotype distribution and haplotype network (inset) for the IGS-trnW locus (mtDNA marker). Haplotypes are widely distributed, there is no distinct southern richness and northern purity pattern. The 20 thousand years since the LGM does not account for the amount of sequence divergence and for the distinct alleles found in northern populations.

A large effective population size (Ne), estimated in A. nodosum to be between 12,000 and 486,000, was sufficient to maintain rare haplotypes.  This ancestral Ne is approximately the same in the present population. The migration rate from Europe to North America is 5 to 60 fold greater than migration in the reverse direction.  However, there is minimal contemporary population differentiation across the Atlantic. Like the findings of other algae and invertebrate studies, Olsen et al. found Brittany was clearly a refugium and hotspot for diversity. Evidence supports an additional mid-range refugium in southern Maine, which preserved private haplotypes and North American genetic diversity (see Figure 2 below).

Figure 2. Neighbor joining tree for the six microsatellite loci. Grey shaded areas indicate refugia locations, upper circle is the Southern Maine (USA) refugium and lower circle is the Brittany (France) refugium. 

Like a “marine tree”, Olsen et al. characterize A. nodosum by its long lifespan, large size, high fecundity, high within population diversity and weak large scale differentiation. Like an old growth forest, Brittany is an important site for genetic diversity and is important in species conservation. A. nodosum is an old species and has survived many glacial and interglacial cycles.

Understanding how a species reacted to climate change in the past can provide a substantial basis for future projections. It has been reported that intertidal species in the North Atlantic have experienced ranges shifts of 50m per decade in response to current climate change (Helmuth et al. 2006). Externally fertilized eggs and sperm produce non-buoyant zygotes, which typically disperse less than 7 meters but to a maximum of 30m (Olsen et al. 2010). A rapid shift in summer sea temperatures could wipe out the southern populations, if unable to disperse to a more northerly range. As the northern locations increasingly become ice-free year-round, the northern population of A. nodosum will likely experience expansion.

The indirect influence climate change has on biotic interactions is particularly interesting to me since I study Vertebrata lanosa, a red algal epiphyte on A. nodosum.   Helmuth (2006) points out that organisms exposed to the same conditions can exhibit very different core temperatures and therefore different levels of physiological stress. A. nosodum is the host to 3 algal epiphytes, 2 brown and the red algae V. lanosa.  Longtin et al. (2009) found that V. lanosa may be less tolerant to high irradiance and dessiation stress compared to both the brown epiphytes. However, V. lanosa is still able to exclude the other epiphytes from the middle frond segments. Some cold-water species, including A. nodosum, have not yet experienced a range shift. With the advance of climate change, V. lanosa may be exposed to different conditions, lose its dominance, and retreat into refugia or experience localized extinctions.

Figure 3. Epiphytic Vertebrata lanosa tufts on the brown algae, Ascophyllum nodosum. The red algae, V. lanosa is found in greater abundance at mid-intertidal elevation and within the canopy of A. nodosum rather than on the periphery (Longtin et al. 2009).

It would be interesting to see if the post-glacial V. lanosa population exhibits the same genetic framework as A. nodosum. The more resilient A. nodosum is better able to survive at the extremes of its limits, and as climate change progresses how will the warming seas affect the distribution of V. lanosa compared to A. nodosum?

Literature Cited

Olsen JL, Zechman FW, Hoarau G, Coyer JS, Stam WT, Valero M, Åberg P (2010) The phylogeographic architecture of the fucoid seaweed Ascophyllum nodosum: an intertidal ‘marine tree’ and survivor of more than one glacial-interglacial cycle. J Biogeogr. 37: 842-856.

Hewitt, GM (1999) Post-glacial re-colonization of European biota. Biological Journal of the Linnean Society, 68, 87-112.

Helmuth, B, Mieszkowska N, Moore P, Hawkins SJ (2006) Living on the edge of two changing worlds: forecasting the responses of rocky intertidal ecosystems to climate change. Annual Review of Ecology, Evolution, and Systematics, 37, 373-404.

Longtin, CM, Scrosati RA, Whalen GB, Garbary DJ (2009) Distribution of algal epiphytes across environmental gradients at different scales: intertidal elevation, host canopies, and host fronds. J Phycol, 45: 820-827.

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