Answer to the question asked by an anonymous visitor of our site on February 11th, 2007.

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Briefly: (1) Unicellular organisms have one property that makes them indispensable for a stable organization of life. They are small and, hence, very numerous. In accordance with the law of great numbers, energy fluxes flowing through populations of unicellular organisms can be made very stable, with minimized spatial and temporal fluctuations. Life does make use of this opportunity -- on land, as well as in the ocean, up to 99% per cent of primary productivity of natural, undisturbed ecosystems is consumed by unicellular organisms. In the ocean, additionally, most primary productivity is due to unicellular organisms (phytoplankton). However, it appears that stable life cannot be unicellular as a whole. The reasons for why it is so are different for oceanic and terrestrial life. (2) In the ocean, unicellular organisms are unable, without a control from the side of the multicellular ones, to ensure maintenance of their own genetic stability, i.e. they are prone to spontaneous genetic degradation. (3) On land, multicellular organisms (trees) are indispensable for the functioning of the biotic pump of atmospheric moisture, which provides all terrestrial organisms with the main one of life essentials -- water.

The main reaction of life, photosynthesis, is performed by multicellular green leaves on land and by tiny, individually invisible unicellular phytoplankton in the ocean.

Literature to statements 1-3

(1) That unicellular organisms dominate biospheric energy fluxes:

Gorshkov V.G. (1980) The structure of the biospheric energy flows. Botanical Journal, 65(11), 1579-1590 (in Russian). Abstract (engl), PDF (1.5 Mb).
Gorshkov V.G. (1981) The distribution of energy flow among the organisms of different dimensions. Journal of General Biology, 42(3), 417-429 (in Russian). Abstract (engl), PDF (700 Kb).
Gorshkov V.G. (1995) Physical and biological bases of life stability. Berlin, Springer-Verlag, 340 pp. (Chapter 5. The Energetics of Biota. Section 5.6 Distribution of consumption by heterotrophs according to their body size.)
Makarieva A.M., Gorshkov V.G., Li B.-L. (2004) Body size, energy consumption and allometric scaling: a new dimension in the diversity-stability debate. Ecological Complexity, 1(2), 139-175, doi:10.1016/j.ecocom.2004.02.003. Abstract, PDF (380 Kb).

(2) On the problem of genetic stability in general and of phytoplankton community in particular:

Gorshkov V.G., Makar'eva A.M. (2001) On the possibility of physical self-organization of biological and ecological systems. Doklady Biological Sciences, 378, 258-261. Full text (PDF, 123 Kb).
Gorshkov V.G. (1995) Physical and biological bases of life stability. Berlin, Springer-Verlag, 340 pp. (Chapter 4. Stability of the Biosphere's Organization. Section 4.8 Productivity and immigration in the community.)
Gorshkov V.G., Gorshkov V.V., Makarieva A.M. (2000) Biotic Regulation of the Environment: Key Issue of Global Change. Springer-Praxis Series in Environmental Sciences, Springer-Verlag, London, 367 pp. (Chapter 5. Ecological principles of biotic regulation. Section 5.8 Immigration in the ecological community.) see the book contents

(3) Trees as drivers of biotic moisture pump:

see here

In greater detail:

Why cannot oceanic life be completely unicellular?

Ocean has two life-important properties. First, light penetrates only to the upper (photic) layer of the ocean, and it is there where photosynthesis is only possible, and it is there where the unicellular phytoplankton have to reside. Second, the upper oceanic layer is well-mixed by wind and waves. This turbulent mixing rapidly extinguishes with depth.

Life in open ocean is based on organic matter photosynthesized by unicellular phytoplankton. Decompositon of organic matter is performed by unicellular bacterioplankton (marine bacteria), multicellular zooplankton and, to a much lesser degree, nekton -- large multicellular organisms capable of autonomous locomotion independent of water currents.

Due to their small size and inability of independent locomotion, phytoplankton cells cannot use intraspecific competitive interaction to tell apart defective from normal individuals and expell the former from the population.

