STUDIES ON PIGMENTED MICROORGANISMS FROM APHOTIC MARINE ENVIRONMENTS. III. EVIDENCE OF APPARENT UTILIZATION BY BENTHIC AND PELAGIC TUNICATA1

Over a 4-month period 158 colonies of the pelagic tunicate Pyvosoma plus one large salp were trawled from depths between the surface and 750 m in an area to the southwest of the island of Oahu in the subtropical Pacific. Examination of individual zooid gut contents, by light and electron microscopy, revealed the presence of the so-called olive-green cells. These cells were abundant in all organisms examined except those from the euphotic zone. Examination of the gut contents of a benthic tunicate collected from 1,500 m in the Indian Ocean also revealed the presence of large numbers of these cells. This is the first direct evidence that olive-green cells are undergoing significant removal by marine organisms. Olive-green cells ( OGCs ) are pigmcn ted, possibly procaryotic organisms of uncertain identity which are ubiquitous and abundant throughout the oceans3 (Hcntschcl 1936; Fournier 1970, 1971). Their unique distribution has been observed in many widely separated regions. They arc essentially absent from surface waters, but a positive gradient exists between -their initial occurrence at about 50 m and their depth of maximum abundance, usually between 300 and 500 m, followed by a negative gradient between the depth of maximum abundance and the greatest depths sampled. Although the pattern of vertical distribution is to date cverywhcre identical, concentrations vary regionally ( Fournicr 1971) . The reason for this variation is not presently known but it appears to be related to regional levels of primary productivity. Since OGCs arc seldom present in the euphotic zone one possible explanation for this apparent relationship may be the ambient levels of dissolved and particulate organic material, which vary positively with production within the upper several hunl This work was supported by National Science Foundation Grant GB 22628. 2 Present address : Department of Oceanography, Dalhousie University, Halifax, Nova Scotia. 3 I can provide prepared slides to interested investigators to aid their recognition of thcsc cells. drcd mctcrs (Riley 1970). The vertical distribution of these cells coupled with their potential use of cuphotically derived organic material suggests a mechanism for the transfer of organic carbon from the surface to the deep ocean ( Fournier 1972). Any consideration of possible consumers of these cells must ncccssarily bc conditioncd by the fact that OGCs average 3.5 p in diamctcr and at maximum concentrations probably do not exceed 1.5 pg liter-l of potentially utiliznblc carbon (Fournier 1971). They have been found in the gut contents of a wide variety of species of bathypelagic copepods ( Wheeler 1970; Harding 1972); however, Harding estimated that OGCs never exceeded 1% of the total number of recognizable organisms present in any gut. This suggests tither that copepods rapidly digest these cells or that they are inefficient collectors of particles that size. Insufficient information prevents evaluation of the former alternative, but it is well known (Riley 1970; Vinogradov 1970) that particles smaller than 5 p arc unsuitable as a source of food