free-living, single-celled eukaryote to becompletely sequenced (the first being that of S. cerevisiae). Wood et al. take advantage ofthis to try to identify genomic properties thatare related to multicellularity. They usedanother conservative approach to comparethe genomes of the two yeast species with theavailable genome sequences of multicellulareukaryotes (a plant and three animals), andfound only three genes that were specific toall the multicellular species.

This may seem surprising, but it proba-bly should not. First, the comparison didnot take into account that multicellularity probably evolved separately in plants andanimals14 (Fig. 1), so different multicellu-larity-related genes may have evolved inthese two evolutionary lineages. Second, thetime interval during which these genescould have evolved is much shorter than for

the eukaryotic versus prokaryotic compari-son — in other words, there has been lesstime to ‘invent’ new genes. Perhaps moreusefully here, the authors found that, evenafter correcting for differences in genomesize, some protein domains are more com-mon in the multicellular than in the uni-cellular organisms, probably reflecting theexpansion of certain protein families. Thisimplies that such expansions may haveoccurred in parallel during the evolution of multicellular animals and plants, but thesame genes were rarely if ever invented byboth groups.

So what next? Clearly, a better compari-son of eukaryotes and prokaryotes requirescomplete genome sequences from a morediverse sampling, not just those eukaryotesfrom the ‘top’ of the tree (Fig. 1). Lumpedtogether as ‘protists’, these other eukaryotes

show remarkable diversity and include manyparasitic species, such as the malaria-causingPlasmodium; species such as Giardia thatlack the cellular powerhouses, mitochon-dria; and organisms with several nuclei andunusual genome-rearrangement processes,such as Tetrahymena. It will also be interest-ing to use the S. pombe and other fungalgenomes to do a more thorough comparisonwith genomes from the Microsporidia, suchas the parasitic Encephalitozoon15. Theseorganisms were once classified with the protists but are now thought to be related to fungi16.

In all these comparisons, it will be import-ant to go beyond simply identifying the similarities and differences between species,and to analyse the origin of the differences,for example the gain, loss and possible trans-fer of genes over time17. But today we should

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NATURE | VOL 415 | 21 FEBRUARY 2002 | www.nature.com 847

Marine archaeology

Acid attackIf you were planning to visitthe impressive restoration of the Vasa, now is a goodtime. The Vasa was a 61-metre,1,210-tonnewarship, which sank inStockholm harbour on itsmaiden voyage in 1628, butwas raised in 1961 andrestored for display in amuseum in Stockholm (see

pictures). A multidisciplinarygroup of researchers,however, has discovered that the ship’s timbers are in danger of disintegrating.As Magnus Sandström andcolleagues report in this issue (Nature 415, 893–897;2002), and describe in anexhibition that opens thisweek in the Vasa Museum,sulphuric acid is beingproduced within the beams of the ship. The acid attacksthe wood both chemically, byacid hydrolysis of cellulose,and physically, as thesulphate minerals expandduring crystallization.

So where is the sulphuricacid coming from? Alerted by sulphate crystals formingon the surface of the Vasa’stimbers, Sandström et al.went on to find largequantities of elementalsulphur inside the wood. They believe that hydrogensulphide — a commonproduct of bacterialdecomposition in anoxicwaters — permeated theship’s timbers and wasgradually transformed toelemental sulphur during the333 years that the ship lay at the bottom of Stockholmharbour. This created areservoir of sulphur, which, iffully oxidized, could produce

as much as five tonnes ofsulphuric acid.

A complicating factorcomes from the legacy of theship’s 9,000 original iron boltsthat have largely rusted away.Iron (III) ions are effectively acatalyst here, graduallyoxidizing elemental sulphur to sulphuric acid. Thereduced iron is then itself re-oxidized by oxygen fromthe air, ready to go throughthe cycle once more.

Museum curators arewell aware of the threat ofoxidation to waterloggedwooden artefacts aftersalvage. As well as stabilizingthe fragile lattice ofremaining wood by replacingthe water with non-volatilepreservatives, conservationmethods routinely involvelimiting further biological and chemical oxidation withsterilizing solutions, andcarefully controlling humidityand temperature. But thenewly observed sulphurthreat calls for differentmeasures. Neutralizing fivetonnes of sulphuric acid isnot really feasible, and themost promising solution liesin tackling the iron catalyst.One proposal of Sandström et al. is to identify an agentthat will form a complex withthe iron solutes, making them

inert, and possibly evenextractable by rinsing theship with alkali.

What about other wrecks?Fortunately, the Vasa seems to be an extreme case. Thedecision in the sixteenthcentury to close off two inletsinto Stockholm harbour, tohinder attack from theRussians, along with centuriesof sewage disposal in theharbour, helped to create an especially stagnant andsulphurous resting place.Moreover, the threat ofsulphur acidification in wrecksthat were completely buried in sediments, such as theMary Rose, now on display in Portsmouth, UK, should besmaller. Indeed, Sandström et al. found that sulphuraccumulation was greatest in

the exposed timbers of theVasa. But in ships that werenot buried — the Batavia ofthe Dutch East India Company,for instance, which waswrecked off Western Australiain 1629 — initial analysesconfirm that the sulphurproblem is more common, ifnot as severe as in the Vasa.

These new investigationssupport recent moves to letsunken ships lie, andpreserve and study themwhere they sank (Nature 415,460; 2002) — virtualtechnology would still allowthe public to view the wrecksand their sites. After all, why not capitalize on thepreserving features of marinesediments, rather than letthem express their acidic sidelater on? Jim Gillon

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