Algae Evolution Resolution

The kingdom-level taxon Chromista means "colored." Members of this group are photosynthetic but are not closely related to land plants or other algae. They contain chlorophyll c, fucoxanthin, lack the plasmodesmata, and do not store their energy in the form of starch. They also contain pigments not found in plants, pigments that give them their characteristic brown coloration. Phaeopheta, the brown algae, are the largest members of the Chromista group and are mostly marine organisms. They are typically coastal, found attached to rocks, coral, or other firm surfaces. Those large kelp forests that you see seals swimming through on nature programs are a good example of this group.

A new paper in Nature takes a look at the brown algal genome to understand the evolution of multicellularity and photosynthesis. Researchers sequenced the genome of Ectocarpus siliculosus, a large brown seaweed that occurs along temperate latitude coastlines. In their paper they report on the 214 million base pair genome sequence.

They found that the E. siliculosus genome includes several features that have evolved for surviving in hostile shoreline environments that vary in light intensity, temperature, salinity and wave action. The researchers found a light harvesting complex (LHC), 53 loci including a cluster of 11 genes with highest similarity to the LI818 family of light-stress related LHCs. Also, the genome is predicted to encode a light-independent protochlorophyllide reductase (DPOR). This allows for the synthesis of chlorophyll under dim light. Together LHC and DPOR allow for survival in environments with highly varying light conditions. The genome encodes 21 putative dehalogenases and two haloalkane dehalogenases that may serve to protect the alge against halogenated compounds produced by kelps as defence molecules. This allows the brown algae to grow epiphytically on these organisms. The cell walls contain alginates and fucans, polysaccharides with properties that help in the resistance to mechanical stress and protection from predators.

The researchers also predicted the pattern of loss and gain of gene families during the evolution of a broad range of eukaryotes. A comparison of genomes showed that the major eukaryotic groups "have retained distinct but overlapping sets of genes since their evolution from a common ancestor, with new gene families evolving independently in each lineage. On average, lineages that have given rise to multicellular organisms have lost fewer gene families and evolved more new gene families than unicellular lineages. However, we were not able to detect any significant, common trends, such as a tendency for the multicellular lineages to gain families belonging to particular functional (gene ontology) groups." They found many genes for kinases, transporter and transcription factors, that are also commonly found in land plants. They suspect that these kinases play a key role in the origin of multicellular organisms. These data also relate to the idea that brown algae arose from the fusion of green alga and red alga as a high proportion of the genes that are characteristic of green algae, including the kinases and transporters typical of land plants, were found in the brown alga.

Here's the paper (its a little dense, especially if you are not a geneticist):

Cock, J. Mark, et al. (2010) The Ectocarpus genome and the independent evolution of multicellularity in brown algae. Nature: 465 (7298), 617-621. (DOI: 10.1038/nature09016)

(image from oceanexplorer.noaa.gov)