Latitudinal phytoplankton distribution and the neutral theory of biodiversity

Aim Recent studies have suggested that global diatom distributions are not limited by dispersal, in the case of both extant species and fossil species, but rather that environmental filtering explains their spatial patterns. Hubbell's neutral theory of biodiversity provides a framework in which to test these alternatives. Our aim is to test whether the structure of marine phytoplankton (diatoms, dinoflagellates and coccolithophores) assemblages across the Atlantic agrees with neutral theory predictions. We asked: (1) whether intersite variance in phytoplankton diversity is explained predominantly by dispersal limitation or by environmental conditions; and (2) whether species abundance distributions are consistent with those expected by the neutral model. Location Meridional transect of the Atlantic (50 degrees N50 degrees S). Methods We estimated the relative contributions of environmental factors and geographic distance to phytoplankton composition using similarity matrices, Mantel tests and variation partitioning of the species composition based upon canonical ordination methods. We compared the species abundance distribution of phytoplankton with the neutral model using Etienne's maximum-likelihood inference method. Results Phytoplankton communities are slightly more determined by niche segregation (24%), than by dispersal limitation and ecological drift (17%). In 60% of communities, the assumption of neutrality in species' abundance distributions could not be rejected. In tropical zones, where oceanic gyres enclose large stable water masses, most communities showed low species immigration rates; in contrast, we infer that communities in temperate areas, out of oligotrophic gyres, have higher rates of species immigration. Conclusions Phytoplankton community structure is consistent with partial niche assembly and partial dispersal and drift assembly (neutral processes). The role of dispersal limitation is almost as important as habitat filtering, a fact that has been largely overlooked in previous studies. Furthermore, the polewards increase in immigration rates of species that we have discovered is probably caused by water mixing conditions and productivity.

Chaetopterid tubes from vent and seep sites: Implications for fossil record and evolutionary history of vent and seep annelids

Vestimentiferan tube worms living at deep-sea hydrothermal vents and cold seeps have been considered as a clade with a long and continuing evolutionary history in these ecosystems. Whereas the fossil record appears to support this view, molecular age estimates do not. The two main features that are used to identify vestimentiferan tubes in the fossil record are longitudinal ridges on the tube's surface and a tube wall constructed of multiple layers. It is shown here that chaetopterid tubes from modern vents and seeps—as well as a number of fossil tubes from shallow-water environments—also show these two features. This calls for a more cautious interpretation of tubular fossils from ancient vent and seep deposits. We suggest that: current estimates for a relatively young evolutionary age based on molecular clock methods may be more reliable than the inferences of ancient “vestimentiferans” based on putative fossils of these worms; not all of these putative fossils actually belong to this group; and that tubes from fossil seeps should be investigated for chitinous remains to substantiate claims of their potential siboglinid affinities.

Evolution of the Diatoms: VIII. Re-Examination of the SSU-Rrna Gene Using Multiple Outgroups and a Cladistic Analysis of Valve Features.

The resolution of the SSU rRNA gene for phylogenetic analysis in the diatoms has been evaluated by Theriot et al. who claimed that the SSU rRNA gene could not be used to resolve the monophyly of the three diatoms classes described by Medlin and Kaczmarska. Although they used both only bolidomonads and heterokonts as outgroups, they did not explore outgroups further away than the heterokonts. In this study, the use of the multiple outgroups inside and outside the heterokonts with the rRNA gene for recovering the three monophyletic clades at the class level is evaluated. Trees with multiple outgroups ranging from only bolidophytes to Bacteria and Archea were analyzed with Bayesian and Maximum Likelihood analyses and two data sets were recovered with the classes being monophyletic. Other data sets were analyzed with non-weighted and weighted maximum parsimony. The latter reduced the number of clades and lengthened branch lengths between the clades. One data set using a weighted analysis recovered the three classes as monophyletic. Taking only bolidophytes as the only outgroup never produced monophyletic clades. Multiple outgroups including many heterokonts and certain members of the crown group radiation recovered monophyletic clades. The three classes can be defined by clear morphological differences primarily based on auxospore ontogeny and envelope structure, the presence or absence of a structure (tube process or sternum) associated with the annulus and the location of the cribrum in those genera with loculate areolae. A cladistic analysis of some of these features is presented and recovers the three classes.