THE IMPORTANCE OF EXOPOLYMERS

The Importance Of Exopolymers (EPS) In Macroinvertebrates



The Importance Of Exopolymers (EPS) In Macroinvertebrates

ABSTRACT

Like many deposit-feeding organisms, the burrowing brittlestar Amphipholis gracillimafeeds on particulate organic matter in surface sediments. Microbial exopolymeric secretions (EPS) are carbohydrate-enriched polymers produced by microalgae and bacteria that bind aggregates and form dense biofilms near the sediment-water interface. EPS are assimilable by some benthic infauna and may be utilized as a significant carbon source. EPS are absorbed by some deposit-feeders, including a holothurian, and may be supplemental sources of nutrition. The burrowing brittlestar. gracillima is a deposit-feeder that was used in a mass balance approach to model the incorporation of radiolabeled EPS by bottom feeders. Absorption of sedimentbound bacterial and algal EPS was similar (92.2 and 90.1%), but bacterial EPS absorption was significantly (p < 0.05) higher in sediment-bound (92.2%) than aqueous (83.3%) exposures. Algal EPS absorption was significantly higher in aqueous (99.9%) exposures. These findings suggest that EPS may represent a significant energy source for this deposit-feeding ophiuroid and other organisms with similar feeding habits. Additionally, A. gracillima appears to be especially adept at utilizing EPS resources from benthic diatom communities.

INTRODUCTION

Extracellular polymeric secretions (EPS or exopolymers) are a class of high molecular weight exudates produced by bacteria and microalgae. For the microbes that produce them, these polysaccharide-like polymers facilitate surface attachment, help trap passing nutrients, exchange genetic material, stabilize sediments, and buffer cells against environmental stressors (Sutherland 1990, Freeman & Locke 1995, Jahn & Nielsen 1995, Decho 2000, De Brouwer & Stal 2001).

EPS form a structuring matrix for ubiquitous-occurring aquatic biofilms, whose biomass can far exceed that of bacterial cells (Barlocher & Murdoch 1989, Lappin Scott & Costerton 1995). Several marine bacteria produce EPS, including planktonic (Corpe 1970, Berkeley et al. 1980) and deep-sea species (Vincent et al. 1994).

This has forced us to expand modern definitions of bacterial and algal biomass to include the mass of EPS that are closely associated with cells (Goto et al. 1999). Deposit-feeding animals in marine habitats are exposed to EPS in surface films, coated particles, and marine aggregates like transparent exopolymer particles (TEP). TEP are formed from biotic (namely phytoplankton in origin) and abiotic processes like chemical precipitation (Alldrege et al. 1993, Passow 2000) and settle rapidly as marine snow (Passow et al. 1994).

DISCUSSION

Amphipholis gracillima clearly has the ability to absorb secreted algal and bacterial extracellular polymeric secretions. Compositional analyses of bacterial polymers demonstrated that they represent an energy source that can be efficiently used by this animal.

Ophiuroids have seasonal and opportunistic feeding habits-they may scavenge, feed on particles or be carnivorous (Dearborn & Edwards 1982, Hendler 1982). In South Carolina, their feeding activity is highest between February and November, when temperatures are rising and organic production increases.

Although A. gracillima has been described primarily as an indiscriminant surface deposit-feeder, omnivore, and scavenger (Singletary 1970), high algal exopolymer absorption efficiencies are appropriate given the ecology of the mudflat this species lives in. Such high absorption efficiencies also make a plausible connection between high seasonal benthic ...