Indeed, large animals, e.g., deer, can find out which male is genetically inferior and deprive this individual from access to females or lichen resources, forcing the defective individual out from good pastures. However, if females and food were chaotically mixed in space, so that all individuals had equal probability of accessing these resources, then such competitive way of genetic stabilization of the population were impossible. Namely this situation is realized for unicellular phytoplankton, which is destined to live in the well-mixed sunlit oceanic layer.

Although defective phytoplankton cells are incapable of keeping their local environment in the optimal state, they do not lose competitive capacity compared to normal cells, because physical mixing equates the environment for everybody. Due to mixing, environmental deterioration caused by accumulation of defective individuals equally impairs functioning of both normal and defective cells. In such a situation the incessant process of decay of life organization (an analogue of the second law of thermodynamics) should ultimately lead to complete genetic degradation of unicellular species and unicellular life in the upper ocean. This would have happened, had it not been for the presence of multicellular organisms capable of autonomous locomotion.

As is well-known, multicellular zooplankton undergoes diel vertical migration from the surface layer to oceanic depth, where turbulent mixing is low. Thus, at depth it is possible to quietly tell apart normal individuals from defective ones and organize effective competitive interaction between zooplanktonic organisms. This done, normal zooplanktonic organisms can migrate upward and eat away defective phytoplankton cells as well as defective cells of other unicellular species living at the surface (in particular, heterotrophic bacterioplankton). The information about what is the norm and what is decay in surface unicells must be contained in the genetic programs of zooplankton and nekton species.

In all regions of the ocean the genetic purification of phytoplankton cells by zooplankton occurs in one and the same way. Normal individuals are those maintaining optimal environmental conditions, including life of phytoplankton -- the basis of oceanic food pyramid. If defective cells start to predominate, all the local ecological community hosting these individuals impairs its environment, loses competitiveness and is forced out by the adjacent normally functioning community.

In coral reefs unicellular photosynthesizing algae are fixed within corals, so that the community as a whole can afford residing in the photic layer at all times. The same with Sargasso Sea, where photosynthesis is performed by multicellular plants, which, similar to corals, greatly reduce turbulent mixing.

Why cannot life on land be completely unicellular?

On land life is only possible if there is liquid water. The continous forest cover of the continents is able to pump oceanic moisture to land via atmosphere, compensating river runoff and keeping optimal soil moisture content at any distance from the ocean. The mechanism of this forest moisture pump is based on the circulation principle saying that atmospheric air flows from areas with lower, to areas with higher, evaporation. Physically, this circulation is conditioned by the fact that atmospheric air drops very rapidly with height, this leads to condensation of water vapor and creates air pressure shortage in the upper atmosphere, so that surface air is sucked upward to compensate for this pressure shortage.

The higher the evaporation flux from the surface, the greater the flux of condensation and precipitation. Intensive upwelling fluxes of moist air, that occur in the region with high evaporation, make the air on adjacent areas with lower evaporation move horizontally to the region with high evaporation. Therefore, to make land moist, it was necessary to "invent" organisms that could evaporate water more rapidly than the oceanic surface. And such organisms were "invented" in the course of evolution -- these are forest trees forming a continous cover with an evaporative surface (cumulative surface of all leaves) up to ten times larger that the evaporative surface of open ocean of the same area. All other unicellular and multicellular organisms on land serve trees to enable them to perform this life-importan work on moisture pumping. Thus, multicellularity on land is a must, since only multicellular green leaves spatially distributed in the air space, can form an evaporative surface exceeding that of the open ocean.

Vast shrublands, savannahs and grasslands appeared on land in the result of anthropogenic activities. These ecosystems are in the state of gradual transition to deserts, because the persistent human pressure does not allow them to recover back to forests. Depending on the character of anthropogenic pressure, time to complete desertification can range from dozens to several thousand years. For a given human generation this can create an illusion of the stationary stability of these ecosystem types in a particular region.