Lipid storage in marine zooplankton

Abstract

Zooplankton storage lipids play an important role during reproduction, food scarcity, ontogeny and diapause, as shown by studies in various oceanic regions. While triacylglycerols, the primary storage lipid of terrestrial animals, are found in almost all zooplankton species, wax esters are the dominant storage lipid in many deep-living and polar zooplankton taxa. Phospholipids and diacylglycerol ethers are the unique storage lipids used by polar euphausiids and pteropods, respectively. In zooplankton with large stores of wax esters, triacylglycerols are more rapidly turned over and used for short-term energy needs, while wax esters serve as long-term energy deposits. Zooplankton groups found in polar, westerlies, upwelling and coastal biomes are characterized by accumulation of large lipid stores. In contrast, zooplankton from the trades/tropical biomes is mainly composed of omnivorous species with only small lipid reserves. Diapausing copepods, which enter deep water after feeding on phytoplankton during spring/summer blooms or at the end of upwelling periods, are characterized by large oil sacs filled with wax esters. The thermal expansion and compressibility of wax esters may allow diapausing copepods and other deep-water zooplankton to be neutrally buoyant in cold deep waters, and they can thus avoid spending energy to remain at these depths. Lipid droplets are often noted in zooplankton ovaries, and a portion of these droplets can be transferred to developing oocytes. In addition to lipid droplets, zooplankton eggs have yolks with lipovitellin, a lipoprotein with approximately equal amounts of protein and lipid. The lipovitellin lipid is predominantly phosphatidylcholine, so during reproduction females must convert a portion of their storage lipid into this phospholipid. Developing embryos use their lipovitellin and lipid droplets for energy and materials until feeding begins. The various functions storage lipids serve during the different life history stages of zooplankton are very complex and still not fully understood and hence offer a multitude of fascinating research perspectives.

Lipid and fatty acid/alcohol compositions of the subarctic copepods Neocalanus cristatus and Eucalanus bungii from various depths in the Oyashio region, western North Pacific

“…The life history pattern of N. cristatus is almost identical everywhere in the subarctic North Pacific Ocean despite the environmental differences (Tsuda et al, 2004). It has been generally accepted that substantial storage of wax esters with long-chain monounsaturated moieties is an adaptation to a pronounced seasonality of food supply (Lee et al, 2006). This lipid accumulation mode of N. cristatus supports the copepod to maintain the stable life history patterns that do not depend on the environmental changes widespread in the subarctic North Pacific Ocean.…”

“…The long-term lipid deposits are stored as wax esters in many boreal and deep-living zooplankton species (Lee et al, 2006). The wax ester storage is especially pronounced in herbivorous zooplankton and related to the fatty acid and alcohol compositions, i.e., the biosynthesis and accumulation of the 20:1 and 22:1 moieties exclusively produced by herbivorous calanoid copepods (Kattner and Krause, 1989;Kattner and Hagen, 1995;Falk-Petersen et al, 2009).…”

“…But this difference can be simply explained by the difference of the timing of peak primary production in the two areas (Tsuda et al, 2004). Triacylglycerols, the principal lipids stored by E. bungii and other copepods, are more rapidly turned over than wax esters (Lee et al, 2006). Thus, the energy supplied by triacylglycerols is probably more suited to flexible life history features as it is for E. bungii and other triacylglycerol-storing copepods.…”

“…In the Oyashio region, both copepods are known to accumulate and biosynthesize substantial quantities of lipids in the surface layer mainly during the spring bloom season, and undergo winter diapause in deeper layers (Ikeda et al, 1990;Kobari and Ikeda, 1999;Tsuda et al, 2001;Shoden et al, 2005) similar to other copepods in cold waters (Dalsgaard et al, 2003;Lee et al, 2006). The major lipid components of the older stages of N. cristatus are wax esters with the important long-chain monounsaturated fatty acid and alcohol moieties, 20:1 and 22:1, whereas E. bungii contains substantial quantities of triacylglycerols with the major fatty acids 16:0, 16:1(n−7) and 20:5(n−3) (Saito and Kotani, 2000).…”

Physiology and Metabolism of Northern Krill (Meganyctiphanes norvegica Sars)

“…WE: Wax esters (WE) are important and frequently occurring storage lipids in many pelagic crustaceans including euphausiids Saether et al, 1986). It has also been suggested that they play an important role as a 'long-term' lipid store in deep sea and high-latitude zooplankton (Lee et al, 2006). However, only small amounts of WE have been detected in M. norvegica (Fig.…”