Roles of Algae in Biogeochemistry

Biogeochemical cycling can be defined as the movement and exchange of both matter and energy between the four different components of the Earth, namely the atmosphere (the air envelope that surrounds the Earth), the hydrosphere (includes all the Earth’s water that is found in streams, lakes, seas, soil, groundwater, and air), the lithosphere (the solid inorganic portion of the Earth, including the soil, sediments, and rock that form the crust and upper mantle, and extending about 80 km deep) and the biosphere (all the living organisms, plants and animals). The four spheres are not mutually exclusive, but overlap and intersect in a quite dynamic way. Soils contain air and exchange gases with the atmosphere, thus causing the geosphere and atmosphere to overlap; but they also contain water, so the geosphere and hydrosphere overlap. Dust from the geosphere and water from the hydrosphere occur in the atmosphere. Organisms are present in water bodies, soils, aquifers, and the atmosphere, so the biosphere overlaps with the other three spheres.

Chemical elements are cyclically transferred within and among the four spheres, with the total mass of the elements in all of the spheres being conserved, though chemical transformations can change their form. The biogeochemical cycle of any element describes pathways that are commensurate with the movement of the biologically available form of that element throughout the biosphere (where the term biological availability is used to infer the participation of a substance in a “biological” reaction as opposed to its simple presence in biota). The most efficient cycles are often equated with a high atmospheric abundance of the element. These cycles ensure a rapid turnover of the element and have the flexibility to process the element in a number of different forms or phases (i.e., solid, liquid, gaseous). Except in a few rare but interesting situations (e.g., geothermal/tectonic systems), all biogeochemical cycles are driven directly or indirectly by the radiant energy of the sun. Energy is absorbed, converted, temporarily stored, and eventually dissipated, essentially in a one-way process (which is fundamental to all ecosystem function). In contrast to energy flow, materials undergo cyclic conversions. Through geologic time, biogeochemical cycling processes have fundamentally altered the conditions on Earth in a unidirectional manner, most crucially by decomposition of abiotically-formed organic matter on the primitive Earth by early heterotrophic forms of life, or changing the originally reducing atmosphere to an oxidized one via the evolution of oxygenic phototrophs. Contemporary biogeochemical cycles, however, tend to be cycling rather than unidirectional, leading to dynamic equilibria between various forms of cycled materials.

Like all organisms, algae provide biogeochemical as well as ecological services; that is, they function to link metabolic sequences and properties to form a continuous, self-perpetuating network of chemical element fluxes. They have played key roles in shaping Earth’s biogeochemistry and contemporary human economy, and these roles are becoming ever more significant as human impacts on ecosystems result in massive alteration of biogeochemical cycling of chemical elements. Just think about the Earth’s initial atmosphere, 80% N₂, 10% CO/CO₂, 10% H₂ (by volume): no free O₂ appeared until the development of oxygenic photosynthesis by cyanobacteria, transforming the atmosphere composition in the actual 78% N₂, 21% O₂, 0.036% CO₂ and other minor gases (by volume); or the petroleum and natural gas we consume as fuels, plastics, dyes, etc. in our everyday life; these fossilized hydrocarbons are mostly formed by the deposition of organic matter consisting of the remains of several freshwater marine microalgae from the classes Eustigmatophyceae, Dinophyceae, and Chlorophyceae.

These remains contain bacterially and chemically resistant, high aliphatic biopolymers (algaenans) and long-chain hydrocarbons that are selectively preserved upon sedimentation and diagenesis and make significant contribution to kerogens, a source of petroleum under appropriate geochemical condition. Moreover, we are still using the remains of calcareous microorganisms, deposited over millions of years in ancient ocean basins, for building material. Diatomaceous oozes are mined as additives for reflective paints, polishing materials, abrasives, and for insulation. The fossil organic carbon, skeletal remains, and oxygen are the cumulative remains of algae export production that has occurred uninterrupted for over 3 billions years in the upper ocean.

In total, 99.9% of the biomass of algae is accounted for by six major elements such as carbon (C), oxygen (O), hydrogen (H), nitrogen (N), sulfur (S) and phosphorus (P), plus calcium (Ca), potassium (K), sodium (Na), chlorine (Cl), magnesium (Mg), iron (Fe), and silicon (Si). The remaining elements occur chiefly as trace elements, because they are needed only in catalytic quantities.

All elements that become incorporated in organic material are eventually recycled, but on different time scales. The process of transforming organic materials back to inorganic forms of elements is generally referred to as mineralization. It takes place throughout the water column as well as on the bottom of water bodies (lakes, streams, and seas), where much of the detrital material from overlying waters eventually accumulates. Recycling of minerals may take place relatively rapidly (within a season) in the euphotic zone (i.e., the portion of water column supporting net primary production) or much more slowly (over geological time) in the case of refractory materials which sink and accumulate on the seabed. In the water column, where there is usually plenty of oxygen, decomposition of organic material takes place via oxidative degradation through the action of heterotrophic bacteria. Carbon dioxide and nutrients are returned for reutilization by the phytoplankton. Ecologically, the most important aspect of recycling in the water bodies is the rate at which growth-limiting nutrients are recycled. Among the nutrients that are in short supply, nitrate (NO^-₃), iron (bioavailable Fe), phosphate (PO₄³²), and dissolved silicon [Si(OH)₄] are most often found in a concentration well below the half-saturation levels required for maximum phytoplankton growth. In particular, algae are important for the biogeochemical cycling of the chemical elements they uptake, assimilate, and produce, such as carbon, oxygen, nitrogen, phosphorus, silicon, and sulfur. We will now briefly consider some aspects of these elements in relation to biosynthesis and photosynthesis.