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Diatoms are one of the most important organisms contributing to aquatic primary productivity and their sterols are frequently used as markers for their presence and abundance. In this study, the sterol composition of .100 diatom cultures was analyzed and its distribution was compared to the diatom phylogeny to identify typical diatom biomarkers. Forty-four sterols were detected, 11 of them being commonly present as major sterols (contributing .10% to the total sterols). 24-Methylcholesta-5,24(28)-dien-3b-ol is the most common sterol in diatoms, being present in 67% of all cultures analyzed, followed by the D 5 sterols, cholest-5-en-3b-ol (cholesterol), 24-methylcholest-5-en-3b-ol, and 24-ethylcholest-5-en-3b-ol. 24-Methylcholesta-5,22E-dien-3b-ol, previously suggested to be specific for diatoms, was only the fifth most common sterol; this sterol was absent in some of the major diatom groups, and high relative concentrations seem to be restricted to pennate diatoms. No sterols are restricted to specific phylogenetic groups of diatoms. Cluster analyses, however, do reveal distinct sterol distributions: Thalassiosirales typically contain high relative abundances of 24-methylcholesta-5,24(28)-dien-3b-ol, high relative abundances of cholesta-5,22E-dien-3b-ol are typical for Cymatosirales, high relative abundances of 24-ethylcholesta-5,22E-dien-3b-ol are characteristic for related Amphora, Amphiprora, and Entomoneis species, and a combination of high relative abundances of 24-methylcholest-5-en-3b-ol, 24-methylcholesta-5,24(28)-dien3b-ol, and 24-ethylcholest-5-en-3b-ol is typical for Attheya species. High contributions of 24-methylcholesta-5,22E-dien-3b-ol (.50% of all sterols) seem to be restricted to pennate diatoms. None of the major sterols found in diatoms can be used as an unambiguous diatom biomarker because all of them have been reported as common sterols in other algal groups.

The TEX ଼ ୌ paleothermometer based on isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) has widely been applied in various marine settings to reconstruct past sea surface temperatures (SSTs). However, it still remains uncertain how well this proxy reconstructs annual mean SSTs. Here, we assess environmental factors governing the TEX ଼ ୌ paleothermometer in the Mediterranean Sea, by studying the distribution of isoGDGTs in surface sediments, suspended particulate matter (SPM), and two sediment cores. A redundancy analysis using the fractional abundance of the six major isoGDGTs indicates that the sedimentary isoGDGTs are mostly influenced by three environmental factors explaining a large part (74%) of the variance in isoGDGT distribution. In order of decreasing significance, these factors are annual mean SST, continental organic matter input as indicated by the BIT index, and water depth. However, when considering only the four isoGDGTs that are used for the TEX ଼ ୌ proxy, water depth is the most significant parameter, explaining 63% of the variance. Indeed, a strong positive relationship between water depth and TEX ଼ ୌ is observed in both surface sediments and SPM from the Mediterranean Sea. This is driven by an increase in fractional abundances of GDGT-2 and crenarchaeol regio-isomer and a decrease in the fractional abundances of GDGT-1 and GDGT-3 with increasing water depth, leading to a bias to higher temperatures of TEX ଼ ୌ in deep-water surface sediments. The fact that the water-depth trend is also apparent in SPM suggests that this change might be due to a change in thaumarchaeotal community thriving below surface mixed-layer waters and that this signal is, at least partly, incorporated into sedimentary isoGDGTs. Interestingly, surface-sediment TEX ଼ ୌ values from >1000 m water depth do not show a correlation with water depth anymore Kim et al., GCA 3 and instead are correlated to annual mean SSTs. A composite deep-water TEX ଼ ୌ dataset of surface sediments from both the Mediterranean Sea and the Red Sea, interconnected regional restricted basins with relatively high bottom-water temperatures and high salinity, forms a distinctive correlation line, statistically distinct from that of the general global correlation. Application of this correlation on two sedimentary records from the western Mediterranean Sea covering the last deglaciation yields SSTs nearly identical to those obtained with the paleothermometer, whereas the global calibration substantially overestimates SSTs. Our results show that the warm bias of the TEX ଼ ୌ proxy in the Mediterranean Sea is not due to seasonality, as previously suggested. Further research is needed to elucidate the mechanism behind the strong water depth trend of TEX ଼ ୌ in the Mediterranean Sea which is not apparent in open ocean settings.

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The development of 2 copepod species was studied with nutrient-limited algae as the sole food source. Thalassiosira weissflogii was grown under different levels of nitrogen and phosphorus limitation. Young copepodite stages of Temora longicornis (Müller) and Pseudocalanus elongatus (Boeck) developed at significantly reduced rates, both when fed with the nitrogen-and the phosphorus-limited diatom. At high levels of nutrient limitation, the copepods often did not reach maturity. The lipid composition of the diatom was strongly affected by nutrient limitation. Both the proportion and the content of long-chain polyunsaturated fatty acids (PUFAs) were reduced, particularly under phosphorus limitation. The dominant sterol, Δ 5,24(28) C28:2, was reduced by about a factor of 2 both under nitrogen and phosphorus limitation. The results suggest that the different growth rates of the copepods observed can be explained by the different lipid composition of the algae due to nutrient limitation.

1Long chain 1,13-and 1,15-alkyl diols form the base of a number of recently proposed proxies used for 2 climate reconstruction. However, the sources of these lipids and environmental controls on their 3 distribution are still poorly constrained. We have analyzed the long chain alkyl diol (LCD) composition 4 of cultures of ten eustigmatophyte species, with three species from different families grown at various 5 temperatures, to identify the effect of species composition and growth temperature on the LCD 6 distribution. The results were compared with the LCD distribution of sixty-two lake surface sediments,

The 18S ribosomal DNA molecular phylogeny and lipid composition of over 120 marine diatoms showed that the capability to biosynthesize highly branched isoprenoid (HBI) alkenes is restricted to two specific phylogenetic clusters, which independently evolved in centric and pennate diatoms. The molecular record of C25 HBI chemical fossils in a large suite of well-dated marine sediments and petroleum revealed that the older cluster, composed of rhizosolenid diatoms, evolved 91.5 +/- 1.5 million years ago (Upper Turonian), enabling an accurate dating of the pace of diatom evolution that is unprecedented. The rapid rise of the rhizosolenid diatoms probably resulted from a major reorganization of the nutrient budget in the mid-Cretaceous oceans, triggered by plate tectonics.

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